VIRGIN ISLANDS RESOURCE MANAGEMENT COOPERATIVE BIOSPHERE RESERVE RESEARCH REPORT NO.21 SEDIMENTATION AND REEF DEVELOPMENT IN HAWKSNEST, FISH AND REEF BAYS, ST. JOHN, U.S. VIRGIN ISLANDS Dennis K. Hubbard, James D. Stump and Brian Carter West indies Laboratory Fairleigh Dickinson University St. Croix, U.S. Virgin. Islands Virgin Islands National Park August, 1987 SEDIMENTATION AND REEF DEVELOPMENT IN HAWKSNEST, FISH AND REEF BAYS, ST. JOHN, U.S. VIRGIN ISLANDS 1987 BIOSPHERE RESERVE REPORT NO. 21 DENNIS K. HUBBARD JAMES D. STUMP BRIAN CARTER WEST INDIES LABORATORY FAIRLEIGH DICKINSON UNIVERSITY ST. CROIX, U.S. VIRGIN ISLANDS U.S. DEPARTMENT OF THE INTERIOR NATIONAL PARK SERVICE AND VIRGIN ISLANDS RESOURCE MANAGEMENT COOPERATIVE VIRGIN ISLANDS NATIONAL PARK P.O. BOX 7789, ST. THOMAS U.S. VIRGIN ISLANDS 00801 LOCAL CONTRACTING AGENT ISLAND RESOURCES FOUNDATION RED HOOK BOX 33, ST. THOMAS U.S. VIRGIN ISLANDS 00802 (NPS CONTRACT NO. CX-0001-3-0048) ABSTRACT In recent years, the potential impacts of sedimentation on reef development have been increasingly recognized. With ever-increasing development stress being placed on the upland areas of St. John, both the National Park and local residents have become concerned with the impacts of development on natural resources, and specifically on nearshore reefs. This study was conduc- ted as a first attempt to evaluate the responses of nearshore reefs in three bays over the past two hundred years. Specifically, the impacts of land use on nearshore sedimentation levels were of special interest. A theoretical study of runoff under natural conditions intimated that the present distribution of reefs around the island are primarily controlled by watershed size, bay geometry and exposure, with recent development exerting a secondary impact. It was established, however, that more frequent (10-25 yr) storms play a very important role in controlling the distribution of reefs around the island. Therefore, it is not necessary to invoke the catas~ trophic event (e.g. 100-yr storms) to explain the occurrence of modern reefs. From this, it can be argued that development can have significant impact under less than heavy runoff conditions. Studies of the present distributions of sediment types and reefs in Hawks- nest, Fish and Reef Bays point to controls identical to those identified by the more generalized study. Of critical importance in reef development was distance from sources of terrigenous runoff (the guts) and degree of exposure. On a local level, these both outweighed development at its present level. Over the long term, there appears to be a gradual decline in the reefs over the entire period of record, based on X-rayed cores through large coral heads. This may be related to a long-term compromise of the soil retain- ing capabilities of the upland watersheds following intensive cane farming in the 1700's and 1800's. With the exception of short-term degradation of the nearshore reefs in Hawksnest Bay, however, reef degradation could not be directly linked to any specific development activity. Based on the findings of this study, specific management strategies are proposed. These include limiting development in main water courses, and establishing reasonable buffers to runoff related to upland construction. Recommendations for future research objectives are also offered. ACKNOWLEDGEMENTS This project was supported by the Virgin Islands Resource Management Cooperative and the National Park Service. The authors wish to further ac- knowledge the valuable assistance of Dr. Caroline S. Rogers and Ms. Vonnie Small (National Park Service) in carrying out the field aspects of the study. Mr. Michael Davis and Ms. Ellen Prager also assisted in the collection of field data. Thanks are also due to James Beets, Allen Putney, Vonnie Small, and George Tyson for helpful criticisms and input throughout the preparation of the manuscript. And finally, we wish to acknowledge all the members of VIRMC and the West Indies Lab staff for input and encouragement throughout the study. ii TABLE OF CONTENTS ABSTRACT cccccccccccccccccccccccccssscccccccccccsssssssccssscssessnscssrcccrceel ACKNOWLEDGEMENTS .ccccccccccccccccccccccccccccccccccsssscccscesessscsecscrsesell TABLE OF CONTENTS. occcccccccccccccccccccccsccvcccsccsvcsssccvcccescsscsssevetid LIST OF FIGURES cc cccccccccccccccccccccccsssescccccccssssssscccsssesosecsssesely LIST OF TABLES cc cccccccccccccccccccccccccccscssccscccsccsssccccscssesseseseViid INTRODUCTION. ccc ccecccccccccccecccccccccccccsccccccsscsscscccccccesccsssoeseoel WATERSHED ANALYSES. ccccccccccccccccccccccccssnscccccccscccssssssescseseseesseey MethodSsecccccccccccccccccccccccvccccsscsccccccssccccssssssssccsecssssessseed ResultScccccecccccccccvccccccccsccscnssscccvccscscesesssscsssessesevcsssceele PRESENT-DAY REEF ENVIRONMENTS. cccccccccccccccccccccccccnccccccceseceesseresee ds INtroductions.sccccccccccccccccccceccccsccccccccccsccesscsssesescucccsssessn ds MethodScccccccccccccccccccccccccccccccccsscvcccseccsccscsssscesscvesvesseesad Hawksnest Bayccccccccccccscccccccccccveccccsscssssscccccccsccscrsssssasececdl Fish Baycccccccecccccscscecccccccccccccccecssscscssccssccsccescscscscscees shh Reef Bays ccccccecccecccececccccccecccecesssesscersssceceseceseseeesseneees eld ENVIRONMENTAL CHANGES. cc ccc ccc cc ccc cccccccccccccsccccnssesecscsevccscccssesseO/ Sediment CoreSccccccccccccccscccccccccccccccvcesssscscccssccccsecssensersesO/ Coral-Growth Study... cccccccccccccccccccccccccccccsssssevsssccccscecascceseslt SUMMARY .cccccccccccccccscctcccccvcnccccccscccsccsessssssecccsccsscsscccceseeedd Ceneraleccececceccecceccccccccccccccccecescssccaccscscccscvcssesecsscsesesedl Management Lmplicationsesseccccavccccccccccccccnccccsccscessccccsccceseneseds LITERATURE CITED. cc ccccc cece ccc cccccnccccccccccceecesesesesssesssesseeeees e090 APPENDIX Loccccccccccccccccccccccececscesesesssscsesecsccssccccessceneesssesedd Lil 10. Il. 12. 13. 14, 15. 16. 17. 18. 19. 20. LIST OF FIGURES Map showing location of St. John and bays discussed in text....cccccscess 2 Nomogram used in calculation of peak discharge rates for watersheds with an average CN of BO. coc eer c wc cnccccecccccccccessscscesell Nomogram used in calculations of peak discharge rates for watersheds with an average CN of 85. This nomogram was used for all CALCULALIONS ccc ececccccccccccccecncencccccccccccescccccccel? Map of St. John showing watershed areas in ACTES ccesccvcccccvcscesevcsceslO Peak discharge rates expressed in cfs for St. John watersheds during a 2-yr frequency storm (3.9"/24 hrs.).ccceccececcccecel9 Peak discharge rates expressed in cfs for St. John watersheds during a 5-yr frequency storm (5.5"/24 hrs.)esccceccccccccceel9 Peak discharge rates expressed in cfs for St. John watersheds during a l0-yr frequency storm (6.6"/24 hrs.).cccccccscccceee20 Peak discharge rates expressed in cfs for St. John watersheds during a 25-yr frequency storm (8.0"/24 hrs.)scsccccccccceeec20 Peak discharge rates expressed in cfs for St. John watersheds during a 50-yr frequency storm (9.2"/24 hrs.).cccccccccccecceldl Average runoff volumes expressed in cfs for St. John watersheds during a 2-yr frequency storm (3.9"/24 hrs.).sccosccesceccece2l Average runoff volumes expressed in cfs for St. John watersheds during a 5-yr frequency storm (5.5"/24 hrse)ecccccccccccccccedld Average runoff volumes expressed in cfs for St. John watersheds during a l0-yr frequency storm (6.6"/24 hrs.).scccccccecccecces22 Average runoff volumes expressed in cfs for St. John watersheds during a 25-yr frequency storm (8.0"/24 hrs.).ccccccccccccees23 Average runoff volumes expressed in cfs for St. John watersheds during a 50-yr frequency storm (9.2"/24 hrs.).cscccucccecececl3 Average runoff volume expressed in cfs for Hawksnest Bay for different intensity storms. Note the consistent decrease in the slope of the curve with increasing SCOFM INCENSLCY...cecccecccreccccccesccncvsscccsccvccscscccscccseccccec elt Peak discharge rate expressed in cfs for Hawksnest Bay for different intensity storms. Note the consistent decrease in the slope of the curve with increasing SCOFM INCENSLLEY eo cececccrcecccvccccvevenceseccsscccccccescccccavecessecs lt Average runoff volumes expressed in cfs for Fish and Reef Bays for different intensity storms. Note the consistent decrease in the slope of the curve with increasing storm intensity. ..cccccecccccevcccescccccccccssesvesveseseseeld Peak discharge rates expressed in cfs for Fish and Reef Bays for different intensity storms. Note the consistent decrease in the slope of the curve with increasing StOrm inteNSity... cc ccescccccccvcccccccccccccccccccecsceseve eld Map of St. John showing average runoff volume for shoreline segments for a 2-yr frequency storm. Dark symbols indicate high runoff values.....ccccccccccccccccccccccccee 26 Map of St. John showing average runoff volume for shoreline segments for a 10-yr frequency storm. Dark symbols indicate high runoff values. The pattern of high-intensity runoff is significantly different than that shown for the 2-yr storm (Fig. 19), but is similar to that of more severe storms (e.g. 50-yr Storm: Fig. 21) .ccccccccccccccccccnccccccccccccceccccccecsccrsescccccceeldl iv 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 4l. 42. 43. 44. 45. 46. 47. Map of St. John showing average runoff volume for shoreline segments for a 50-yr frequency storm. Dark symbols indicate high runoff values. Note the similarity to the pattern seen for less severe, 10-yr storms (Fig. WW) ecccccacccccccccssccccccccessses lh Base map of Hawksnest Bay. Locations of transects, sediment samples and coral cores are shown. Arrows delineate entrances of major guts into the Days ccccecceccecccccccecccascescecsesesenssseee tod Base map of Fish Bay. Locations of transects, sediment samples and coral cores are shown. Arrows delineate entrances of major guts into the DAY sce ccccsccccccccccesccvesccvccesesesse Base map of Reef Bay. Locations of transects, sediment samples and coral cores are shown. Arrows delineate entrances of major guts into the Dayscccccsccccceccccccessssccssecesesee30 Map of Hawksnest Bay showing grain-size characteristics of surface sediments along each LLANSECEecesecccoccvccceecscccceceeeesee ad Map of Hawksnest Bay showing the percentages of terrigenous material in the surface SECIMENES ceccccccccccccccscccscccecccessseseces c40 Hawksnest Bay profile HB-Ol. A key to profile symbols is included in Figure Wesccccccescccccccceccccccscsecccccscsccosessescces el Hawksnest Bay profile HB-02 (left). The symbols shown in the key (right) are consistent on all profiles in this LEPOTteccececeee 42 Hawksnest Bay profile HB-03. A key to profile symbols is included in Figure Wecececnccccccceccescvesssvcccscccccscssvecssssectd Hawksnest Bay profile HB-04. A key to profile symbols is included in Figure WDWiecrcesccccccccccccccccccsvccccsssccsccccssvcvesss ss shh Map of Fish Bay showing grain-size characteristics of surface sediments along each EKANSECEececcccccccccccccecncceccccccncccccseesses el Map of Fish Bay showing the percentages of terrigenous material in the surface SedimentS..ccccscccccccsrccccccceccsecssceccccesc48 Fish Bay profile FB-01. A symbok key is included in Figure V2: ee) Fish Bay profile FB-02. A symbol key is included in Figure 2B owaeceeeee D0 Fish Bay profile FB-03. A symbol key is included in Figure WBessesecceccrl Fish Bay profile FB-04. A symbol key is included in Figure Woseseceseccrd2Z Map of Reef Bay showing grain-size characteristics of surface sediments along each EVANSECEececccccccccccccecsssserecescccsccsesesessede Map of Reef Bay showing the percentages of terrigenous material in the surface Sediments...cccsecscccccccccccccccccacsecesesces lO Reef Bay profile RB-Ol. symbol key is included in Figure 28.......2+.+57 Reef Bay profile RB-02. symbol key is included in Figure 28...........58 Reef Bay profile RB-03. symbol key is included in Figure 28.....+..+++59 Reef Bay profile RB-04. symbol key is included in Figure 28...........60 Reef Bay profile RB-05. symbol key is included in Figure 28...........61 Reef Bay profile RB-06. symbol key is included in Figure 28...........62 Reef Bay profile RB-07. symbol key is included in Figure 28...........63 Generalized circulation within Reef Bay. To the east, wind and wave-driven currents carry turbid lagoonal waters toward the bay head. This sediment, along with that derived from runoff, flows toward the western reef face where it is confined by incoming waves. This flow pattern likely accounts for the often-turbid conditions encountered in this region. Flow vectors are based on observations during several field ViSLCtScccccccccccccccccsevcvcccccccccccsccsssseess s00 Sediment cores from Reef and Hawksnest Bays. See Figure 48b for key to symbols for core LOgS.cecscccccccccccccccccercccccsssssesces 208 > PP > > > > 48a. 48b. 49a. 49b. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. Sediment cores from Fish Bay. See Figure 48b for key to symbols for core LOSS cree ccccccecccsccrccessccerccvcccccescccccceseee o69 Sediment cores from Fish Bay (continued) on left. A key to symbols in Figures 47 and 48a is located on the bob 6:40) ey 2 Fish Bay profile FB-04 showing percent terrigenous material horizontally in cores. Note: Cores are exaggerated vertically to enhance clarity of contour lines.............71 Fish Bay profile FB-04 showing percent terrigenous material horizontally in COTES cee ec eccsescccvccsccrecccccnsncsccessccccscsccvcccell Reef Bay profile RB-07 showing percent terrigenous material in sediment cores. Note: Cores are exaggerated vertically to enhance clarity of contour LineSeccccccccccccvaccccccccccccseccceccced2 X-ray positive of a typical core segment showing annual growth DaNdS.ecesececcrecccccccccccccccccccevccccccccccecccccecce?5 S~year average growth rate for cores from Fish Bay. All values have been normalized to remove variability between corals due primarily to depth of occurrence. To obtain actual growth rates, multiply values derived from the graph by the number in the upper left-hand COLNE esccccccvccevesceselO 5-year average growth rate for cores from Reef Bay. All values have been normalized to remove variability between corals due primarily to depth of occurrence. To obtain actual growth rates, multiply values derived from the graph by the number in the upper left-hand COTNCT es ecscccveccccsseceels 5-year average growth rate for cores from Hawksnest Bay. All values have been normalized to remove variability between corals due primarily to depth of occurrence. To obtain actual growth rates, multiply values derived from the graph by the number in the upper left-hand corner... ...eccccccccceeee78 Annual plots of growth rates for selected corals from the three bays.cercccccccccccccccnccccccccccccccscccccncccccccccccccccee?9 Average yearly rainfall for St. John. Each point is a 5-year average of data provided by the National Park Service on St. John. Data are normalized in the same fashion as for coral growth. To obtain actual rainfall, multiply values derived from the graph by the number in the upper left-hand corner... .ccccscccccecccccccccccccccecccccccceccceeSl Maximum yearly rainfall for St. John. The maximum monthly rainfall was averaged over the five-year periods. Data are normalized in the same fashion as for coral growth. To obtain actual rainfall, multiply values derived from the graph by the number in the upper left-hand corner. Data provided by the National Park Service on St. John...cccscoccccccccccccec8l Graph displaying amount of annual 9" rainfall. The value reported is the sum of all monthly rains equal to or OXCENding Mec eecceccccrccseccscccccenccccccscccccrsecccececcscccees 82 Graph displaying amount of annual 12" rainfall. The value reported is the sum of all monthly rains equal to or exceeding 12"... . cece ccc c cece ec ee cen vescceccscccrccsccescccceccecceec82 Annual growth rates for small coral heads collected from three sample sites in Hawksnest Bay (for sample-site locations, see Figure 22)... ... ccs cca ccccc ccc ccccccccnccccccccccccccccee83 vi 62. 63. 64. Summary of annual growth data from coral cores in the three bays on St. John. Within 5-year periods, growth rates are grouped in intervals between 0-0.5, 0.5-0.75, 0.75-1.0 and >1.0 Cm/Yressccccccecsccccceccvssccsscecsscscasecsssssssccccsssesscesse Od Graph of maximum coral age versus water depth in the St. John cores. Note the trend toward increasing age with depth (and away from sources Of runOff)...ceceesssseccecccccesereccrcccrers Graph of coral-growth rate vs. depth in the St. John cores. The dashed line describes the range of growth rates for corals from similar depths at Cane Bay on St. Croix (Hubbard and Scaturo, 1985). Changes in average-growth rate during the three development periods are also ShOWNececccccccccccccccccccccsssssccccccccssessssosese eds Graph of coral-growth rate vs. depth in the St. John cCoreS.cesesecceeees 288 22 085 LIST OF TABLES Runoff curve numbers for agricultural areas. Higher curve numbers indicate less infiltration and increased TUNOLE. ccc ccecccsecseors Runoff curve numbers for urban areas with development completed and vegetation established... . cc ccccccccccccccccccccscvcccccc e& Runoff curve numbers for urban areas with development underway and no vegetation established... cccscccccccccccccccccccccecesesS Hydrologic soil groups, based on infiltration and ETANSMISSION LACES. ceecececcccrcccsccccccccvccccsccusccccvccccccccuccececd Chart used to determine slope factor. ...ccccccccccccccccccceccecceccceccel3 Chart used to determine average runoff depth per 24 hours...scecceccceeceld Data used in calculation of runoff for St. John watersheds.....seccccecceel? List of bays containing shoreline segments with runoff values in excess of 50 cfs/ft under different storm conditions...scecccccceccee30 Watershed: Bay-area ratio for St. John bays, and their relation to reef development... ...scccccccccccccccccccccccccceccccseccece3l Sediment sample treatment procedures..cccccccccccccccccscecccceccvceceese 38 INTRODUCTION The history of St. John (Fig. 1) is rich and varied. Since its settlement by Europeans in 1718, it has been the site of many kinds of development stress. From 1718 to 1850, the island was cultivated for agriculture, the primary crop being sugar cane. During this time period, cattle grazing was also important in certain areas. Through much of the late 1800's and early 1900's this environmental pressure was largely removed after the demise of the local cane industry in the late 1850's. In Reef Bay, cane production continued until the early 1900's, but at a greatly diminished capacity. With the gift of land to the National Park Service in 1956, Laurance Rockefeller placed much of the island under federal control with the hope of preserving at least part of the natural beauty of the island. The remaining areas, however, are coming increasingly under new, and potentially more severe pressures. Developers, both large and small, have discovered the marketing potential of a tropical area wiht a National Park in its back yard. The influx of people has resulted in both direct development in the terms of new homes, condominiums, etc., and indirect development as an outgrowth of the developing infrastructure necessary to support population growth. And finally, the impact of the Park facility cannot be ignored. While the control exerted by the Park Service over parts of the island minimize the human impact, creating a facility for the public to appreciate those natural environments necessarily takes its toll. Because the Park must operate within the overall community of St. John, it has raised justifiable concerns about minimizing the impact of Park opera~ tions, and development in adjacent areas, on the integrity of the Park itself. Likewise, St. John residents are concerned over the impact of the Park in the areas within and adjacent to its jurisdiction. Coincident with this concern is the existence of the Virgin Island Biosphere Reserve, which includes the entire Park area. The purpose of this project was to assess the longer-term impacts of historical development in the area. To address the problem, three separate, but related studies were conducted. In the first, a theoretical approach was used to model runoff within individual watersheds on St. John. The purpose of this exercise was twofold. First, it was intended to establish the primary controls of reef occurrence on St. John. Secondly, it tested the usefulness of the technique as an interpretive tool. The remaining studies were conducted on three bays that are different both in terms of natural environment, and presumed levels of development stress. Because of their different locations and histories, as well as varying degrees of control exerted by the Park Service, each bay presents a unique set of environmental and management problems. One study described the present character of the bays, and related that to the parameters identified above. The other measured changes in the bays over the past 100-200 years. 5 VaS NV3AEaluvo He “tela 4 NHOF ‘18s NV390 SILNVILV Qe °3x84 UT passnostp sheq pue uyor *4g Jo uoTyeooT Butmoys dew “t,t aundty i B8@q@ 1B10 BUuoT MDE LZ BL 4 WT N9b,b9 Because of the relatively steep nearshore slope around St. John, reefs are confined to narrow strips close to shore. Because of their positions, these fringing reefs are very susceptible to environmental stresses related to upland areas. All three bays receive runoff from large watersheds (Hawksnest = 233 acres; Fish = 1448 acres; Reef = 1391 acres). During storms, rainwater flows through well-defined "guts" that empty into the bays so as to form a point source of upland sediment. The locations of these guts exert a profound effect on the distribution of reefs and live organisms within the bays. The present condition of the reefs is the result of many complex factors in combination with one another. It is not always obvious which control is primary and which are secondary in nature. With respect to watershed management, the obvious point of interest is the degree to which changes in runoff will affect these factors. Therefore, it is important to be able to first, separate the effects of natural runoff from other controls of reefs development. Secondly we need to address what changes in the watersheds might impact those reefs, either positively or negatively. In bays wholly within the Park boundaries, a posture of total protection of the watersheds might be appropriate. Within areas where at least part of the watershed is outside the control of the Park, and development is likely to occur, it is important to establish at what level those changes become a negative factor in the development of the reefs. Of critical importance here is understanding what levels of stress might result in degradation of the marine resource. Because our understanding of the organism-level responses of reefs to sediment stress is in its infancy, this study concentrates on documenting two things: 1) present environmental conditions within individual bays relative to coral distribution; and 2) changes in those reefs through historical time, which might reflect changes in environmental conditions over the period of record. This investigation is only a very small first step in understanding sedimentary controls on reefs on St. John or in tropical areas in general, In addition to the more obvious limitations of cost, reef researchers will remain at a severe disadvantage until quantitative information is finally available about the metabolic responses of reef organisms to various types and levels of stress. Nevertheless, our nearshore environments are coming under severe and increasing stress in the meantime. This study is an attempt to address some very basic questions, albeit in a simplistic fashion, until those data eventually become available. This report is divided into four sections. The first is a generalized discussion of the watersheds on St. John based on the theoretical calculations mentioned above. The second describes the three bays within which this study was concentrated. The primary focus is on reef character, sediment distribution, and the relationship between the two. The third section deals with changes in the environment over the recent past. Sediment cores were taken to examine changes in the amount of terrestrial material introduced into the bay over time. Cores through large coral heads record changes in the responses of reef organisms to those and other stresses. A final section briefly discusses elements of the study that are relevant to management strategies within the Park/Biosphere Reserve and surrounding environs. A discussion of future research stategies is also offered. Throughout this report, metric units are generally used. One exception is the section on Watershed Analyses. Because all the nomograms, graphs and tables available are in English units, use of metric units within the text would create confusion between values cited in the text and those in the tables. Therefore, English units are used in this section, and conversion factors for all units used in the watershed studies are included as Appendix I. The conclusions of this study must draw heavily on those of other companion VIRMC projects. Data on historic changes on St. John are derived from the work of George Tyson, a local historian. Generalized information on the location and character of reefs in bays not examined in this study is drawn from Beets, et al. (1985). It is hoped that by using this approach, we will be able to integrate information from many disciplines, and provide a reasonable picture of natural and man-induced controls on the reefs within the three bays. While an attempt is made to provide information that might be useful in watershed management within the Biosphere Reserve in general, great care must be taken in applying these recommendations beyond the three watersheds from which they were developed. WATERSHED ANALYSES The reef system surrounding St. John is an important ecological asset as well as an energy buffer zone which helps reduce beach erosion. With | increased human development on St. John, this resource has come under increasing pressure. In this section, general patterns of runoff from St. John watersheds are discussed. The principal goal of this exercise was to evaluate whether the present pattern of reef development around St. John can be explained by variations in natural sedimentation, or whether obvious development impacts play the primary role. At the outset it was recognized that sedimentation patterns around St. John are far too complex in their origin to be categorized with one model. Nevertheless, it was felt that some attempt to relate potential physical controls to reef development would be useful. The major problem in this approach is in separating all the different kinds of modern development activities into groups that would have equal impact on the reefs. In addition, it is impossible to accurately describe the levels of those activities over the time period during which the present-day reefs have developed. Given an inability to accurately and objectively characterize development impact, an approach was chosen that eliminated those stresses from consideration. In our calculations, a condition of complete forestation (presumably the case prior to colonization) was chosen. It was felt that if the present-day pattern of reefs was responding on a large seale to recent development activities, then the reef distribution emerging from this exercise would vary noticeably from those occurring in nature today (i.e. in areas of development, reefs would be more poorly represented than under the forested conditions of the model). Such an approach has obvious limitations. With the present data base, we ean only reliably relate the hypothetical runoff patterns to the presence or absence of reefs in any given area. Valuable information exists in aerial photos and a recent report by Beets, et al. (1985). Information from these sources was used to extend the reef classes to "Absent, Scarce, Present or Abundant", but these are only qualitative terms, and this must be kept in mind throughout the discussion that follows. If we are to extend the usefulness of our exercise beyond general descriptive terms, data will eventually be needed that specifically quantify reef development in areas that are identified by this model as naturally stressed, or where man-induced stresses in recent times can be quantified. Methods Estimates of the peak rate of discharge and the average runoff volume for storms of various magnitudes were made, based on methodology outlined in the Engineering Field Manual for Conservation Purposes (Kautz, 1975). Variables used in the calculations included: 1) watershed area (in acres), 2) rainfall (in inches over a 24-hour period), 3) average watershed slope (in percent), 4) watershed vegetative cover, 5) the hydrologic soil group (e.g. clayey soils with low infiltration and high runoff rates), 6) the antecedent moisture condition of the soil (from the previous 5-day accumulated rainfall) and 7) the conservation practices within the watershed (e.g. contouring and terracing of the land). The relative importance of one factor over another, and the specific means used to measure them (e.g. using average vs. maximum slope below), can be argued for specific cases in individual watersheds. It should be recognized at the outset, however, that these methods represent a time-tested standard, and that redesigning this engineering methodology is well beyond the scope of this exercise. Furthermore, the purpose of this study was to evaluate whether the present distribution of reefs around St. John can be generally explained by runoff patterns under natural conditions. It was not to evaluate either specific impacts of development within individual bays or to establish critical levels of rainfall at which runoff will impact specific reefs, While these are important pieces of information, it is unreasonable to attempt these without substantial field data from each watershed and a much better knowledge of the specific sedimentation levels that given organisms can tolerate. Watershed area was measured from the most recent USGS topographic map of the island. Watershed boundaries were delineated, and watershed acreages were measured using a Houston Instruments HiPad Digitizer and an Apple IIe computer. Watershed areas on the map were measured in square inches, and converted to acres using the topographic map scale (1:24000). Rainfall data for St. John were acquired from U.S. Weather Bureau Atlases for 2-year, 5-year, 10-year, 25-year and 50-year frequency storms. 24—hour rainfall values for these storms on St. John were 3.9, 5.5, 6.6, 8.0 and 9.2 inches respectively. 100-year storms were not modeled, as relationships between runoff patterns and reef development had already emerged at lower levels of storm intensity. Given the accuracy of the runoff predictions, it was felt that the variability at the 100-year level would be too subtle to be discriminated by the technique. Watershed slope was calculated using an average from at least two profiles within the watershed. Total relief was measured along each profile, and divided by the distance along that profile. Slope was expressed in percent. Soil conditions (vegetative cover, hydrologic soil group, antecedent moisture condition of the soil, and conservation practices - Tables 1-4) were used to determine a runoff curve number (CN) for each watershed. This value is necessary to choose from a family of nomograms provided to determine discharge estimates. For a more detailed discussion of curve number, the reader is referred to Kautz (1975). All soils on St. John were considered to be in hydrologie soil Group D (Table 4), those soils with a high runoff potential (i.e. due to flashiness of rain, thin sediment cover and eclay-rich soils, infiltration is generally low). Table 1. Runoff curve numbers for agricultural areas. Higher curve numbers indicate less infiltration and increased runoff. From Kautz (1975). Description Hydrologic Hydrologic Soil Group Condition A B Cc D Fallow Straight row tated 77 86 91 94 Row Crops . - \ . Straight row " Poor * 72 81 88 91 Straight row Good 67 78 85 * 89 Contoured Poor 70 719 84 88 Contoured Good 65 75 82 B86 Contoured & terraced Poor 66 74 80 82 ___ Contoured & terraced Good 62 71 78 81 Small Grain Straignt row Poor 65 76 84 88 Straight row Goad -63 75 83 87 Contoured Poor 63 74 82 85 Contoured Good 61 73 81 84 Contoured & terraced Poor 61 72 719 82 Contoured & terraced Good 59 70 78 81 ' Sugar Cane- ~~ Straignt row Trash burned 43 65 77° =~. 82 Straight row Mulched 45 66 77 83 ‘Contour : “—-- 32 58 72 79 Straight row. Poor 49. 69 79 84 Cotfee . . ; No ground cover -—--- 48 68 79 83. Terrace & ground cover _——— 22 $2 68 75 No terraces Good 25 55 70 77 Close-seeded Legumes or Rotation Meadow - Straight row . Poor 66 77 85 89 Straight row Good 58 72 81 85 Contoured . Poor 64 75 83 85 Contoured Good 55 69 78 83 Contoured & terraced Poor 63 73 80 83 Contoured & terraced Good §1 67 76 80 Pasture or Range - No mechanical treatment Poor 68 79 86 89. No mechanical treatment . Fair 49 69 79 84 No mechanical treatnent Good 39 61 74 .80 Contoured Poor 47 67 81 88 Contoured Fair 25 $9 75 83 Contoured Good 6 35 70 719 Meadow Good 30 - + +#2+58 71 78 Woods Poor: 45 66 q7 83 Fair 36 60 — 73 719 Good 25 §5 70 77 Farmsteads ---- 59 74 82 86 Roads. Including FRights-of-vav OS Dirt -—-- 72 82 87 89 tL Hard surface ---- 74 84 90 92 Table 2, Runoff curve numbers for urban areas with development completed and vegetation established. Description Hydrologic Soil Group A B io D Lawns, Parks, Golf Courses, Cemeteries, etc. . : 39 61 74 80 Pavement and Roofs:- Commercial oo and Business Areas . 98 98 98 98. 4: Row Houses, Town Houses, and Residential with Lot Sizes 1/8 Acre or Less 80 85 90 95 Residential = Lot sizes of 1/4 acre 61 75 83... 87 Lot sizes of 1/2 acre . et) 70 80 85 Lot sizes of 1 acre 60 68 79 84 Lot sizes of 2 acres 47 66 77 81 Table 3. Runoff curve numbers for urban areas with development underway and no vegetation established. Description Hydrologic Soil Group ; A B Cc D Newly Graded Area 81 89 93 95 Pavement and Roofs - Commercial anc Business Areas 98 938 98 98 Row Houses, Town Houses, and Residential with Lot Sizes 1/8 Acre or Less . . ; 93 96 97 - 98 Residential _~ Lot sizes of 1/4 acre 88 93 95 97. Lot sizes of 1/2 acre 85 91 94 “96 Lot sizes of 1 acre 82 90 93 95 Lot sizes of 2 acres 81 89 92 94 Table 4. Hydrologic soil groups, based on infiltration and transmission rates. Over 8,000 soils have been classified into four hydrologic soil groups as shown in Exhibit 2-1. The hydrologic soil groups, according to their infiltration and transmission rates, are: A. (Low runoff potential). Soils having high infiltration rates even when thoroughly wetted. These consist chiefly of deep, well to excessively drained sands or gravels. These soils have a high rate of water transmission in that water readily passes through them. ° Soils having moderate infiltration rates when thoroughly wetted. These consist chiefly of moderately deep to deep, moderately well to well-drained soils with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission. Soils having slow infiltration rates when thoroughly wetted. These consist chiefly of soils with a layer that impedes down- ward movement of water or soils with moderately fine to fine texture. These soils have a slow rate of water transmission. (High runoff potential). Soils having very slow infiltration rates when thoroughly wetted. These consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious material. These soils have a very slow rate of water transmission. Conservation practices~- The procedure does allow for adjustments in the runoff calculations based on such things as terracing, paving, etc. As discussed above, however, because it would be difficult to 1) characterize general conservation practices by watershed, and 2) almost impossible to track these practices through historical time, a forested condition was chosen. In this sense, the model approximates "natural" conditions. Peak discharge rates were derived for each watershed area using nomograms such as those shown in Figures 2 and 3. Rainfall (in inches per 24-hour period), drainage area (in acres), and the runoff curve number for the watershed areas are the variables used in these figures. Curve numbers for wooded areas in soil group D varied between 77 and 83. (Table 1). To model worst possible conditions (i.e. very flashy runoff), hydrologic condition was set at poor, and a curve number of 85 was chosen for all bays. After a value was determined for peak discharge on the appropriate nomogram, that value was multiplied by the slope factor derived from Table 5. Average runoff volume (in cubie inches) was determined by multiplying runoff depth (Table 6) by the watershed area. Within each major watershed, sub-watersheds with separate drainage streams were defined. This exercise was undertaken to determine whether there were shoreline segments across which a relatively large percentage of an individual watershed was emptied of rainfall and sediment. The calculations described above were repeated for the sub-watersheds. The percentage of the watershed which drained into the bay along a designated length of shoreline was determined. This percentage was multiplied by the average volume of runoff from the entire watershed to determine runoff from the sub-watershed. This number, in cubic feet per second, was divided by the length of shoreline that drained each portion of the watershed. There is a small degree of error in using this simplified approach, whereby each sub-watershed slope is derived from the previously calculated average slope for the entire watershed. The degree of error is not large enough to warrant the recalculation of the slope for each sub-watershed, however. Values recomputed for Fish, Reef and Hawksnest Bay sub-watersheds using the slope of each sub-area varied only 9% from those based on average slope of the entire watershed. . A ratio of watershed area to bay area was calculated to investigate whether there was a relationship between the presence or absence of reefs and this ratio. Bay area was defined by the water surface landward of the projecting headlands on either side. It was generally anticipated that the greater the ratio, the lower the probability of there being extensive coral reefs within the bay. Admittedly, bay volume would be a more sensitive parameter. Problems with this approach, however, include: 1) determining "average" depth for the bays given the poor data base on small~scale bathymetry around St. John, 2) assessing the impact of bay geometry on sediment retention, and 3) determining the effect of bay orientation on the ability of waves to affect flushing. In the absence of any way to objectively quantify these variables, bay area was considered equally appropriate for the general level of comparison in this study. 10 SLOPES - STEEP CURVE NUMBER - 80 24 HOUR RAINFALL FROM US WB TP:40 PEAK RATES OF DISCHARGE FOR SMALL WATERSHEDS TYPE I. STORM DISTRIBUTION 89g2BR29R 8 Cow : 1 | ovat | pis Ba | 006 =|=: ous +42 | ve | | oot ft cf] 29 BZ AES nT 11 | 05 op & ORAINAGE AREA IN ACRES 8 02 pho L Ln OL Nomogram used in calculation of peak discharge rates for watersheds with an average CN of 80. we shoves bt seats toh we me ee ee wee rh” bhal Hoy” vows N q N nan ne sy - GNOO3S Y3d 1334 DIGND NI 3DYVHOSIG AW3d stop z N -N N qj NJ NJ NS N N., Ni Se Me wmworaa 7 wwonr BaNnG Figure 2, PEAK RATES OF DISCHARGE FOR SMALL WATERSHEDS TYPE IL STORM DISTRIBUTION SLOPES - STEEP CURVE NUMBER - 85__ 24 HOUR RAINFALL FROM US-WB TP-40 2000 TTT __ it: ' weeds roe | iy? fads Jf ‘ ate en ee Z PEAK DISCHARGE IN CUBIC FEET PER SECOND ; | 19 aoa A —>~ — Eon _— = 24 Aa a a 5 — rae : 60 alae ae > ar a — 50 N 1 NY % LD ont win DS : & & §€ 8 Sees g 300 400 wan 600 mon ano 9m) 100 2000 ORAINAGE AREA IN ACRES Figure 3. Nomogram ‘used in -ealculations of peak " discharge rates for watersheds with an average CN of 85. This nomogram was used for all calculations. 12 Table 5. Chart used to determine slope factor. Flat Slopes - round to nearest % Slope shown (Use discharge values from ES sheets labeled "Flat") Slope ' Acres 4 1-50 51-500 501-2000 5 -80 ~~ .80 -78 7 87 089 -91 1.0 1.00 1.00 1.00 2.0 1.13 1.16 1.18 Steep Slopes - round to nearest % slope shown (Use discharge values from ES sheets labeled "Steep”) Slope Acres x 1/50 51-500 . §01-2000 8 .92 -88 83 9 -93 .90 85 10 (194 91 87 11 .95 -93 289 122-96. 94, 91 13 97 .96 294 14 -98 .97 -96 1s ~ 99 .99 -98 16 1.00 1.00 1.00 17 1.01 1.02 1.02 18 1.02 1.03 1.04 19 1.03 1.05 1.06 20 1.04 1.08 1.08 25 1.08 1.14 1.17 30 1.11 1.20 "1.23 35 1.13 1.24 1.28 40 1.16 1.29 1.33 45 1.18 1.31 1.37 s0 1.21 1.34 1.40 55 1.23 1.35 1.43 60 1.26: 1.37 1.46 65 1.28 1.39 1.48 70 1.30 1.40 1.50 75 1.32 1.42 1.52 Moderate Slopes - round to nearest % slope shown (Use discharge values from ES sheets labeled "Moderate") Slope , Acres : % 1-50 51-500 501-2000 . 3 96 695 94 4 “1.00 "2.00 1.00 5 1.04 1.05 1.06 7 1.09 1.13 1.15 Example: Given l. D.a. = 1000 acres 2. Rainfall = 4" 3. C.N. = 80 4. Slope = 41% Determine Peak flow Solution 1. Since the slope is greater than 8%, look up flow on ES 1027 - CN 80 - slope - steep = 1320 cfs 2. -From table for steep slopes and under 501-2000 acres drainage area and on line with 40% slope (41 rounded to.40) find factor 1.33 3. Peak flow = 1320 x 1.33 = 1750 cfs 13 Table 6. Chart used to determine average runoff depth per 24 hours. Rainfall Curve Number (cn)L (inches) 60 65 70 75 80 85 90 1.0 0 0 0 0.03 0.08 0.17 0.32 1.2 8) 0 0.03 0.07 0.15 0.28 0.46 1.4 0 0.02 0.06 0.13 0.24 0.39 0.61 1.6 0.01 0.05 0.11 0.20 0.34 0.52 0.76 1.8 0.03 0.09 0.17 0.29 0.44 0.65 0.93 2.0 0.06 0.14 0.24 0.38 0.56 0.80 1.09 2.5 0.17 0.30 0.46 0.65 0.89 1.18 1.53 3.90 0.33 0.51 0.72 0.96 1.25 1.59 1.98 4.0 0.76 1.03 1.33 1.67 2.04 2.46 2.92 5.0 1.30 1.65 2.04 2.45 2.89 3.37 3.88 6.0 1.92 2.35 2.80 3.28 3.78 4.31 4.85 7.0 2.60 3.10 3.62 4.15 4.69 §.26 5.82 8.0 3.33 3.90 4.47 5.04 §.62 6.22 6.81 9.0 4.10 4.72 *5..34 5.95. 6.57 7.19 7.79 10.0 4.90 5.57 6.23 - 6.88 7252 8.16 8.78 11.0 5.72 6.44 7.13 7.82 8.48 9.14 9.77 12.0 6.56 7.32 8.05 8.76 9.45 10.12 10.76 1 To obtain runoff depths for CN's and other rainfail amounts not shown in this table, use an arithmetic interpolation. 14 Data on reef development were derived from recent aerial photographs, in conjunction with information from Beets, et al. (1985). Defining the degree of reef development was necessarily sub jective. Reefs were categorized as Absent, Searce, Present or Abundant. Results Watershed areas for St. John are shown in Figure 4. Table 7 summarizes the data on watershed character. Generally, the larger watersheds are located on the south shore of St. John (e.g. Fish Bay and Reef Bay). Peak discharges for 2, 5, 10, 25 and 50-year frequency storms are shown in Figures 5-9. Average runoff volumes of the St. John watersheds are shown in Figures 10-14. As storm intensity increases, peak discharge and average rates of runoff volume also increase dramatically up to a certain point. Peak-discharge and runoff-volume rates do not increase as rapidly between 10 and 25-year or 25 and 50-year events as is the case between 2 and 10-year storms (Figs. 15-18). Likewise, there is a much greater change in runoff intensity per foot of shoreline between the 2 and 10-year frequency storms than between 10 and 50-year events (Figs. 19-21). The shape of these curves (Figs. 15-18) is related primarily to the patterns of rainfall change between these storms. The rainfall increase between the 2 and 10-year frequency storm is from 3.9 to 6.6 inches per 24-hour period, an increase of 69%. The rainfall increase between the 10 and 50-year frequency storm is from 6.6 to 9.2 inches per 24-hour period, an increase of only 39%. This pattern is important, as the spatial variations in runoff that appear to control modern reef distribution can be largely established under higher-frequency conditions. While more severe storms (i.e. 100-year) can have substantial impact on individual reef areas, the general reef distribution around St.John appears more related to events with a periodicity of 10-25 years. The three most critical factors for reef development around St. John appear to be: 1) runoff concentration, 2) exposure to wave energy and 3) the type and extent of development within individual watersheds. Runoff - Two points are repeated at this time. First, vegetative cover was assumed to be uniform forest. This approach was taken to assess whether the present reef community could be explained by natural conditions or, conversely, whether development impact might be implied by anomalies in the data. Secondly, the runoff values and reef characterizations reported here are considered as only relative. It would be very difficult to accurately characterize variability in soil conditions and other factors that might impact runoff. Therefore, it is not realistic to take these runoff values as anything more than estimates of likely conditions. Nevertheless, the general relationships between runoff and reef distribution that emerge are still useful. 15 Table 7. Data used in calculation of runoff for St. John watersheds. Bay Watershed Area Average Slope Borek Creek 139 acres 28%, Brown Bay 294 15 Caneel Bay 219 27 Chocolate Hole 204 25 Cinnamon Ray 452 34 Coral Harbor 1475 18 Cruz Bay 24 16 Denis Ray AT 2) Devers Bay 19 35 Drunk Bay 75 4g Durloe Channel SE 22 11 East End Bay 28 39 Elk Bay tou 26 Europa Bay TAY 27 Fish Bay 1448 18 Francis Bay 162 34 Frank Bay 2 16 Friis Bay a7 29 Great Cruz Bay 542 14 Great Lameshur Bay 587 2u Grootpan Bay 298 Hy Hansen Bay 127 36 Hart Pay 7 35 Haulover North 83 28 Haulover South 31 24 Hawksnest Bay 233 37 Johns Folly Bay 133 ev Johnson Bay 188 3A Kiddel Bay 47 8 Klein Bay 54 34 Little Lameshur Bay 339 22 Long Bay 62 37 Maho Bay HO6 33 Mary Creek 429 23 Mary Point North 112 46 Mennebeck Bay 38 27 Monte Bay 85 47 More Hill North 117 38 Newfound Bay 127 37 Otter Creek 31 29 Pond Bay 59 26 Popilleau Bay 4] 35 Princess Ray 131 34 Privateer Bay 117 23 Reef Bay 1391 19 Saltpond Bay 17 17 Sanders Bay 148 ay 17 Table 7 (continued). Bay Watershed Area Average Slope Solomon Bay 88 38 Threadneedle Point Tt2 39 Trunk Bay 176 32 Turner Bay 153 24 ' Turner Point West 22 38 Turtle Bay 7 26 Water Creek 79 28 Watermelon Bay 173 33 18 . 849 TIS ISS “ore Bay 136 oS SS & 25 Ggom nel Figure 5. Peak discharge rates expressed in efs for St. John watersheds during a 2-yr frequency storm (3.9"/24 hrs.). Figure 6. Peak discharge rates expressed in cfs for St. John watersheds during a 5-yr frequency storm (5.5"/24 hrs.). 19 Hawkenest Bay Figure 7. Peak discharge rates expressed in cfs for St. John watersheds during a 10-yr frequency storm (6.6"/24 hrs.). Figure 8. Peak discharge rates expressed in cfs for St. John watersheds during a 25-yr frequency storm (8.0"/24 hrs.), 20 Figure 9. Peak discharge rates expressed in cfs for St. John watersheds during a 50-yr frequency storm (9.2"/24 hrs.). Hewkenest Gay Figure 10, Average runoff volumes expressed in cfs for St. John watersheds during a 2-yr frequency storm (3.9"/24 hrs.). 21 Figure 13. Average runoff volumes expressed in cfs for St.John watersheds during a 25-yr frequency storm (8.0"/24 hrs.). AY 449 10 Berg ol A ( Kg if | Figure 14. Average runoff volumes expressed in cfs for St. John watersheds during a 50-yr frequency storm (9.2"/24 hrs.). el Z 23 HAWKSNEST 108 t 1 i | I i U t i 6 $ 10 15 20 2 30 35 40 45 58 TIME (YEARS) Figure 15. Average runoff volume expressed in efs for Hawksnest Bay for different intensity storms. Note the consistent decrease in the Slope of the curve with increasing storm intensity. HAWKSNEST 2ces 1660. 1608 1400 1200 i loca eco] 600__ 400__ 206__| t i ! t i | U q ' G S 10 1S 20 25 30 35 40 45 So TIME (YEARS) Figure 16. Peak discharge rate expressed in cfs for Hawksnest Bay for different intensity storms. Note the consistent decrease in the slope of the curve with increasing storm intensity. 24 506 450_ 400_ _ 350. 3001 iS 250. 200_] iso. 100 50 t i } | T 0 s 40 15 20 2 30 35 40 4S 58 TIME (YERRS) Figure 17. Average runoff volumes expressed in cfs for Fish and Reef Bays for different intensity storms. Note the consistent decrease in the slope of the curve with increasing storm intensity. 6ocs $400 4800 4200 3600_) iG 300a_ 2400 1800] 1200] 600] I t 1 I t i i | Lf 0 5 10 15 20 25 36 35 40 45 so TIME CTERRS) Figure 18. Peak discharge rates expressed in cfs for Fish and Reef Bays for different intensity storms. Note the consistent decrease in the Slope of the curve with increasing storm intensity. 25 *santea Jgounu YBTYyY aqyeotpul sToquks yueq *wWu0zs Aouenbauy 4A-Z & AOS SqUSWBaS aUTTe4OYs UOJ SUNTOA JJounu aBeueae BuymMoys uyor °4g Jo dew °6| aunzTy uuiojs Aouendey LeOk-Z OwNOA youru EBRIBAY "(Le ‘3Tq twu0ogs 4A-QG °3°3) suu0qs auaAas auow JO VEY OF UBLTTWIS ST 3nq ‘(6L °3T4q) wuoys 4A-Z |yZ OZ UMOYS FeYY UEYY JUseUETJTP ATZueoTJTUBTS st jgouns Aypsuaequt-yZtTy jo uaaqqed ayy ‘saenztea jyound usTY aqzeotTpuy sToquAs yueq *wWuoqs Aouanbaus 4Af-O, @ 4JOJ SquaeMBes aeuT[Seu0YysS 4OJ auiNTOA gJyounu aBeusaAe BuymMoys uyor *4sg Jo dey °O? aunty es0;° 08 usyy 10480306 ) 8 o/0)9 09-02 Ey o70s0 08-018 KY os830 o8-9 (ES osej0 o-t [J e7e)0 t-o [_] uuoys Kouenbes Jeok-O} eWNjOA youru eGeeAy a 50-yr e nts for Note the similarity to the pattern seen segme runoff volume for shoreline off values. ge cy storm. Dark symbols indicate high run 10-yr storms (Fig. 20). 21. Map of St. John showing avera less severe, Average runoff volume §0~year frequency storm frequen Figure for Behind watershed size, geometry plays a critical role in determining the intensity of runoff within specific shoreline segments. Generally, the areas of highest runoff intensity are found in large watersheds with relatively high slopes. Also, these shoreline segments tend to drain areas that funnel a high percentage of the watershed through them. Shoreline segments with high runoff concentrations are listed in Table 8. There is good correlation between areas of high runoff (greater than 50 efs/ft) and the absence of reef development (generally at the heads of bays with moderate to large watersheds). In these areas, large amounts of sediment introduced into the adjacent bay waters during storms are detrimental to the reef organisms. Shoreline segments with low runoff intensity (less than 50 efs/ft; ideally less than 20 ecfs/ft) are more likely to contain better developed reefs in the nearby bay waters. The watershed:bay-area ratio exerts a less predictable and secondary control upon reef development. Shore segments with no reef cover had an average value of 4.96 (Table 9). In areas with well-developed reefs, the ratio averaged 1.96. Ratios for the scarce and present categories were 4.21 and 3.76, respectively. Lower values consistently eccurred in areas where reef development was extensive. Although the ratio of watershed area to bay area decreased generally as reef abundance increased, numerous exceptions accurred and this ratio is not likely to be useful as a quantitative predictor of reef occurrence. As is discussed below, bay exposure exerts substantially more control than bay size. Exposure - Wave energy is important as it affects resuspension and/or removal of fine-grained sediments. In protected areas such as inner Cruz Bay, Great Cruz Bay, inner Fish and Coral Bays (Fig. 1), waters can be very turbid as a result of both high runoff input and low prevailing wave energy. Along the eastern section of Hawksnest Bay, fine-grained bottom sediments in the eastern and central bay occur primarily due to sheltering from the point of land to the east. Sheltering within individual bays therefore provides for poor flushing during runoff events and subsequent settling and retention of fine-grained sediments in the interim. Exposure can be affected by either the location, orientation or geometry of a specific bay. Bays on the western end of St. John subject to regular sediment input will tend to be more turbid because of the diminished wave action along that less-exposed coast. The inner areas of deeply incised bays, like Fish Bay, Hawksnest Bay and Coral Bay, receive lower levels of wave energy, and are therefore subject to poorer flushing. As will be discussed in later sections, exposure plays a very prominent role in controlling the degree of reef cover in the three bays studied in detail. For example, the inner margins of Reef Bay are equidistant from a major gut that delivers runoff from the upland watershed. Yet, the more exposed reefs on the western side have noticeably higher coral cover than those to the east. 29 Table 8. List of bays containing shoreline segments with runoff values in excess of 50 cfs/ft under different storm conditions. 2~YEAR FREQUENCY STORM Central Coral Harbor Central Reef Bay Central Fish Bay 10-YEAR FREQUENCY STORM Central Coral Harbor Central Reef Bay Central Fish Bay Central Grootpan Bay West~-Central Great Lameshur Bay Central Little Lameshur Bay South-East Fish Bay Central Great Cruz Bay Central Cruz Bay 50-YEAR FREQUENCY STORM Central Coral Harbor Central Reef Bay Central Fish Bay Central Grootpan Bay West-Central Great Lameshur Bay Central Little Lameshur Bay South-East Fish Bay Central Great Cruz Bay Central Cruz Bay East Coral Harbor Central Johns Folly Bay East-—Central Great Lameshur Bay Central Europa Bay Central Caneel Bay Eastern Hawksnest Bay Central Denis Bay Central Cinnamon Bay Central Maho Bay Central Brown Bay Central Mary Creek 30 Table 9. Watershed:Bay-area ratio for St. John bays, and their relation to reef development. ABSENT WA/BA SCARCE WA/BA Borek Creek 2.9 Brown Bay 4.3 Caneel Bay 9.5 Cinnamon Bay 3.7 Coral Harbor 15.5 Fish Bay 11.1 Cruz Bay 6.6 Monte Bay 2.4 Francis Bay 1.6 Pond Bay 4.9 Frank Bay 2.1 Princess Bay 2.1 Friis Bay 3.8 Solomon Bay 4.0 Great Cruz Bay 5.6 Turner Bay 3.6 Klein Bay 1.0 Water Creek 2.9 Little Lameshur Bay 10.3 Watermelon Bay 3.1 Maho Bay 7.7 Otter Creek 2.1 Popilleau Bay 1.6 Sanders Bay 4.1 PRESENT WA/BA ABUNDANT WA/BA Chocolate Hole 5.4 Drunk Bay 0.96 Denis Bay 6.1 Hart Bay 1.4 Devers Bay 1.7 Hawksnest Bay 1.7 East End Bay 1.3 Johns Folly Bay 2.3 Elk Bay 1.5 Mennebeck Bay 0.79 Europa Bay 6.0 Newfound Bay 3.6 Great Lameshur Bay 6.5 Privateer Bay 1.6 Grootpan Bay 11.1 Reef Bay 3.9 Hansen Bay 3.4 North Haulover Bay 1.4 Johnson Bay 4.1 Kiddel Bay 5.2 Long Bay 3.9 Mary Creek 4.6 Saltpond Bay 1.4 Trunk Bay 3.5 Turtle Bay 2.1 South Haulover Bay 5.2 Threadneedle Point Bay 3.3 West Turner Point Bay 1.6 SE Durloe Channel 1.1 31 Development - The degree of development within the watershed affects the potential for runoff absorption. In general, more intense development and construction translate into higher runoff intensities with large amounts of sediment input into the adjacent bays. It is often difficult to separate the effects of development from exposure, as development has typically centered around bays protected from wave action. Examples of this are Great Cruz and Cruz Bays on the western shore of St. John. These areas have been affected by recent development, and the reefs have likely suffered as a result. In Cruz Bay, this has involved dredging, port expansion and population growth. At Great Cruz Bay, dredging has created an artificially quiet area in which fine-grained sediment derived from runoff has been trapped. Periodic resuspen~ sion results in typically poor water clarity. Without adequate baseline data before development, however, it is difficult to assess the real magnitude of the environmental change, and to separate development impacts from stresses due to naturally low levels of wave energy and concentrated runoff. Impacts on Modern Reef Development - The purpose of this exercise was not to provide thresholds for precisely predicting various levels of reef degradation in the face of development. Before any such accurate model even can be considered, we must come to grips with our impressive lack of knowledge ~ concerning what types and levels of stress individual organisms can tolerate. What this study was able to do, however, was to examine the distribution of stresses from natural sedimentation and to relate that to the present~ day pattern of reef development around St. John. In general, the distribution and abundance of reefs can be explained as results of shelf configuration, runoff distribution and exposure to wave action. Fringing reefs dominate because of the abrupt drop in the shelf close to land. Accordingly, these reefs are highly susceptable to natural levels of sedimentation stress. While exceptions do occur on a small scale, the position and character of the reefs around St. John generally follow a pattern that would be expected under natural conditions of total forest cover. It would be naive to say that development bears no influence on modern reefs, but development impacts appear to still be exerting a secondary control behind the factors of watershed size and geometry. While not directly addressed by the model, very important in runoff reten- tion and therefore reef development, are shoreline features such as ponds and mangrove complexes. In several instances, lowered watershed slopes in our calculations reflect substantial areas behind the shoreline occupied by these features. Their most important roles with respect to this discussion however, are as sediment-retention features. It has become common practice in recent decades to fill these areas to either create new land or to remove standing water that breeds insect larvae. In addition to the ecological losses whose enumeration is beyond the scope of this discussion, substantially increased runoff and nearshore sedimentation can result from their removal. Their role in a natural setting is probably reasonably approximated by the slope reductions in the model. Because of their tremendous importance in a development setting however, they bear special mention. 32 PRESENT-DAY REEF ENVIRONMENTS Introduction From a management perspective, the simpified model described above, and a little logic give us a reasonable starting point for assessing potential future impacts. The present condition of the reefs infers the ability of these communities to have at least in part withstood the levels of stress over time. The key to addressing the question of "thresholds" of impact, therefore, lies in: 1) separating real from perceived impact in areas of purported change 2) determining the changes in environmental conditions that might be responsible for real changes (i.e. are they cyclic changes or real stress events?) greatly broadening our knowledge base concerning the responses of specific organisms to types and levels of stress that are likely under conditions of development ( e.g. How much sedimentation stress is "shading" vs. "smothering" of organisms? Is sand or mud more harmful to corals?). The remainder of this report addresses the first two points in three bays studied in detail. Unfortunately, the third element must await a time when our basic research on reef metabolism catches up to our applied needs. This section describes three bays as they exist today. The purpose of studying these bays was twofold. First these bays are of specific interest to the National Park Service, and this investigation, therefore, provides useful baseline data. Secondly, the study uses new methods to assess changes in those bays over time. The investigation thus provides a critical test of these methods, and illustrates the kinds of inferences that can be made. The three study sites were: 1) Hawksnest Bay, on the north shore, 2) Fish Bay, on the south shore, and 3) adjacent Reef Bay (Fig. 1). Hawksnest Bay was chosen because it is presently under use by the Park Service, and had recently come under presumed increases in sediment stress with the initial construction of the St. John Medical Clinic near the head of the watershed in 1981. Fish Bay is bordered by both Park Service land and private property. Indications of imminent and potentially substantial development within this large watershed made it an obvious candidate for study. And finally, Reef Bay is at the terminus of a large watershed, in this case however, controlled almost entirely by the Park. It is unlikely that this area will come under substantial development pressure in the near future, and its management is tied more solely to the operation of the Park. Thus, each bay is associated with a large watershed, but has a different natural and management situation, Methods Three to six transects were established across representative environments in each bay (Figs.22-24). A tape measure, compass and depth 33 *Aeq ay} OFUT s4n8 uofew so saouBuqua ayeeuTTeap SMOudy *UMOYS aue Ssdu00 Teu00 pue seTdues qusutpas ‘sqoesueu4 Jo suotyeoo’y “Aeg qseusymey go dew aseg °22 aunBtTy wonrk 006 oot 0 ee oe SjB109 4BWeE 10; Su0TIBOO] O1f8 Bugpdw G *BU0T M LZ Ey neers WIN Zr p9 2 34 ABQ OFU] BOUBIIUS INH ~ f DE CANT settted sample PPV and YVRIGH——— ORY and WEIG! ———— i CALCULATE mean and sorting y SFPAPATE 1 g mud WREECY PISSOLVE CO; in 19% ue} PRY and WEIGY % TEPPIGEMNCUS in mud (mud) Sediment sample treatment procedures. SAMPLE (50-150 g¢) WET SLEEVE ™ 24 hrs.). \, Bay 4 FILTEP® unsettied sampte (1.45 4m) TATAL TERRIGEMOUS 38 > 46 (sand and grave!) PPY STEVE (-28to 49 at 0.5 intervals) SEPARATE 1 go sand and gravel WETGH PISSOLVE CO, in 19% HC! PPY and “FIGH % TERPIGEMNCUS in sand and gravel *qoesue ly yoes ZuoTe s UuaUiTpes soejuns JO SOTysTuaqoeueyo azy{s-uteus BuTMous Aeg ysousymey go dey °Sz aunsTy WOOE 00e Oot i) puss o814809 pues wnjpew puss euy) pues euygy AOA 39 Aeg }seusyMeH fered Pa pene ae JOJYOVICYD OBZIS UleIH so *squeutpas WwOoEe 002 ool i) %S°°1'D Aeg }sousymepy snousbisia] jyuadIOY 40 “Qc aun3T4y UT papntouyt st stoquids attyoud 04 Aey WY “LO-GH aTtgoud Aeg ysousymey *12 aunty ° JOA0D GAIT JUADi0g gL z Does ape eT TIT te ences cece Bi scee eee eee a Bev a ¢ zt -- on gt g0 os 1 os Sz vo 4) (%) iw) ~\ i ot ° o —so sz ~ 3 ww youeg *quodau Stuy UT SeTTyoud [Te uo quaysTsuod aue (4UusTu4) A@y ayy UT UMOYUS STOGWAS AaUL *(43eT) ZO-GH aTtgyoud Aeg ysausymey *g2e ounBry ‘ds S3ifldg (euapeg Ayensn) Ul. LIS - Lo a 2 i P 3 40 Bissejeq Ayensny ssez6 ees tf] 02 lo - UR} BOS Ps _ < diya eas . —ve a A st Burcs“¥ 5 ° SaySNebE Boneby Ww ‘ v . a ° rl (juaoied us) = N ee r—F% $ n y SMOTS CITSS TTY fepoyew oN +—- SL - Lop ° ™ U ot snoveinayF — Re 2", ye109 peay U v sz @euked G1sdoHy peo 7 wv peep gt bios ¥ % S Ady Of}fO1d SI vO pica 1 ‘Ns —s » ) g C] Fal 5 x 5 ~~ 3 3 Ww -0 —-0 *@z aun3ty uy papnytouT st sToquiAs aTtgoud 09 hey y “£O-GH eTTyoud Aeg ysousyMeH °62 aunsTt4 ¢ oe JOA0D GA[} }UODSId m o 43 4 (w)yy3deq “82 OunBty ut papntouTt st sjtoqwAs attjoud 03 Aay y “hO"GH eTTgzoud Aeg qseusymey “of aun’ Ty v o- pa eo @ 3 ~ Led < ea Qa °o < @ . (wjyuydeg Terrigenous content appears to be related to two factors: 1) wave energy and 2) proximity to the source. In general, sediments highest in terrigenous material were located in low-energy, deep-water areas. The most terrigenous-rich sediments were located in the deep, highly protected eastern portion of the bay (Fig. 26). This environment is conducive to sedimentation by fine-grained material characteristic of much or most of the land-derived sediments. With respect to source, an eroding volcanic headland near Transect HB-02 is probably responsible for the high percentages of mostly sand-sized terrigenous grains found there. Similarly, the seaward ends of Transects HB-01, HB-03 and HB-O4 extend into an area along which a major drainage gut empties into the bay (Fig. 22). The abundance of land-derived, fine-grained material, in combination with the relatively low-energy area along the eastern edge of the bay, results in significant terrigenous-sediment deposition in the central part of the bay. Reefs - Of the three bays, reef cover is least developed in Hawksnest Bay. The best developed reefs are along the central and eastern shore, Profile HB-01 (Fig. 27) traverses the middle reef along the central shore (Fig. 22), and is dominated by Acropora palmata. The corals along the reef crest are a mixture of columnar fragments and upright live and dead colonies. Live cover reaches 20% immediately in front of and behind the main reef crest (Fig. 27), and drops rapidly on either side. The forereef, out to a depth of about 20 m, is a carbonate hardground occasionally covered by a thin veneer of sand or rubble. Cover is light and dominated by widely scattered, meter-sized heads of Montastrea annularis, with a sharp decline seaward of the swimming-area boundary. The channels on either side of these reefs (e.g. profile HB-03; Fig. 29) are areas of low coral cover. The presence of several colonies of "knobby" M. annularis in the area along Transect HB-03 likely reflects high periodic stresses as will be discussed below. The lack of cover between the central and eastern reefs is related to the presence of two major drainage guts from the Hawksnest watershed. The eastern, larger gut has been the site of the construction for.the St. John Medical Clinic. The potential impact of this recent development is discussed in a later section. The eastern reef complex is much more poorly developed. This is a response to both lower energy levels and the proximity to the major drainage gut. Percent cover is generally low (ca. 5%), and is discontinuous in nature. The bottom is primarily hardgrounds with a thin sediment veneer. To the west, bottom slopes are much steeper. Despite the higher wave energy, the slope provides a relatively narrow zone of reef growth, and cover is generally low. A possible exception to this occurs along the seawardmost point of the western shore. The most recent NOAA aerial photogragh shows a deeper reef separated from the shore. Beets, et al. (1985) estimated coral cover in this area at 25~30%. As this area was not likely to reflect sedimentation stress from the guts primarily to the east, it was not examined. 45 Fish Bay (Figs. 31-36) Sediments - A total of 41 surface samples were collected along four transects within Fish Bay (Fig. 23). Two transects (FB-01 and FB-03) are located on the southern, more-exposed portion of the bay and extend perpendicular to the shoreline. Transect FB~02 extends across the narrow opening which separates the inner and outer bays. Transect FB-04 dissects the interior bay, and extends slightly into the outer bay. Sediments along Transects FB-01 and FB-03 are composed of predominantly poorly-sorted coarse to very coarse sands (Fig. 31). The character of these sediments reflects both the proximity of the reefs and the high wave energy of these environments. Along Transect FB-02, the coarsest sediments are similarly associated with the reefs. Behind the eastern reef, sediments are poorly-sorted, medium to coarse-grained sands. Grain size increases over the emergent reef, with a gradual increase in sorting seaward. Beyond a distinct slope break at the base of the forereef, the channel-—floor sediments are the finest and most well-sorted (fine to medium sands) found along the transect. The absence of coarse-grained, reef-derived sediments in this area contributes to the fine-grained, well-sorted nature of the sediments. Along the western reef, sediments increase in grain size and decrease in sorting. Sediments found along Transect FB-O4 (Fig. 36) are the finest-—grained, most poorly-sorted of any observed in Fish Bay. Coarse sands dominate the landward end of the transect. Deposition along this end of the transect is influenced by runoff from the major gut from the watershed. A small delta has developed where the coarse sediments carried through the gut have settled in the bay (sediment-—core samples FB-01, FB-02; Fig. 23). The seaward end of this transect is also coarse and moderately well-sorted, reflecting an increase in energy and proximity to the reefs of the outer bay. Beyond the influence of the delta, sediments become progressively finer in a seaward direction. A lack of appreciable energy levels within this part of the bay causes deposition of poorly-sorted, fine to very fine sands. Because of the narrowing of the bay (near Transect FB-03) which restricts wave energy, and the rapid settling of coarser sediments in the delta, the only sediments available to this portion of the bay are fine-grained. There are few reef-derived sediments found in the interior portion of the bay. The sediments are either derived from terrestrial sources carried into the bay through the guts or are formed in situ by epibionts within grassbeds or by the breakdown of calcareous algae. The higher terrigenous content of inner Fish Bay sediments may in part be due to increased construction activity in the area over recent years. This includes bulldozing of numerous roads and clearing of mangrove areas near the bay head. While this activity has undoubtedly had some impact, two lines of evidence indicate that the primary controls of sedimentation patterns within the bay are still natural. First, the terrigenous content within the cores 46 *qVoasueuy yokes BuoTe squsui~pas aoejuns Jo soTqstTuaeqoeueyo azys-uteus BSutmous Aeg usty jo dew "LE aunBiwy N WOOE 002 oot Oo poaesb pups e81809 AIOA pues e81809 puss wnajpew puss out) puss oupy AIGA AT Aeg ysis JOLOBIBYD 9ZIS UleIyH *squoulTpes aoejuns 944 UT TeTueqew snouaZtuuay jo seBequeoued ayy Zutmous Aeg usty Jo dew °Z& aunty Aeg usi4 .Ssnousbliiay pusdsed *@z eundTy uy papntouy sy Aey Toquds y “1O-Gd eTTJoud Aeg usta “EE eundty Vv fe) « -o8 3 a @ Pj i - Lo © < be o | 0 o Loe < @ ” oz gt 91 vt zi OL D * ty x it * e ¥ o * ® 2 y ~ ry, wt qt —s5 WZ ,, ~ 4 ~ Xf Aeeky, a —O “gz aunZt4y ut papnpout st Aa Toquds y *zO-dd eTTyoud Aeg usta ‘we eun8ty Pe ee mnagentonesl, @. e be x4 " “ -- OS L ° 4GAOD GAZ] 2UGTIOd LOS (wyyrdeg °Q@Z aunBT4y ut papnpout st Aey Toquds y ‘€O-dd eTTsoud Aeg usta 4Q0A0D OAL] }UO910d (wyuideg ge oun Ta UT pepntTouT st Aexy TOqWAS Y ‘HO-dd eTTjJoud Aeg usta “°QE aunBTy g°e J9A0D GALT 2UE910g 52 (w)yurdeg taken from the bay floor does not show a dramatic increase near the sediment surface. Second, the distribution of coral types within the bay (revealed by coral cores - discussed later) intimates that the general pattern of turbidity has remained relatively constant over the past 100-200 years. Terrigenous content of Fish Bay sediments (Fig. 32) was as much as three times higher than that from any of the other areas studied. Terrigenous percentages in Fish Bay ranged. from greater than 57% at the landward edge of Transect FB-04 to less than 2% in several places in the outer bay. The most dominant trend is a constant decrease in terrigenous sediment in a seaward direction across the inner bay. High terrigenous content is related to the influence of the major gut draining the watershed, and the morphology of the bay. Inner Fish Bay is a semi-enclosed bay protected from most wave activity. The gut at the landward end of Transect FB-O4 (Fig. 23) drains 1057 acres of land. During heavy rains, nearly 300 million gallons of rain fall on the watershed in a 24-hour period and must be ultimately absorbed or exported through the gut. The coarse fraction of the sediment being carried is deposited in the delta at the head of the inner bay. The fine-grained, terrigenous sediments settle out in the quiet environment beyond, and cause the high terrigenous component seen in samples FB-07 and FB-08 (Fig. 32). Although quantitative data regarding the actual output of terrigenous sediments through the gut are lacking, some generalizations can be made. While Reef Bay to the east has a comparably sized drainage basin, terrigenous percentages within sediments from that bay are much less than those observed in Fish Bay (see discussion below). Therefore, it appears that bay morphology is a major factor in terrigenous sedimentation in inner Fish Bay. Terrigenous content decreases dramatically near the mouth of the bay (Fig. 32), and is comparable to that at similar locations in Hawksnest and Reef Bays. Energy is again the major factor influencing depositional patterns, as increases in wave and current activity preclude the deposition of mostly fine-grained terrigenous sediments. Values in the outer bay vary between 1.5 and 14%. Exceptions to this pattern can be seen in the southwest portion of the bay where abnormally high values, some greater than 40%, occur. In this instance, the high terrigenous component is dominantly confined to the coarse fraction. The sample sites are located near a rocky headland where an abundant supply of sand-sized terrigenous material is produced. The degree of protection provided by the surrounding topography prevents the movement of most of the sand-sized terrigenous component of the sediment. Reefs — Fish Bay is comprised of two bays, an outer, exposed body of water and an inner, more protected one (Fig. 23). Within the inner bay, levels of turbidity are much higher, primarily due to the quieter environment. The coral community within the inner bay is sparse, and is confined to a few colonies of M. annularis and Siderastrea sp. in shallow water along the western shore, and scattered heads of Manicina and 53 Meandrina (V. Zullo, pers. comm.) in nearby grassbeds. In the outer bay, wave exposure is higher, and levels of terrigenous sediment are correspondingly lower. The best coral cover is generally separated from shore and on the forereef slope in somewhat deeper water. Along the eastern shore, the reefs are generally separated from land by a shallow lagoon or a submerged reef flat covered with cobble-sized coral debris (Figs. 33 and 34). To the west, the reefs form veneers over steeply sloping bedrock. Variations from a uniformly sloping profile (e.g. central portion of profile FB-03; Fig. 35) are probably the result of accretion by corals, and are associated with channels which store and serve as pathways for the removal of sediment produced locally. Reef growth is most active along the western, more-exposed margin. Along both Transect FB-02 and the western end of FB-03 (Figs. 34 and 35), the highest degree of cover is related to exposed areas of high slope near the base of the forereef. With the exception of a narrow band of A. palmata close to the shore on western Transect FB-03 (Fig. 35), these areas of higher cover are associated with head corals and Agaricia sp. Reef Bay (Figs. 37-45) Sediments - Within Reef Bay, 56 surface samples were collected along seven transects (Fig. 24). Along the high-energy northeast side of Reef Bay near the White Cliffs area (Transect RB-01; Fig. 39), no lagoon environment occurs. The sediments range from a very coarse sand on the landward end of the transect to a medium sand on the seaward end of the transect. Sorting decreases seaward from a moderately well-sorted sand landward to a poorly sorted sand seaward. Transects RB-O2 and RB-03 (Figs. 40 and 41) differ from RB-01 in that they each have a shallow lagoonal environment behind the reef erest. In both instances, lagoonal sediments are coarse to very coarse sand, and the most poorly-sorted found along the transect. This is probably due to accumulated "backreef rubble" in this environment. Along the forereef, grain size decreases slightly seaward until, at the base of the reef, medium sand dominates. Sorting is generally poor with only a slight increase downslope. On the west side of Reef Bay, Transect RB-O4 (Fig. 42) is similar to RB-02 and RB-03. The sediments in the lagoonal environment are analagous to those encountered in similar environments on the east shore. Mostly poorly-sorted, coarse sands are again the result of accumulated backreef rubble. In a seaward direction, the sediments become slightly more well-sorted and finer-grained. Just seaward of the forereef, a slight depression (1 m relief) occurs where the sediments are extremely poorly sorted and much finer-grained than the surrounding sediments. This area seems to be a small depository for fine-grained sediments which probably flow through the channel that separates the eastern and western reefs. Seaward of this depression, the sediments are moderately-sorted, medium sands. 54 ; *qoosueuy yoeo BuolTe Ssquawutpas soeJunsJoSoFAsTAayoeueyo azts-upeus Butmous Aeg joaoy go dew “LE aunsTy wooe 00% OOL i) | peaes6 pues essvod AIGA. puee ee1809 puss wnypew puss eu pues euyy Asa 55 Aeg j990yY JOJORBIBYS OZIS useIyH “squaulTpes soejuns ayy UT Tey~uaqzew snouast4ueq Jo saBvequsoued ayy Butmous Aeg jooy jo dew “ge aunty 56 Aeg ja0y snousbissa, JUsdIaY vr OL *@z ouns Td UF pepntouy st fay Toquks y “LO-qy aTTgoud Aeg gaoy “6€ aun3Ty zt 19A09 OAI] 3U0940d (ujyydeg 57 "SZ aunByTy uy pepntouy sy sey ToquAs y °ZO-GU STTyoud Aeg jooy “Oh aunBTy r—Ov | ° JOAOD BAIT 100940q S 9) 20 wow ie] (pita Wo 9 ® —~gGov =z ~ 3 vz Sz 9Z e e e —0 "gz aun3Ty ut pepntout sq Aey Toquds y ‘€O-qu eTTJoud Aeg Jooy “Lh eundta 2 JOA0D GALI] }UG910d (w)yyuideg 59 e QZ aunZBTyq ut pepntoutT st Aay ToquUAS Y “HO-GH eTTsoud Aeg jooy °7h aun3Ty 48A0D SAI] 2UBDI0g (W)4YIdeg 60 “gz eunBtq ut papntouy st Aey Toquds y “GO-GN eTTJOUd Aeg gaoy “Eh SunBTy a] @ pa a @ 3 - r < ® a ° < ® 3 cujurdeg *@z aun3yq uy papntouy st fey Toqwks y °gO-qy aTTjJoud Aeg Jaay “hh aundt4 4Q0A0D GAIT 2UEDI0g 62 "Se aundyTq uy papntouy sz Asay Toquds y *LO-qy aTTJoud Aeg gaoy *Gh eunBTy 2 SEAOD BAIT JUEZIEYg io rad . b tepeuttts stu gad guast tuthgtensigd atten, (w)yurdeg 63 Transects RB-05 and RB-06 (Figs. 43 and 44) are both located on exposed headlands, in areas of high wave action. The landward ends of the profiles are typically barren, encrusted pavements with no lagoon. Reef growth starts in 1-3 m of water. Very little sediment is encountered along the reef crest or upper forereef environments. Seaward of the pavement surface, sediments range from very coarse or coarse sands on the lower forereef to a medium sand along the bay floor. No appreciable change in sorting values exists along the transects. All sediments are poorly-sorted. Transect RB-O7 (Fig. 45) is located in the channel separating the east and west reef systems. The surface sediments are mostly medium and fine sands, and are constant in character along the transect. Sediments along the inner transect are moderately well-sorted. Sediments found near the break in the reef contain appreciably more coarse-grained material (probably small amounts of reef detritus), and are thus more poorly-—sorted. In summary, general trends in mean grain size and sorting of sediments within Reef Bay are predictable. In areas where there is a lagoonal environment behind the reef crest (RB-02, RB-03 and RB-04), very poorly~sorted sediments are found. This occurrence is due to coarse-grained backreef rubble mixing with the generally fine sediments occupying this low-energy area. In areas where there is no lagoonal environment present (RB-01, RB~OS and RB-06), the best-sorted sediments are generally found along the landward end of the transect. High wave energy causes well-~sorted material to deposit in the nearshore environment. On the reef itself, grain size tends to decrease in a seaward direction from coarse to very coarse sands along the reef crest to medium sands at the base of the forereef. Percent terrigenous material in the surface samples shows several marked trends (Fig. 38). Values on the eastern side of Reef Bay are consistently higher than those on the western side. Values of greater than 60% terrigenous material oceur on the landward end of easternmost transect RB-O1, while values along the western shore rarely exceed 5%, and are often nearer 2%. Terrigenous sedimentation from the White Cliffs area on the eastern shore has a dramatic effect on terrigenous content in the surface sediments. This probably reflects deposition in the lee of the exposed point to the southeast. Terrigenous material on the western shore of Reef Bay is relatively scarce. The western shore contains no major drainage guts. Those guts that are present drain only a small percentage of the basin, and would not be expected to carry large amounts of terrigenous material. Despite extensive construction near the point on the western shore, terrigenous content along Transect RB-06 (Fig. 24) remained below 5% (Fig. 44). Terrigenous content of sediments near the bay mouth adjacent to the major guts ranges from 5-15% (Fig. 38). Much of the terrigenous sediment in the bay mouth area is due to runoff from the guts, but some of the sediments are due to lagoonal currents carrying in material from the White Cliffs area, Because this area is more exposed than inner Fish Bay, sediments can be attributed to lagoonal currents carrying in material from the White Cliffs area. Because this area is more exposed than inner Fish Bay, the flushing action by the waves results in lower percentages of terrigenous material. 64 Reefs Large-scale bay geometry is related to the underlying antecedent topogra- phy. The shape of the embayment is such that it is deepest along a line bisecting the bay and shallower to each side. Reasonably continuous fringing reefs form veneers along the steep sides of the bay. A channel which separates the two reef complexes is related to the major gut draining the upland water- shed. The reef development on either side of the channel illustrates the effect of exposure on reef development. Profiles RB-03 and RB-04 (Figs. 41 and 42) are located on either side of the channel (Fig. 24), approximately equidistant from a large gut draining the adjacent watershed. Coral cover on the more exposed western transect (RB-04; Fig. 42) reaches 30% while on RB-03 (Fig. 41) it never exceeds 5%. With the exception of the outermost profile, the eastern reef complex is characterized by a shallow lagoon, a narrow, exposed reef crest dominated by dead and encrusted corals (primarily Porites sp. and A. palmata), and a sloping forereef. In general, reef cover increases toward the southeast, in part a response to increasing wave action, but more likely as a response to increasing distance from the major drainage gut at the bay head. Maximum coral cover is 40% and corresponds to the head coral zone along Transect RB-Ol and mixed head corals and A. palmata along profile RB-02. The western reef is a veneer over the underlying bedrock, similar to that found in western Fish Bay. Given the exposure of this shore, the low coral cover is surprising. This may be related to the lack of a lagoon to separate the reef face from shore effects. Probably more important is the higher turbidity observed on several occasions along this shore. This suspen- ded material is probably derived from the lagoon to the east, and is held up against the reef face by incoming waves, as illustrated in Figure 46. Reef cover is highest along the inner section of the margin. In this area, the steep slope, which is probably related to erosion by the central channel, provides an environment where sediment settling on the reef is easily shed. As in Fish Bay, the richest cover is related to higher slopes in the lower forereef, removed somewhat from nearshore processes. A distinctive morph of M. annularis predominated along the innermost portion of the western bay. Coral colonies were composed of a selection of "knobs" which effectively divided each colony into smaller sub-colonies. Because of the environmental conditions associated with these corals (higher turbidity, and proximity to a source of terrigenous bedload material), this morphology is felt to be an adaptation to sediment stress. It is possible that this partitioning of the colony surface results in a shorter path along which to move settled sediment. Whatever the explanation, this colony type seems confined to areas of high sedimentation near the bay head. The persis- tence of this morph in the area over the time period represented in our coral cores intimates that the present levels of sediment stress have prevailed in this area for a considerable length of time. This is discussed in more detail in the next section. 65 “SZISTA PTOTZ [e4eAes Butunp suorTyeadasqo uo paseq aue S409O9A MOTY ‘UCTZau sTyy UP PaveqZUNooUS SUOTYTpUuod ptquny—-uazyjo ayy wos squnoooe ATOHXTT uuayqed MOTZ stTul *‘saaen Buywoouy Aq pauysyuod ST FT SuaymM aoey Jaou U4azSeM |YY P4eMOY SMOTZ ‘Jyounu WOuJ paAtuap 384u4 YYIM Buole ‘quewtpes stul *peay Aeq auq puemMoy SuaqzemM Teucosey Ptquny Auuwed sqyUuauuno USATAP-SABM PUB PUTM *4See ayy OL *AbG Jaaey UTUXTM UOTZeTNoOUTO paztTTeuauay "Ot aunty yuoWIpES 3 a ENVIRONMENTAL CHANGES The final part of this study examined the three bays described above with respect to possible changes in environment over time, and related those perceived stresses to temporal variations in the reefs. This study focused on two things: 1) changes in the amount of terrigenous material in the sediments over time, and 2) changes in the growth rates of individual corals on the reefs, Sediment Cores Methods - Sediment cores were extracted from each of the Study areas, six from Reef Bay, eight from Fish Bay and one from Hawksnest Bay (Figs. 22-24), These cores were taken to determine 1) the controls of the sediment distribution within the bays, and 2) whether there has been a sudden increase in terrigenous sedimentation associated with recent development. Three-inch (7.6 cm) irrigation pipe was pounded into the sediment by hand. A rubber piston attached to a fixed vertical staff provided suction during coring and extraction. Core penetration reached lengths of over 1.6 m in some cases. On the surface, the length of each core was measured, and the sediments were extruded. At 15 cm intervals, 100-150 gram samples were collected. As a result of the extrusion procedure, the cores became compacted, Compaction was considered as the difference between core penetration and the length of core recovered. The sample locations within the cores shown in Figures 47-50 have been corrected using these values and assuming compaction was uniform throughout each core. Samples from the cores were wet-sieved through a .0625 mm screen to separate the sand from the mud (Table 10). Each fraction was dried and weighed to determine its importance in the sample. A representative subsample of each fraction was exposed to 10% HCl to dissolve the carbonate fraction. The remaining insoluble residue was dried and weighed to yield the amount of terrigenous material in each sample. 50-1060 grams of the sand fraction was dry-sieved at 0.5-phi intervals (Folk, 1974) to determine mean grain size and sorting (calculations assume a mean grain size of 0.044 mm for the material smaller than 4—phi). Data on grain-size, Sorting and terrigenous content are summarized in Figures 47 and 48, Hawksnest Bay - A 1-m long core was recovered near the seaward end of Transect HB-03 in approximately 10 m of water (Fig. 22). Only one core was taken, as this was the only area with sufficiently thick sediment for coring. The sediment becomes finer-grained and more well-sorted toward the top of the core (Fig. 47). Terrigenous content increases slightly upward from 11.0% at the base to 13.7% at the top of the core. These values are similar to the terrigenous content of nearby surface samples. The eastern edge of Hawksnest Bay is an important site for fine-grained, terrigenous sediment deposition because of its protected location and its proximity to the gut on the eastern shore of the bay. This is reflected in the relatively thick (at least 3 m) deposits of terrigenous-rich, 67 *sBOT *sheg JsousyMey pue jaoy Woy Sauo0d JUST pas *Lh a4n3ty auoo woz sToqudAs 04 Aay sos ABH aUNnZTty 3ag oz OF 0 zz OF gE lay os F 4 ' ‘ é hi fo: a: pi ro: ro! a ve Zau ¢-10du i f i i 1 oz Ob 0 © ! B17 9% 3S i Vey y" 06 4 ost oe + aH H My ft oz 1 ra E ' 09 r -09 09 We iy: ne f tf: v1 }oe }ie HOE HOE +08 H rh: { ! i t ! { i ' i j ! | p-10ae ahha ©-208u z-10du 1-208 auoo “OJ SToquks 04 Kay 4OJ QQ PunBTy veg ‘Aeg YSTYA WOuZ Sau0d YuoUTpag “egy aunry *sSOT ve gece ror 09 09 "7 a ' U i / Le oe + 4 * i i f i t t i t i + ; F ; oped y-yo"as Tt HZ FZ GZCE o9 OS a tt 06 bed Cs ’ 1 t t y é é iy 09 09 4 08 | ‘ ki: * F I pe i ' t F Pe i: i ! : 3 V: : ran wa} gle ma, a: Wyo dh ddede 1-008 e083 do dh du th dh du dz cz ve * 8 *qUuZT4 944 UO paqzeoOT st 8h pue dh seundTy ut stoquds 03 sey y *4geT uo (panutquoo) Aeg ysty wous sau0o queWTpsg *qQh eunBty aa 70 ~ 06 8 (uO) yRIEp 2200 9- soqUITY 8109 8-084 iif oe diay Terrigenous content Profile FBO4 5— Depthim) [?] 250m 500 750 C.l.: 5% Vertical exaggeration (profile): 35x Vertical exaggeration(cores): 116.7x Figure 49a. Fish Bay profile FB-04 showing percent terrigenous material horizontally in cores. Note: Cores are exaggerated vertically to enhance clarity of contour lines. | Terrigenous content- Profile FRO4 . ' 0 250m 500 750 C.1.: 5% Figure 49b. Fish Bay profile FR-04 showing percent terrigenous material horizontally in cores. 71 aoueyus 04 ATTBOTIUaA paqyeuasBexo aue sau0g *SOUTT unoquod Jo AqyTueTO LO-Gu aTTgoud Aeg joaay *OG aun3Ty ° 3040N "“SOU0D YUdUWITpes UT [eTuaqell snousBtuueay yusoued ButTmous XE'ES 1(Sas0d)UONesIOHHeXa [BSIIOA XGZ :(OWJOsd) UON}FBIOHHexXe JBOyISA | %b FO WwOSl 72 x4 Giyysdeg I To) ZOEY 2Old yuayUu0d Snouebie] » fine-grained sediments found here. Although it is difficult to reach any far-ranging conclusions based on data from one core, the upward increases in both fine-grained sediment and terrigenous content are not sufficient to make a case for significant environmental change over time. Fish Bay -— Eight sediment cores were taken from Fish Bay along Transect FB-O4 (Figure 23). Cores were taken in water depths ranging from 1m in the interior portion of the bay to over 10 m at the seaward end of the transect. The percent of terrigenous material in these cores was the highest encountered in any of the three bays studied, ranging from less than 9% in the seaward cores to greater than 60% in the interior cores (Fig. 49). As discussed for surface sediments, grain size and sorting generally correspond to differing energy levels within the bay. In the nearshore environment near the major gut, cores FB2 and FB1 (Fig. 48) are composed of fairly well-sorted, medium to coarse sand. Cores FB6, FB7 and FB8 located in the central part of the inner bay contain the poorest-sorted and finest-grained sediments along the transect. These sediments are positioned in a low-energy environment, away from heavy influence of the major gut and protected from most wave activity by the geometry of the bay. In the more exposed channel between the inner and outer bays (cores FB3,. FB4, and FB5; Fig. 48) the sediments again coarsen and become much better sorted. Laterally, there is an expected decrease in terrigenous percent away from the gut. This trend occurs at the surface as well as at corresponding Levels within all cores. The two most interior cores (FB2 and FB1; Fig. 48), contain almost 60% terrigenous material. In the channel cores (FB3, 4, 5 and 6), that value drops to less than 10%. More significant, however, is an upward increase in the percentage of terrigenous material in each core. Exceptions are the landwardmost core, FB2, and core FB5, which have fairly consistent values throughout. This trend may be explained by an increase in the amount of fine-grained terrigenous runoff from the major guts within the bay over recent time. Alternately, reef development on the southeast shore of the bay may have reduced wave energy, thus inhibiting the removal of the terrigenous material from the bay. If this is the case, and the trend continues, then inner Fish Bay would eventually become a stagnant salt pond, completely separated from open-ocean circulation. Without conclusive data on 1) the time span represented in the sediment cores (i.e. radiocarbon dates) and 2) the development of the bay morphology during that period (i.e. longer cores through the entire reef system), choosing between the two alternatives is difficult. Reef Bay ~- Six sediment cores were extracted from Reef Bay, five along the N-S Transect RB-07 and one inside the reef complex (Fig. 24). Water depths ranged from near 1.5 m at core RBZ to approximately 10 m at core RBS5. Percent terrigenous material in the cores ranges from 6.5% at the base of the landwardmost core to 18% in the middle of the seawardmost core (Fig. 50). Grain size within cores along Transect RB-07 increases slightly in a seaward direction (Fig 47). The landward cores are composed primarily of fine sand 73 while the seawardmost cores are predominantly medium to coarse sands. There is little grain-size variation within the cores. Sorting along the transect tends to decrease seaward. The finest, most well~sorted sediments are found in the landwardmost cores. Terrigenous content is significantly lower than in Fish Bay despite a similar watershed area. This is probably due mostly to the differing degrees of exposure in the two bays. Fish Bay is protected, and fine-grained sediment from the major guts can settle out in a relatively quiet environment. Reef Bay is more open. Nevertheless, there is sufficient sheltering of the innermost portion of the bay head to allow fine-grained sediments to be deposited at stations RB3 and RB4 (Fig. 45). Terrigenous percentages within the cores increase in a seaward direction, most likely a function of water depth. Closer to shore, there is a slight increase in terrigenous content toward the sediment surface, but percentages remain low throughout the cores. The seaward cores reflect relatively constant conditions and no systematic variation in terrigenous content was seen. Coral-Growth Study Methods = Fifteen large (greater than 1m) coral heads were eored in the three bays (Figs. 22-24). The sites were located to provide representative samples of the diverse habitats and marine conditions in the area, These include: (1) proximity to drainage guts, (2) wave exposure on alternate sides of the bays (3) openess of the bays and, (4) water depth. The largest available corals were selected in order to maximize the length of time recorded by their growth. In highly stressed areas, Montastrea annularis often grows in a knobby morphology. Rather than a single hemisphere, the colony is subdivided into numerous, smaller lobes. Cores through these provided a very discontinuous record due to the segmented character of the colony, and porous nature of the skeleton. Therefore, during the latter part of the coring program, only massive forms were cored. The cores were taken with a hydraulic drill similar to that described by MacIntyre (1975). The cores were slabbed longitudinally along the bias of the corallites. 5 to 6-cm thick slices were X-rayed on a Faxitron X-radiograph machine using Dupont Cronex 4 medical X-ray film. Positive contact prints were made from the negatives, and the spacing of the annual growth bands was measured using a Houston Instruments HiPad digitizer. A typical core segment is included as Figure 51 to illustrate the bands revealed by X-rays. This banding pattern has been shown to be annual in M. annularis (Knutson, et al., 1972). Density-band analyses have been used elsewhere to interpret. ‘environmental controls in both modern (Hudson, et al., 1976; Dodge and Vaisnys, 1977) and ancient (Runcorn, 1967; Hubbard, et al., 1985, in press) corals. The underlying prineipal is that “any “degradation of the water quality (here, a decrease 74 Figure 51. X-ray positive of a typical core segment showing annual growth bands. in light due to an increase in turbidity or sedimentation rate) will result in compromised coral metabolism and lowered growth. The age of an individual coral was calculated by counting the annual bands. Continuous segments of the cores were assigned absolute ages. Where intervals between segments were ground away by drilling, growth bands were matched as well as possible across the discontinuity. In extreme cases, a reasonable estimate was made of the period missing based on the length of the core recovered versus the penetration by the drill. The total ages of such discontinuous cores were calculated by adding the number of years from each individual segment. This undoubtedly introduced minor errors in the actual dates towards the bottom of some cores. The ages of colonies with the knobby morphology, therefore represent minimums, while those from the more massive corals are closer to absolute. Growth rates were plotted for all corals against colony age for 5-year average intervals (Figs. 52-54). Selected colonies (R1, R2, R6, F2, H3) were plotted annually (Fig. 55). Corals will vary their growth rates under normal conditions in response primarily to light (usually with depth). Therefore, it is difficult to compare growth rates between corals, because those growth rates are expected to be different, with or without stress. To eliminate this variability, the raw data were normalized for each coral by dividing the 75 igen T i a 1 ! L 1 b t o ! Lg Woe eee eee 1 A i 1 al + er = ios r t reo aI re eee ee eee mewn eee ee a x ' Ww H Pow _ ‘ + te Le bee eee _ ' i ' H [ ' be ‘ [3 t o L enn ennnnneneee- es a H . f iy] be us t T T T T “Tt T T = o - o WON SA ; a a L. = Hi t t 1 t a H [8 be ww wee Pecos ene e ewww ee 1s ‘ be ' ' pe > t [ x H _ i i Se | anew n eee en ene | = ‘ L x= ‘ wi r it ' r ' be Ww HK eee ee ee ee ee r ewe owen eee ta | . H Ls Len nn ere cemnecnce Mee cece ne eeeee r L i i 5 H L + B r T T T t T T T r - “ ~ o WYON SA TEARS YEARS FISH BAY Trt soo a a 1935 t TT TEARS Figure 52. 5-year average growth rate for cores from Fish Bay. All values have been normalized to remove variability between corals due pr imarily to depth of occurrence. To obtain actual growth rates, multiply values derived from the graph by the number in the upper left-hand corner. 76 REEF BAY REEF BAY Z nov 4 g o 9 9 ~ t =” wt deo ret Port | 5 g25 F 2 < & a et PT em meee a a 7 f L > 4 Hen — r ti} r art r yy 5 i eo v> }> ¢ r ma 3 c © ~ ' re so ag F } . 2 i. re & a awww nnn ar ed Sprorrrereeeseee bE B a > aie i & on St = ori « § Ls ~N r me oP sn . q i. ow ee > es a ee & eSzZe wo ' H oO FS r H § es - 0 L ; . Hu Sw Lg ' | 8 pas) a re benecencnneceees Ls avez na H F o oO 0 ) 0 i j % Poe is " 0 ‘ { @ ag tg ' PS o" Sa r vr LU tw >t tO) ~ © “ 7 o ov ont uot SA WION SA wok L 00 ord wv i) Pad bt a 55 tf Do wo a Oo os g é e588 . Q Ys : s tg f a Sek iateted aan powctnerecennn er Setahaletatahetatatetatatatatadel wl o Vn gd } ? i to * o8 § wy F So = re eo Ae +8 Ko 8 s , oN ' > Oo 2.0 Seg | rg a og 28 r 7 oO r L hEL Lg Ww a NO OG LS -- ad Jf oF ‘ me 2% w r 3 NOG yg | 3 8 CO © = we eew enn n enn = a - 27.8 Sz L: o © 7 Sopvpg 2 © § t 7 wm > ad : 8 os wee 1885 WON SA LJ i [ aly L J nn as r 4 = oo am +a ye POT r ' =a { o r 8 rom A Sitateekeeeeneeenaneneeel posse neers rene aa ud i a \ i v2 ' 8 PZ L------- ene ee eee Menace enone wee - < ' xz L woe eeeeeee es: poeooees r PY § ° l nn “” t -} WBON SA w & in - | TS r » & wow ae gee Ln ° Pe a eS al ie ' fa 3 : 2 en oer Leg a 3 | ae 8 : se) i - i & oa 8 rm ip 8 oO i is OO | 2-2 ---- ee eee \ mewweneceeennne = Ap rv stecermn rrr enpe rower rere eres 5 tr i Z i 3 tg be # 9 Le a es eee ® 1 ; q - £. Is . ‘ b ; 1 : 5 ¥ ° * nous * vO NE MLNS 4 NI HA nn | 8 : 2 i SS s ee - = ” 2 = | 2 | E, 4 ee led belie bon Pan f ° ” ; 3 a eee erro err rere L. 5 ‘ pa D aad 5 4 J ga x oc a i oe { Cl b--~~---n---ee--- | ewe Magra nc nna nee . 3 cf a uJ L ud } 9 i r 3 8 | cence ennenenee a & © L...--.---------- eS. re = ae pot ro r a “ r Ly ‘ a r a 2 na in Kee ORE SSS SK PH THe Tee SS Be emer wna i secon er eaw r o b. . an 3 3 > ~ . ns: ) - ° tn 0 bad oe « NI Hinoys growth rate in any given year by the average growth rate between 1960 and 1964 for that colony. Rainfall was low during this time period, and it was felt that under these conditions, the effects of land-derived runoff would be minimal and uniform among the three bays. The 1960-64 growth rate for each coral is given on the figures. To convert normalized growth rate to actual growth rate, the reader should multiply the value from the graph by the normalization factor provided. Rainfall data provided by the National Park Service on St. John were averaged in a similar manner, and are summarized in Figures 56 and 57. The raw data were inspected to identify months in which more than 9 and 12 inches of rain fell on St John (not to be confused with 9 or 12 inch/24 hour storms). Figures 58 and 59 summarize the rainfall under those conditions on an annual basis. The nature of the study also required the comparison of the growth rates with selected historical periods having characteristic patterns of land use. Based on discussions with George Tyson, the following periods were chosen: Early cultivation period: pre-1910 Post-cultivation period: 1910-1949 Modern (development) period: 1950-1985 For each coral core, the means of the growth rates within each period were statistically compared to each other for significant differences by a one-sided t-test. In Hawksnest Bay, 12 smaller corals were also collected intact. These coral heads were collected from three sample sites within the bay (Figure 22). Annual growth rates were determined for these corals in the same manner as for the cores. Growth rates from the small coral heads were compared to recent rainfall data to determine if the small coral heads were more susceptible to runoff influence (Fig. 60). Results (cores) ~ Fifteen Montastrea annularis cores were taken in Hawksnest, Fish and Reef Bays (Figs. 22-24, 52-55). Seven of the cores provide a continuous record into the 1910-1949 post-cultivation period, and four cores (R2, R4, R7,F5) extend into the cultivation period prior to 1910. In addition, some of the cores have discontinuous records that, although not precisely datable, do provide a record of minimum ages prior to the turn of the century. The ages of the cores are shown in Figure 61. The youngest cores are those from Hawksnest Bay (post-1910's) and those from corals with a knobby morphology in Fish and Reef Bays. The absolute ages of the latter may be somewhat biased, due to the discontinuous nature of the cores. The oldest cores tend to be present at greater depths and further from the gut (Fig. 62). This patern reflects Significant stress events during more recent time near the shallower corals closer to shore. 80 AVE YEARLY RATNFALL 2 2 T T 44.6 i ' H H H ' t ' ' t ’ i t ' t ' ‘ ' t t ' t 1 ' 4 fl 4 t ' t i i 1 t ' i 1 t ' ' 1 1 ‘ ' ’ ' ' 4 t 1 ' ' t t i i t b 1 ' ‘ ‘ 1 a i q A ' i z= ' “ WY a! t ~. t i 1 ‘ ‘ 2 ! LN Poms fs are 1 of 21 o4-.-.--- S Sotatae atciams in ahataieiads bed tebroene ere ~weee Se haat w ' ’ 1 i 172 ey GN 1 < 4 , ‘ t . t view fy s 1 ' i t { Nf t ‘ ' 1 I my ' t 1 t i 4 ' ' t 1 t Ls t i] 1 i] t t ’ ' i I t 1 ’ t 4 f 1 i 1 ' ‘ ' ‘ : ‘ 1 1 5 t t J ' ‘ t t 1 ! ' ' t ' 1 ‘ t t 1 a ' t ' ' I Lj L Ll Lj in Li t T 7 iH Lu T Lu i | Ld i T T | ¢ La + i i v Ly Ls 1835S 1860 1885 1916 1935 1960 1985 YEARS Figure 56. Average yearly rainfall for St. John. Each point is a 5~year average of data provided by the National Park Service on St. John. Data are normalized in the same fashion as for coral growth. To obtain actual rainfall, multiply values derived from the graph by the number in the upper left-hand corner. AVE HIGH RAINFALL 2 T 8.94 \ } ‘ i 1 1 1 1 1 t 4 1 1 t t t 1 1 ‘ 1 1 1 1 i ‘ t 1 { t t t 7 1 t 1 ' t t t 1 1 i 4 ( ‘ f 4 ' ‘\ t t \ 1 iY oR At : 7 \ roy ih y \ ee At g bi AQ IE ALL UPS = 1 4a donna poeta begin da- pope“ -b ae f= t H i ‘ t Pou / pe! EY ' H 1 ' \ j a \ A 4 t i t 1 XG \ ;! ‘ ' t t ‘ 1 5 1 ) ] ‘ t ' ' 1 i t ' t 1 t t 4 ' t i 1 1 1 i 1 1 I i ‘ ' 1 1 1 1 1 t t 7 ' t ‘ ’ i i t t ' | { 1 G T T v + 1 T 7 Ll | T Li T Li 1 L LU Li t { LJ T Ly r | oe + TT 1835 1860 1885 1910 1935 1960 1985 YEARS Figure 57. Maximum yearly rainfall for St. John. The maximum monthly rainfall was averaged over five-year periods. Data are normalized in the same fashion as for coral growth. To obtain actual rainfall, multiply values derived from the graph by the number in the upper left-hand corner. Data provided by the National Park Service on St. John. 9 INCH RAINS 30 7 20 z q z q < J a _ io t) 4 Ly 7 T v i T t Li 7 1835 1660 1885 1919 1935 1968 1985 TERRS Figure 58. Graph displaying amount of annual 9" rainfall. The value reported is the sum of all monthly rains equal to or exceeding 9", 12 INCH RAIN 30 4 a 20 = z : < J e “ 10 J 6 4 Ld i Li Li i T Ly T . 1835 1860 1885 1919 1935 1960 1985 YEARS Figure 59. Graph displaying amount of annual 12" rainfall. The value reported is the sum of all monthly rains equal to or exceeding 12", 82 SITE 1 SITE 2 2 2 4 4 4 4 4 4 4 q e) 4 ; z 4 z 4 4 q x = 1 J “4 5 14 E4 4 4 3 , o 4 pots « 4 « J 7” . ° 4 9 ] 7 4 q 4 4 e lL : ST wean 0 L_—- a a 6a 6s 18 78 30 8s 60 6s 78 60 85 YEARS YEARS ; SITE 3 = 4 Fs 4 z 4 Zt nm—™ OK ' “AS | S ; 18 8 T T | ook need 2 oe rr—7 T T+ r 8 65 70 7S 60 8s YEARS Figure 60. Annual growth rates for small coral heads collected from three Sample sites in Hawksnest Bay (for sample-site locations, see Figure 22). 83 *Auo[Too pauoo ayy yo AZoToyduocw ayy SsayeopTpuy e400 ay. Jo aseq oy4 7e& [TOqUAS BYU] “SqQUeWZesS auod [Te UT spueq YAMOUZS Jo uequnu ayy Zuywuns uo paseq ST saqunu sqTYyL *Tewoo ayZ JO ave wnwyupTW ayy aqeotput (€qH JOS CHG6L °B°e) spTeAsaqut B400 94 UTUZTM Sazep sul *SuosTuedwOD ayeu—YyyMO4Z sy UT pasn auam (patddt4s) saenrtea asoyuy ATuo ‘auojyauaul “Teotquapy AT[eyquesss aue [eoTyueA Jo saausap O€ uTyATM sexe BuolTe paatsusp seyeu yyMoud qzeyy aqzeotpuy spesey Teuoo peqqets wou eqeq ‘sueuBo4ysTYy ayq ut uuaeqqged patddtqs pue oqtuM pT{os 24uq Aq UMOYS a4e 2100 |YY JO SFXE GYR 09 DATQZETSA YYMOUZ [e40D JO UOTZOSUTp ayy UF SUOTIeTUeA *Us/WO O'lL< pue QO°L-GL°O *SL°0-G°O *S°0-Q UeeMqeq STeAUSqZUT UT pednous due saqeu yymMous ‘spotuad ueak{-G UTUZIM “UYOr *4g UO SAeq Fau4Q BY4 UT Sauoo TewoOo WOU eVep YAMoU, Tenuue jo Auewung *19 aunBTy W oo jeoveudsiw0H 5} W Y ecet sq ey oy eh ZOt —OL8t un” ayeuuneyopUL Gy eyes yal 929g ~—Oosel spowSey 8109 - | —0961 oat (.0¢%) “yuo ~oret sad OO6E ~—~oeel eeu O16! Wm nol st f Ad» 4 —oz6t 2zes —O€6t 2e61 evel ~over —~OS61 ~—O96I —O261 —08éi ase $ 'aa 84 puauy a4 e4ON “Sauce uYyor *4g 344 UT Uydep wayzeM SNSudA 93e Teu0d wWNWTXeW Jo ydeuy °Zg9 aun3TYy sjes0g ABg jooYy y sj8109 ABg ysi4 @ $s1Bi05 ABg JSOUSYMEH wo *(jgouns Jo saounos wouj Aeme pue) yqydep yqqmM aZe Supseasouy puemoy (SYALSW)HLd30 YILUM L 5) S v € (4 I 0 | | | | | | goat zu ; vest v ed Sd ‘ e 4 Drs oO 4 user s rc) = : et eu meet a : wri. ra 4 ou a ede, oe m nn aa tH _St4 * | Beer Gh suv-~ ZH fH y mid tf) -U96 1 . US6T HidAad YSLVYVM SA SDV WNWIXVN on0z The absolute growth rates for the corals during the modern period (1950-1985) are similar to those cited in the literature (Dustan, 1975; Gladfelter, et al., 1978). Figure 63 shows a clear decrease in growth rate with increasing depth for the modern period (1950-1985). At any depth, there is a 10-20% difference in growth rates for corals from the three bays. Nearly all the corals have present-day growth rates similar to corals from the open-shelf margin of Cane Bay on St. Croix (Hubbard and Scaturo, 1985). Short-term fluctuations in coral-growth rate likely represent acute variations in water clarity. These short-term fluctuations were compared to rainfall records to determine whether extreme runoff events from major storms could be correlated with decreases in coral growth. Within some cores, decreases in growth rate can be correlated with rainfall events on occasional instances. The relationship, however, is by no means consistent, nor can similar patterns be discerned in even adjacent cores. For example, cores R2 and R7 (Fig. 53) do show reasonable correlation with some rainfall events, but are located well away from gut-related sediment sources. While the apparent correlation in coral R2 might be explained by currents during heavy rains depositing excess sediments at that site, this is difficult to imagine at site R7 (Fig. 24) on the exposed east point of the bay. The patterns of growth occurring in adjacent cores R5 and R7 (Fig. 53) are almost completely opposite to one another. Also, corals located near guts, such as Rl, R3, Fl, F3, and H1-4 (Figs. 52-54), all have ambiguous records when they should be expected to display the most profound effects. On a time scale greater than 20 years, more consistent trends emerge. The mean growth rates for each of the three time periods listed above are summarized in Figure 64. Based on a one-sided t-test, five cores (R2: .005, R4: .005, R6: .005, F5: .005, H3: .15) have significantly diminished growth rates of 10-20% between the post-cultivation period and the present. Coral F2, showed no significant difference, and coral R7 had higher growth rates in recent times. It is tempting to atrribute this decrease in growth rate in 5 of the 7 cores to recent development. However, in four of these corals that extend back further into the cultivation period, three had higher growth rates during cane production. Thus, the long-term trend appears to be one of a gradual decrease in growth over time, regardless of land use. To test whether changing temporal boundaries between land-use periods would affect the trends just discussed, the initiation of the post-cultivation period was extended back to 1860, and the recalculated means were compared. The overall pattern remained the same (Fig. 64). With the exception of core R7, the growth rates in the latter part of the 1800's and early part of the 1900's were significantly higher (by 10-20%) than the present (1950-85) growth rates, and a pattern of consistently decreasing growth rates through time was still evident. If this is to be related to man's influence, then some mechanism must be recognized whereby sedimentaiton has gradually increased over the past 100-200 years. This is clearly contrary to our initial hypothesis that reef development would have been greatest during the post cultivation fallow period (1910-1949), and lessened by more intense land use on either side. 86 *uMOUSs OSTe aue spotuad queudoTeaAap aauyy oy} Buyunp ayeu yyAMOUS—aveusae ut saBueyg * (SR6L *‘ounqeosg pue pueqqny) xfToug “4g uo Aeg aueg ye syuqdep seTWTs Wouy STeu0d UOJ saqeu yyMou® jo asueu ayy SoeqT4osep sUuTL peysep ayy ‘seuo0o uyor *4g 9344 UT Yyqdap *sA aqeu YyBMOUZ-[Teu00 Jo ydeuy *€9 aunty OLGL-O81d O 6¢P6L-OLGIL V vySE6lL-OG6I ®@ (SYSLIN)HLd30 YILUM 8 Ll 9 S 14 J l l _t ! € Zz t l I J ed 9 sue Ss‘ Hid3d YSLVM SA ALVU HLMOHD T1VHO9O (YA/WI) SLY HLMOYS NUSW 87 sen[TeBaA uNWTUTW aue (seToOuzo uado) saqed YyyMous sapTO ‘auozauayL *(STXE TBOTIAAA 943 JO apts ueyyTe uo gyo sdoup ay4eu U4MOUR *a°T) TBOFWUaA |aYy4 Wous saauZep OF ueyq auowW payTueA sexe YIMOUT |9yy auayM squaWBes Teu0D WOU eyep esn 03 Auessaoau sem 4 *OQ9GL puckaq sevo0d awos Jo aBuei ay4 pua4zXxXe O] ‘alles ayy UTeWau spusu4 ayy ‘saTuepunog Tesoduiay paBueyo aqtdsap yey 930N ‘sSpotued queudoTaAap aauyy ayy usemMyeq saTuepunog quadajjJTp uo paseq asoyq sue sutT peysep e Aq pagoauuod squtog *€9 aunBypy wouy uayeqz aue sauTT PTTos Aq payoauuod syutog *‘sawoo uyor “4g 343 UT YAdap “SA aqeu YyMOuB-—Teuoo Jo ydeuy "hg oun3Ty (SYSLSW)HLd30 YALUM 6 8 L 9 S v € A | 0 I i i i fi | ! ; S 0981-84d O-- = g* 6y6L-O9BL W-- — L° p86L-OS6L @-— — 9: peyep Ajjsesyique awos YIM — 6° pe epnapeocay 0161-81d O— t 6y6L-OLGLY— mt pe6l-OS6l e— a. Hidad YaLVM SA SLVH HLMOHD IvuOO &'! (YA/WI) ALYY HLMOYS NYAW 88 A long-term response to intensive agriculture in the 1800's is a probable cause, but it is puzzling that the decline to the present growth rates typi- cally started in the post-cultivation era, when the direct effects of cane should have been at a minimum. At this point, the most promising possibility is that early forest succession since the end of cultivation may have been detrimental to the ground-holding understory, and thereby caused increased erosion. This, coupled with the degradation of any water-retaining terraces built for cultivation could explain the long-term decrease in coral growth. This is supported by several lines of evidence. First, the curve numbers (CN) ASSIGNED TO "woods" are slightly higher than those for "meadow" (Table 1; a higher CN corresponds to greater runoff). Also, recent experiments at Coweeta Experimental Forest in Franklin, North Carolina have shown that "grass cover in a watershed results in less sedimentation than forest" (A. Putney, written comm.). It is thus possible that the gradual reforestation of the former plantation areas lies at the heart of the pattern seen in the coral cores. In Light of this, VIRMC studies of forest succssion being con- ducted on St. John by the College of the Virgin Islands take on particular significance. Coral heads - No significant long-term trends for annual-growth rates of the smaller coral heads in Hawksnest Bay can be established over the 10-20 years that they record. There is a strong indication from the data, however, that recent construction within the watershed has had important short-term effects. There was a significant decrease in annual growth rates of corals found in sites 1 and 3 (Fig. 22) during 1981 and 1983. These declines corres- pond with relatively high rainfall values within the same time frame (e.g. Figs. 56 and 57: 13.87" rainfall in May, 1981, 9.72" rainfall in December, 1981, 13.58" rainfall in April, 1983). Within the same corals, however, no decrease in growth rate corresponded to the much more severe rains in 1979 (13.34" in May; 16.64" in September; 11.55" in November). During October, 1980, ground was broken for the construction of the hospital at the head of the Hawksnest Bay watershed. Guts which drain the construction site flow into Hawksnest Bay near sample sites 1 and 3. The fact that the post-construction rains in 1981 and 1983 corresponded to drops in growth rate, while pre-construction rains showed no consistant correlation, strongly intimates higher levels of runoff and lowered water quality after clearing of the upper watershed. Direct comparison of the record of these small corals with larger colonies is impossible as cores could not be obtained from within the swimming area close to the presumed stress. Core H3 from just beyond the eastern swimming area, however, did not show a drop in 1981 corresponding to that of the smaller corals closer to the gut. Therefore, if an impact to the larger heads has resulted, it appears to be confined to the immediate vicinity of the eastern Hawksnest guts. It is encouraging to note that in most instances, the coral-growth rates have shown recovery since the decreases discussed above. It is likely that subsequent revegetation of the upper watershed has returned its sediment-retaining properties to near normal. Thus, this example has 89 demonstrated both the potential for upland development to cause a short-term degradation of the nearshore environment, and the ability (at least in this case) for the natural environment to mitigate the distrubance over time. At the heart of assessing the potential impact of this kind of spot development is the ability to determine 1) the extent of the short-term environmental response (e.g. will the corals just slow down or will they be killed?), and 2) the time frame within which the system will return to "normal". There is evidence that significant levels of sediment stress have occurred at least twice at Hawksnest in the recent past. The initial coral growth reflected in the core's clusters just after the 1910's. This likely represents an episode of recolonization after chronic stress related to either cane cultivation, which ended at that time, or to reef destruction by the 1916 hurricane. The dominance of 10 to 20-year-old corals in the nearshore zone may reflect yet another resurgence of the reef after a prior stress event. 90 SUMMARY General The position of the U.S. National Park within the overall community of St. John has raised concerns on both sides over the impacts of development activities (including those of the Park) on the nearshore environments around the island. With increasing development pressure, we seem to constantly find ourselves in the quandary of predicting environmental impact with frighteningly little data. At the crux of the problem is separating real from perceived change in the absence of any baseline data with which to start. This project was a first attempt to investigate ways that we might assess long-term environmental changes given these extreme limitations. In the first section of the report, a methodology generally accepted within the engineering community was used to evaluate runoff patterns on the island in general. With the lack of quantifiable information on the impacts of specific development practices (e.g. what is the impact of a condominium development vS. construction of single-family dwellings? What are the relative impacts of 10 vs. 20 homes per square mile?), a hypothetical case of no development (i.e. 100% forest cover) was evaluated. Inasmuch as the pattern of reef occurrence predicted by the runoff calculations for a "natural" system reasonably matched that of the present-day reefs, it is likely that watershed size, bay geometry and exposure still maintain the primary control over runoff and, therefore, reef development. That is not to say that-:recent construction practices are without impact or that they are of small significance in causing localized degradation. Ideally, the runoff data should be compared to information on not only reef distribution, but reef character as well. Reef data available at this time for St.John, however, are not sufficiently detailed to address island-wide reef development on anything but this gross scale. In three specific bays, surveys were conducted to describe the distribution of reefs and reef-associated environs within those bays. The purpose of the study was twofold. First, it provided baseline data on these bays of interest to the Park Service. Second, it tested whether sufficient data could be gathered on a bay-—by-bay basis to explain localized patterns of reef development (and “quality") in light of natural and man-induced stresses. As was the case in the larger-scale study of the entire island, the degree of reef development is primarily controlled by the size of the watershed and the location of the natural guts that funnel water and sediment into the bays. Also important is the degree of exposure within different areas of the bays. Terrigenous content of the sediments (which presumably reflects sediment stress near the sample sites) varies between and within bays primarily as a function of their degree of exposure and flushing ability. While features such as mangroves and low-lying areas behind the beach exert controls on the total amount of sediment entering each bay, they appear to be less important in determining the 91 distribution of those sediments once they reach open water. For example, the low-energy environment of inner Fish Bay is often turbid, despite being totally surrounded by mangroves. In Reef Bay, cover along profile RB-03 is poor, despite the low-lying area between it and the nearest source of terrigenous runoff. In both these instances, bay geometry and exposure play the dominant roles in dictating the patterns of circulation, and therefore, reef development. While development in the upland watersheds has likely affected runoff characteristics, it is not readily apparent in the distribution of land-derived materials found in the bay sediments. Terrigenous content of the sediments along western Reef Bay remains low despite recent clearing of land in that area. A substantial increase in runoff is clearly evident from the slumps and scars at the development site. The higher wave energy and flushing levels seem to be able to counter this, however, at the present level of development. The growth patterns within the coral cores (Figs. 52-55) provide information about the way those corals, and hence the reefs they form, have fared through the last two hundred years. When plotted against water depth, the ages of the corals. generally increase with increasing water depth and distance from sources of runoff, demonstrating consistent responsiveness of those organisms to change in environment. The long-term decline of coral-growth rates in all three bays appears to reflect a long-term response to forest succession. If this is the case, then exceedingly long-term repercussions of extensive agriculture clearly (e.g. historic cane clearing on St. John; present clearing for agriculture on St. Croix) emerge as important controls over the amount of sediment reaching nearshore reef environments. The consistent decline in coral-growth rates in all three bays clearly dominates over any patterns that might be related to recent development. This is further supported by the apparent constancy in zonation of Montastrea annularis over time. In Reef and Fish Bays, multi-lobed or "knobby" morphs of M. annularis consistently occur near the bay heads, closer to the sources of sediment stress. The common occurrence of this morph on backreef environments and opposite drainage paths in many other reefs implies a possible link to sediment stress. Since these potential indicators of high sediment flux formed in and persisted through the post-cultivation period, they seem to deny any recent and dramatic worsening of conditions, and suggest that chronic sediment runoff problems are a persistent component of these inner bay environments. Even if a direct link between sedimentation and colony morphology is not the case, the shape of the corals is nevertheless the direct result of some environmental factor(s), and they have recently remained constant. 92 Management Implications The watershed analysis revealed generalized patterns of reef occurrence consistant with runoff under conditions of total forestation (i.e. the reefs are still responding to primarily natural controls). Within the three watersheds investigated, both the large-scale reef configuration and the distribution of live organisms on those reefs are again consistent with the presumed pattern of natural sediment stresses within the bays. The sediment cores do show a gradual increase in terrigenous content over time. Given probable rates of sedimentation, however, it is more likely that this reflects changes over hundreds or thousands of years. Corals show a general decrease in their growth rates over time, which appears to relate to gradual changes in ground cover since cane cultivation. Despite their colonization at differing times and presumably under very different conditions of land use, the initial morphology of individual corals is consistently comparable to both the present morphology of that colony as well as that of surrounding colonies of varying ages. The inescapable conclusion is that the distribution of sediment stresses within the bays have not changed sufficiently over the past two centuries to noticeably alter that large-scale zonation pattern. While impacts of local development are suspected, they occur at a very small scale, and cannot be separated from larger-scale, natural controls. The only impact that could be readily attributed to recent development is the decline in growth rates in the Hawksnest Bay corals after 1980. Even in this case, however, coral-growth rates subsequently recovered, and the temporary declines were quite localized. At this point, the crux of the problem is twofold. First, we must separate real from perceived changes in reef conditions. Everybody always remembers "the good old days" of healthier corals and more reefs. Without quantitative and repeatable baseline data, this problem cannot be addressed. The second part of the problem relates to equating change with degradation. Many changes on the reefs are cyclic in nature (e.g. the recent Diadema mortality, El Nino, ete.). Even with reliable baseline data, we must be careful in assigning blame for real changes in reef conditions over time periods of a few tens of years. With respect to either of these problems, the data from this study hint at a long-term decrease in the growth rates of the corals within the three bays studied. And in Hawksnest Bay, there is strong evidence for short-term reef damage related to development. Great care must be taken however, in moving from these conclusions to management strategies within the entire Biosphere Reserve. Management strategies and potential development impact must be assessed on an individual basis. Nevertheless, some generalized recommendations can be made. 93 It is common to link environmental damage to very large, infrequent storms. Under natural conditions, this is often the case. Nevertheless, our runoff calculations have shown that large-scale reef distribution is responding to much smaller, more frequent events (ca. 10-25 yr storms). Given this fact, it is likely that substantial damage is done by smaller storms, but at a level more subtle than we can easily detect in the time scale of most biological studies. This fact, combined with the tendency for developers (and some scientists) to rationalize that big events are going to destroy everything no matter what we do (e.g. the recent drainage plan for the development of Fountain Valley on St. Croix), lies at the heart of many potential problems with future development on St. John. Development in major water pathways must be discouraged. Any compromise in the water and sediment-holding capabilities of these areas will likely result in the long-term degradation of water quality in adjoining bays. At Hawksnest Bay, construction near the head of the watershed appears to have adversely impacted nearshore reefs for several years. The Subsequent recovery of the reefs is related to the fact that 1) siltation was not sufficient to kill major coral populations, and 2) the watershed was able to recover over a relatively short period of time. The confinement of siltation to levels below lethal values is likely related to two factors. First a high location of the construction within the watershed minimized the amount of water traveling through the disturbed area. Secondly, the maintained integrity of the lower watershed was apparently sufficient to buffer the effects (although not totally) of the increased runoff and erosion. Critical points from this example are: 1) Try to discourage development in the major drainage guts of larger watersheds, especially those that funnel water into confined areas (i.e. high runoff values in Figures 19-21). If development must occur in such areas: 2) Try to limit the acreage of the development (i.e. high density may, in some instances, be better than a wider spread, less dense development). 3) Attempt to locate the development as high in the watershed as possible. 4) Attempt to preserve the integrity of the lower watershed. In these situations, the buffering ability of the lower watershed is the last defense against reef damage. Included in this is the retention of nearshore ponds and mangroves that efficiently trap substantial quantities of runoff within relatively small areas. 5) Consider accentuating the water and sediment-holding capabilities of the watershed. This involves things like weirs to slow down flow, ponds to collect water and sediment, etc. Specific suggestions are not offered here, as this is a very complex undertaking, and must be studied very earefully before proceeding. 94 6) To the extent possible, confine ground-clearing activities to just after "rainy seasons", Then, attempt to replant ground cover prior to periods where heavy rainfall might be expected. This approach minimizes the potential for heavy runoff and siltation during construction, while speeding up thes recovery of the upland watershed. If ground clearing must continue through rainy periods, some sediment-retaining structures should be considered until the watershed has returned to its prior state naturally. Development in watersheds emptying into low-energy embayments should be discouraged. The highest levels of turbidity and terrigenous material found in the three bays studied occurred in inner Fish Bay, where wave energy is low and flushing is poor. Likewise, in the general, island-wide calculations, such low-energy areas typically correspond to poor reef conditions. Such development along the shoreline in low-energy areas should be earefully examined before being allowed. This is one of the most difficult areas for which to provide hard-and-fast rules. Areas where poor circulation has resulted in nearshore environments of little ecological value might be acceptable for development. The problem here is deciding on “ecological value" (e.g. the unwise infilling of Krauss Lagoon on St. Croix and the many stagnant nearshore ponds all around the Virgin Islands). At the other end of the spectrum, high-energy areas can often mitigate sedimentation by rapid removel of terrigenous debris. For example, recent development near the western point in Reef Bay appears to have had little impact on nearby reefs. The decision to develop in these instances necessarily falls upon criteria other than nearshore sediment damage. The crux of the problem lies in those intermediate areas where the nearshore environment is of considerable worth (criteria for this "worth" are beyond the scope of this discussion), and increased sedimentation under protected local conditions could cause significant environmental damage. Therefore, an immediate research goal in this respect would be to: 1) Identify areas with low to moderate flushing under conditions likely to accompany severe rainfall events. This can be done without extensive field work, given some knowledge of basic oceanographic processes. 2) Assess the natural environments within the nearshore zones of those bays. 3) Designate development areas only in those regions that either: A. occur in high-energy zones, or B. occur in areas of already degraded environment. The main problem with this approach, is that prime areas for development are those with low enough energy levels to allow water-related sports, and pristine nearshore environments that are aesthetically pleasing. In other words, development will tend to accumulate in those areas where one might 95 least want to place it in this context. The final point of discussion relates to the relative impacts of high-density development vs, less dense, but more extensive,land use. The example from Hawksnest Bay illustrates the ability of the watershed and nearby marine environments to at least partially mitigate localized development. At the same time, the long-term but gradual decrease in the growth rate of corals in all three bays studied demonstrates the long-term impact of large-scale ground clearing. In the case of cane cultivation on St. John, the aftermath of the cane industry appears to be more significant than the impacts during cane farming itself. The importance of these two examples relates to the size of the development activity as opposed to its intensity. On St. Croix, large tracts of land are cleared, ostensively for agricultural purposes. The apparent lack of concern for this type of activity is evidenced in the exemption of agriculture from the Coastal Zone Management permitting process. If we are to learn anything from this exercise, it is likely to be the potential long-term impacts of such activities. This is in addition to the concern that should be raised by the often-extensive sediment plumes emanating from those areas that are typically cleared just before the rainy season to maximize the revegetation potential. The worst case lies in extensive, and often excessive clearing of entire watersheds or drainageways prior to commercial development. Developers often clear entire lots rather than selectively removing ground cover only in areas of construction. This permits easier access for contractors, and requires substantially less foresight in clearing. The cost, however, is often environmental damage. In this respect, the following recommendations are made: 1) Limit large-scale agricultural clearing to situations where it is absolutely necessary, probably never on St. John. 2) Provide substantial sedimentation buffers below areas where clearing is allowed on this scale. This may include artificial sedimentation ponds, 3) Require (or, outside the Biosphere Reserve, request) that the larger developments institute a clearing plan that restricts vegetation removal to areas of construction and access. It may be necessary to restrict development from areas where this approach cannot reasonably mitigate erosion. 4) Confine clearing activities to low-rain periods, and require revegetation strategies, as discussed above. Future research strategies should center around two areas: 1) Establishing permanent monitoring areas that can be resurveyed in such a way as to permit quantitative comparison at some future date. The transects measured during this study probably represent the absolute 96 minimum of reproducibility required. While the transects were accurately located, and points of observation were carefully measured and recorded on the profiles, reef cover was only estimated. This is useful for documenting gross changes, but will miss more subtle variations often important in assessing environmental damage. Permanent quadrats should be established with reference to these transects, and careful documentation of those quadrats should follow. The objective of this study was not to establish permanent transects, but the existing transects do provide an opportunity to do so in an area where generalized reef distribution has been documented and related to physical factors. 2) Determining the levels of stress in a field setting that will adversely impact reef quality. While runoff volume, bay circulation, and other factors that impact water quality are not without use, they cannot be related to reef damage (except in the most basic way....post-mortem) until the levels at which sedimentation becomes detrimental at the organism level are understood. This is not to say that VIRMC should ignore such factors, or not be interested in programs that might monitor them over time. The point here is that, if reef degradation due to sedimentation is to remain an interest of VIRMC and the Park, then the first priority should be to determine specifically at what levels stress will do damage. Only at that point will all these other studies of contributory processes become applicable in a predictive way. Of critical importance are: A. What are the metabolic responses of reef organisms to varying concentrations of suspended matter? B. How does the character of those suspended materials (i.e. size, composition) affect the level of impact? C. What are these levels near the organisms under natural conditions and during storms of various intensities? This last measurement is both more crucially tied to reef development and easier to measure than runoff from adjacent watershed areas, or the host of environmental factors such as currents or flushing that link runoff to the nearshore reefs. 97 REFERENCES CITED Beets, J., Lewand, L. and Zullo, E. S. 1985. Marine community descriptions and maps of bays within the Virgin Islands National Park/Biosphere Reserve. Biosphere Reserve Research Report No. 2. U. 8S. Dept. Interior National Park Service, Dodge, R. and Vaisnys, J., 1977. Coral populations and growth patterns: responses to sedimentation and turbidity associated with dredging. J. Mar. Res. 35: 715-730. Dustan, P., 1977. Growth and form in the reef-building coral Montastrea annu- laris. Mar. Biol. 33: 101-107. Falk, R., 1974. Petrology of sedimentary rocks. Hemphill Publ. Co., Austin, TX, 182p. Gladfelter, E. H., Monahan, R. K. and Gladfelter, W. B., 1978. Growth rates of five reef-building corals in the northeastern Caribbean. Bull. Mar. Sci. 28: 728-734. Hubbard, D. K., Burke, R. B. and Gill, I.P. in press. Styles of reef accretion along a steep, shelf-edge reef, St. Croix, U. S. Virgin Islands. J. Sedim. Petrol. 56. Hubbard, D. K. and Scaturo, D., 1985. Growth rates of seven species of sclerac~ tinean corals from Cane Bay and Salt River, St. Croix, USVI, Bull. Mar. Sci. 36: 325-338. Hudson, J. H., Shinn, E.A., Halley, R. B., and Lidz, B., 1976. Sclerochron- ology:: a tool for interpreting past environments. Geology 4: 361-364. Kantz, E., 1975. Engineering Field Manual for Conservation Purposes. Full citation unavailable, Xerox copy available from McComb Engineering, Sub Base, St. Thomas, USVI 00802. Knutson, E. R., Buddemeier, R. W. and Smith, S. V., 1972. Coral chronometers: seasonal growth bands in coral reefs. Science. 177:270-272. Macintyre, I. G., 1975. A diver operated hydraulic drill for coring submerged substrates. Atoll Res. Bull. 185: 720-738. Runcorn, S. K., 1967. Corals and the history of the earth's rotations. Sea Frontiers. 13: 4-12. 98 APPENDIX I. Conversion factors for English units used in discussions of engineering calculations. 1 inch 2.54 om = .0254 m 4O46 m- 1 acre 1 foot = 0.305 m 1 cfs. 2.832 x 107° m>/sec 1 efs/ft (in diagrams cfs/s) = 9.29 x 107° m°/sec-m 99