Ecological Assessment of
Antigua and Barbuda Reefs:
Maya Goreau and Thomas J. Goreau
August 31 1996
Between August 25th and 29th,
1996, a rapid ecological assessment of coral reefs was carried out in areas
along the north, west, south, and east coasts of Antigua, and in the north and
south of Barbuda. Reefs were examined with regard to species of corals and
algae, coral bleaching, coral and algae bottom cover, and other abundant marine
organisms, while water samples were analyzed for seven water quality parameters.
All sites were found to have to have clean ocean water, with little or no
detectable nitrogen and phosphorous, yet despite excellent water quality the
conditions of the reef were generally fairly poor. Most sites were dominated by
dead coral rubble and had live coral cover of only between 5% and 20%. Algae
cover was very low at most sites, but reached up to 80% in some areas. The
healthiest reefs seen were In the north of Barbuda, and the worst in the
southwest of the same island. Previous studies and interviews with longtime
divers and fishermen indicate that reef deterioration preceded the large
hurricanes of recent years, and its spatial and historical pattern is not
consistent with either land-based pollution or overfishing being a major cause.
Diseases which attack branching corals are suspected of playing a critical role
in reef decline in the area. Corals were in the early phases of a coral
bleaching event, with up to 70% of corals affected by partial bleaching at some
sites. Unusually high sea surface temperatures, around 30 degrees Celsius,
appeared to be the only possible cause. These findings suggest that the major
stresses to reefs in Antigua and Barbuda are not of local origin.
Antigua and Barbuda are distinguished by the finest and most extensive white sand beaches in the eastern Caribbean, which provide the foundation of their tourism-based economy. The abundance and purity of the limestone sand results from the large areas of coral reef which virtually surround both islands, since every grain of sand is the remains of a skeleton of a living coral reef organism, primarily of sand-producing calcareous algae and of hard corals. Besides producing sand for the beaches, the coral reefs act as wave breaking barriers which protect the beaches and cliffs from erosion, and act as homes and food sources for a wide variety of fishes and other marine organisms such as lobsters, providing the basis of the islandsí fisheries and their attractiveness to sport divers. The economic future of these islands is clearly very closely dependent on the health of their coral reefs.
Antigua and Barbuda have the most extensive coral reef habitat of any Caribbean islands east of Hispaniola because of their unique geological and climatological conditions. Located on top of the Antillean outer limestone island arc, along with Anguilla and Barbados, they differ from the high islands of the inner volcanic arc in being generally low lying and dry, thus lacking the high rainfall and rivers which wash eroded mud, silica sands, and human-generated wastes and pollutants into the narrow coastal zone in the higher and wetter islands to the west. The location of Antigua and Barbuda in the extreme northeast corner of the Lesser Antilles exposes them to open ocean waters transported by the trade wind driven currents from the Atlantic Ocean, and places them up-current of all but locally generated sources of land-based pollution, ship-based sources, or materials transported across the entire ocean from Western Europe or North Africa.
With the exception of the layered marine volcanic ash materials which make up southwestern Antigua, the rocks of Antigua are predominantly made of gently northward sloping limestone sediments of Tertiary age. These formations formed in the deep sea from deposition of microscopic limestone skeletons of single-celled plantonic organisms. No fossil coral reefs, such as those found around the shores of the central and western Caribbean, were seen anywhere around Antigua. Antiguaís layered limestones, made up of the mineral calcite, are fairly soft, in contrast to the more resistant recrystallized and altered aragonite minerals of fossil coral reefs, and they have been eroded to form a large shallow limestone rock shelf which is ideal for growth of corals and sand-producing algae. In contrast, the limestones of Barbuda appear to be largely made up of calcareous algae sands with shells of clams and snails which live in back reef lagoonal sands and seagrass beds such as conch, Strombus gigas, and the West Indian top shell, or whelk, Cittarium pica, with fossil corals of Plesitocene age found in the higher areas of the northeast limestone ridge.
Despite their great local importance and intrinsic scientific relevance, there have been only a few studies of the coral reefs of Antigua and Barbuda. A guidebook and assessments of aerial photographs by H. G. Multer and his student M. Weiss focused on bottom sediment types rather than on reef ecology. A survey of reefs done just before Hurricane Hugo in 1989 by a team of Royal Navy divers was largely impressionistic and contained little ecological documentation. The most detailed work done to date was assessment of a dozen transect sites by L. Bunce and colleagues prior to Hurricanes Luis and Marilyn in 1995. Our study, done in conjunction with the Environmental Awareness Group of Antigua, extends these earlier studies with more detailed characterization of coral and algae species and water quality assessment.
Our approach differs greatly from that of previous investigators, being based on detailed identification of all coral and algae species seen and interpretation of their abundance and condition based on knowledge of their natural history and ecological preferences gained in over 40 years of experience in other parts of the Caribbean, primarily in Jamaica, Panama, Turks and Caicos, Florida, Barbados, and St. Kitts.
Our method is based on maximizing coverage by swimming as rapidly as possible over the widest range of habitats, across the major physical, habitat, or stress gradients in each area. In contrast, traditional assessment is based on detailed counts in very limited areas, usually along a measured line or quadrant. Our approach sacrifices detailed accuracy in a small area for a broad overview of a much larger area. In almost every area examined very dramatic differences in coral and algae abundances were seen on each dive going from more protected and shallow habitats to more exposed or deeper habitats. A line transect approach would have yielded very different results in each sub-area, and might not have been representative of areas only a few metres away from a transect line. Because we could assess areas more than 10 times as large as could have been assessed by counts in a line transect or quadrant in the same time, our approach allows a much broader general assessment of overall reef conditions in a short time, and identification of the sources of stress by assessing their gradients. Because the greatly expanded coverage is obtained at the cost of extremely detailed counts or percentages at very limited sites, we generally estimate the average coverage of hard bottom (rock or hard coral substrate, ignoring sandy or muddy areas) to the nearest ten percent, indicating the range seen in sub-areas. Bottom coverage by live coral, fleshy algae, calcareous algae, and dead coral rubble was estimated.
The approximate percentage of bleached coral was also estimated. We observed bleaching at every site, but it was in early phases, and almost all corals were discolored and pale rather than bleached to full whiteness. Interviews with local divers indicated that they were not familiar with partial bleaching, and greatly underestimated its prevalence. Our estimates are based on familiarity with partial bleaching based on observation of around a dozen bleaching events in other parts of the Caribbean.
Water sampling was done at selected sites around the islands. Due to limitations of time and materials analyses were not conducted at all sites at which reefs were assessed, since many of them were exposed to very similar waters. On the other hand samples were analyzed from several locations at which reefs were not present, since we were looking for evidence of land-based sources of sewage, nutrient, and other pollutants, and therefore sampled the dirtiest waters we could find, in areas which no corals occurred due to excessively turbid waters.
Water temperatures were measured in-situ using calibrated thermometers read to the nearest tenth of a degree celsius (centigrade). Half litre plastic water sampling bottles were filled to the top, sealed tight, and returned to land for sampling within hours of collection. Salinity, pH, and dissolved oxygen were measured using an electrode probe. Results were expressed in parts per thousand by weight, as the negative logarithm of the hydrogen ion concentration, and as percentage of saturation with regard to the atmosphere respectively. Chlorophyll was measured using a fluorometer which measured the photometric voltage output detected from the narrow band red fluorescence given off by chlorophyll in response to excitation by a blue light emitting diode. Nitrate and phosphate levels were determined using Hach kits. These use addition of chemical reagents to produce red and blue colours respectively, which are compared to standard concentration scales.
The ecological significance of these water quality parameters is outlined below. Excessively high water temperatures cause corals to lose the symbiotic algae which provide their colors and most of their food. The coral tissue then turns pale or transparent, revealing the white limestone skeleton beneath the thin layer of tissue. This phenomenon, called coral bleaching, does not kill the coral unless conditions are too hot for too long, but causes corals to starve, halting growth and reproduction for many months after the high temperature has vanished. Excessively high or low salinity is also stressful to corals, while low salinity also indicates the addition of fresh water runoff to the coastal zone, which is invariably a major source of nutrients and other pollutants derived from sewage percolation, leaching of fertilizers, and runoff of industrial wastes from land. pH is a measure of acidity of the water, and low values prevent corals and other calcareous (limestone producing) organisms from growing their skeletons. Low pH is an indicator of excessive acidity produced by decomposition of organic matter or by acid mining or industrial wastes. Dissolved oxygen is critical to support the respiration of all organisms higher than the most primitive bacteria, and low levels of oxygen, produced by excessive decomposition of organic matter or by oxygen-consuming chemical wastes, can cause death of corals and other attached marine organisms, and mass fish kills. In general this is a problem only in rivers and marine habitats which are enclosed and have restricted circulation with open ocean surface waters and the atmosphere. Chlorophyll is a measure of the abundance of phytoplankton, microscopic single celled plants living in surface waters. High values of chlorophyll result from excessive fertilization of surface waters with nutrients, and cause the water to become green, limiting light penetration to bottom dwelling coral reef communities. The concentrations of nitrate and phosphate represent the major forms of nitrogen and phosphorous respectively, which are the major nutrients in ocean water whose excessive abundance can cause eutrophication, the massive growth of nuisance algae species which overwhelm and kill normal aquatic organisms. Healthy coral reefs have extremely low nutrient levels, and the major source of nutrients in reef habitats comes from land-based sources of sewage and fertilizers. Reefs around the Caribbean have become eutrophic along with the spread of coastal development without tertiary sewage treatment facilities.
SITES EXAMINED: DESCRIPTION, RESULTS, AND ECOLOGICAL ASSESSMENT
1. Prickly Pear- at sand barge grounding
This shallow area, northwest of Prickly Pear Island consisted of a coral community growing over a substrate composed primarily of coarse Acropora palmata (elkhorn coral) rubble covered by encrusting red coraline algae (primarily Porolithon on upper surfaces and Mesophyllum in interior crevices). The live coral cover was very low, around five percent, and dominated by head corals such as Porites astreoides, Diploria strigosa, and Montastrea annularis. Also abundant was the fire coral Millepora alcicornis. Some small plate corals, Agaricia agaricites, were found in protected crevices and encrusting rubble, and the encrusting coral Diploria clivosa was present as well. Most of the corals were small and the community was a relatively young one. However, the species present were clearly survivors of the 1995 hurricanes due to their size. The Prickly Pear reef today has very few living Acropora palmata colonies left though the vast amounts of rubble indicate that it made up the main reef structure in the past. Several surviving patches of this species (a few inches across) remained on largely dead branches, but larger standing colonies of Elkhorn coral were extremely rare.
A few hours prior to our visiting this site, a sand barge had run aground over the reef, and had taken a great toll on the few remaining large live Acropora palmata colonies. Many living elkhorn stands had been razed flat, or had had large branches broken off. In addition, a zone of bright white scrapes and gouges in freshly broken head corals and rubble was visible for at least a hundred meters.
The algae at Prickly Pear were primarily encrusting reds, the brown alga Dilophus alternans, and Liagora mucosa, which proliferates dramatically in the months following hurricanes, slowly disappearing within a year or two. Most of the latter was found on sand at depths of 10 to 15 feet. There were also small amounts of the calcareous alga Halimeda incrassata. None of the algae present served as indicators for eutrophication (excessive nutrient) problems. The temperature at this site was 29.6 degrees Celsius, and small-scale coral bleaching was observed in individuals of most of the species observed (Porites astreoides, Millepora alcicornis, Montastrea annularis, and Agaricia agaricites).
2. Hurricane Island rubble shoal
The reef off this small island, located northwest of Prickly Pear, was built on tall 30 foot hills of Acropora palmata and Porites furcata (finger coral) rubble. The rubble island was created by Hurricane Luis. The live organism bottom cover of the reef varied from 0 to 60 percent live coral and 0 to 50 percent algae. The submarine hil tops generally had many standing dead Acropora palmata, and a few live colonies of Acropora palmata and smaller head corals and hydrocorals. Overall living coral cover in waters shallower than 20 feet tended to be extremely low, less than 10%. The reef framework was largely barren of algae, but had discrete patches overgrown with thick, dense mats of blue-green and turf algae, as well as Dilophus alternans and Halimeda copiosa. The presence of discrete dense algae patches could be due to proximity to fish schooling or resting sites. The amount of live Acropora palmata was greatest at the eastern end of the shoal and least at the west, the side most affected by Hurricane Luis. With increasing depth, many healthy, large corals covered most of the bottom. Montastrea annularis in at least three morphological types (lobate, massive, and bumpy) dominated below depths of 20 feet, some heads reaching 20 feet in diameter. Other major coral species included Diploria strigosa, Diploria labyrinthiformis, Diploria clivosa, Porites furcata, Porites astreoides, Siderastrea siderea, Montastrea cavernosa, Millepora alcicornis, Millepora complanata, Millepora squarrosa, and three other palmate/fenestrate forms of Millepora. The algae Dilophus alternans, Liagora mucosa, Halimeda copiosa and Halimeda opuntia were found in this site. Partial bleaching was observed in Montastrea annularis, Montastrea cavernosa, Agaricia agaricites, Diploria strigosa, and Diploria labyrinthiformis.
A water sample was taken for chemical analysis at this location because it was representative of waters inside a large bay with moderate influence from hotels, marinas, port facilities, residential effluents, and industrial activity. The shallow bottom was very turbid, due to large amounts of resuspended sediment resulting from the spoil banks produced by extensive dredging of Parham Harbour since the Second World War for military bases and for shipping and industry. Although this bay is likely to be subjected to more human influence than any part of the island other than St. Johns Harbour, values measured were indicative of clean water (Appendix). Samples of the effluent from the desalinization plant were not taken, but since the process of desalinization uses the reverse osmosis method, the saline effluents should not have elevated temperatures or nutrients.
4. South Bird Island
This shallow area, composed mostly of sand and Thalassia testudinum, supported many surprisingly large corals in the lee of the island. Many moderately large heads of all the Montastrea and Diploria species, and Siderastrea siderea dominated this inshore area, with many reaching 3-4 feet in diameter. There were numerous 20-30 foot patches of standing dead Porites furcata, encrusted with crustose coralline algae. Living colonies of Porites furcata seemed to be diseased, covered in bare splotches void of polyps. This appeared similar to a recent unidentified disease which has been observed by T. Goreau to attack this species in Jamaica over the past year. Though there was little Acropora palmata, many areas further offshore were dominated by healthy Acropora cervicornis (staghorn coral) in small, dense patches. The large amounts of Acropora cervicornis seen here was unique in all areas observed. Normally this species dominates in much deeper waters from 20 to 60 feet deep, and although considerable amounts of old dead rubble of this species was seen at other sites, living colonies of staghorn seen at all other sites could be counted on the fingers of one hand. There seemed to be a high recruitment rate for this species at this site, since both older corals which had survived recent hurricanes, and small young colonies only a few inches across could be found. Unfortunately, both these and other corals were plagued in places by mats of turf-like yellow algae. The high cover of live corals, the large sizes of some, the large amount of young corals, the protection from waves provided by the island and surrounding reefs, and the clear shallow water make the area ideal for snorkeling if human impact to the regenerating coral can be minimized by managing the area as a marine park and reserve, as is proposed by the Environmental Awareness Group.
5. Off mangroves at the end of Mercerís Creek Bay
The red mangroves (Rhizophora mangle) in this area were fairly short, no more than 10 to 15 feet tall at most, suggesting that they were limited by a lack of nutrients, namely nitrogen and phosphorus, in the water. We were unable to pick up any significant concentrations of these by our testing methods. Salinity was typical of ocean water, indicating lack of freshwater runoff with associated nutrients. The water was extremely turbid, hence its higher temperature of 30.9 degrees. The moon jellyfish, Aurelia aurita, was extremely abundant, probably feeding on zooplankton in the turbid water. Plant branches in the middle of the channel were covered with large masses of the weedy algae Acanthophora spicifera, which could potentially be used in fish mariculture projects, since it is one of the preferred foods of the surgeonfish.
6. Tire FADs- Mercerís Creek Bay
Several tire FADs (Fish Attraction Devices) were set up by Foster Derrick and the EAG in order to increase the fish population in the bay by giving them suitable habitat. The bottom substrate, about 20 feet deep was composed entirely of fine mud sediments, with virtually no algae. Visibility was extremely low, around five feet. The tires were covered in a thin layer of mud, and large colonies of bryozoa and hydroids were the only organisms growing on the tires. Porcupinefish seemed to like the areas deep inside the tire FADs, and small schools of spadefish congregated there as well. However, this area of Mercerís Creek Bay seemed ill-suited for fish due to lack of algae or invertebrate food supplies and the high turbidity, and the FAD would probably work much better as a fish habitat in area with clearer water.
7. Rope FAD- Mercerís Creek Bay
This area had the same muddy bottom and high turbidity which were encountered at the tire FADs. However, the ropes had been colonised by huge mats of three species of algae, Caulerpa racemosa- forma peltata, Caulerpa taxifolia, and Caulerpa sertularioides, as well as various blue-green algae. The extremely dense growth of these three Caulerpa species on the rope contrasted dramatically with sparse growth of Caulerpa prolifera on the surrounding sand. Many black and pink sponges and Clavellina tunicates were also present. The rope was clearly a far superior substrate for marine organisms of all kinds compared to the rubber tires. This structure had far more fish than the tire FAD, possibly because of the plentiful algae. Results would probably be even more dramatic in a site with clearer water.
8. Experimental Fish Condominium FAD- west Mercerís Creek Bay
These FADs, made out of drift nets and milk crates, were placed closer to the side of the bay, and the water had a much higher clarity, and far greater biodiversity of corals, invertebrates, fish, and algae. The net FADs abounded with large numbers of young fish, and were clearly the most successful of the three locations. Inshore, an impressively biodiverse coral community could be found, dominated by small Manicina areolata, and mid-sized Diploria strigosa, Porites astreoides, Porites furcata, Millepora alcicornis, and three other palmate/fenestrate growth forms of Millepora, Siderastrea radians, Siderastrea siderea, Montastrea annularis, Millepora complanata, Diploria clivosa, Agaricia agaricites, Porites branneri, and the soft corals Zoanthus sociatus, and Zoanthus pulchellus. This area had an extremely high level of coral recuritment, with many young, small head corals, and abundant numbers of Manicina areolata in the young cup-shaped phase. Also present were many gorgonians, mainly Briareum sp. and Plexaura sp. Despite the high temperature of 30.7 degrees, only modest bleaching was observed. Many calcareous algae were present, namely Halimeda monile, Galaxaura oblongata, Neogoniolithon spectabile, and to a lesser extent, Halimeda incrassata, Halimeda tuna, Halimeda opuntia, Penicillus capitatus, a finely branched Amphiroa sp., and Udotea sp.. Large amounts of non-calcareous algae were observed, overgrowing corals in many instances. These included Acanthophora spicifera, Laurencia obtusa, the blue-green Schizothrix calciola, Dilophus alternans, Caulerpa taxifolia, Caulerpa racemosa, Caulerpa racemosa- forma peltata, Caulerpa languinosa, Caulerpa cupresssoides, Padina sanctae-crucis, Sargassum polyceratium, Sargassum platycarpum, Dictyota divaricata, Rosenvingea sp., Avranvillea, andVentricaria ventricosa. Also alarming was the presence of moderate amounts of the weedy algae Dictyosphaera cavernosa and Lobophora variegata in waters near the shore line mangroves. The large amounts of algae, especially calcareous algae, coupled with certain weedy algae seems to suggest that there are moderate levels of nutrients in this area. However, we were unable to find measureable nutrient concentrations, despite moderate chlorophyll levels.
9. Reef crest and fore reef off Mercerís Creek Bay (left of channel)
The fore reef area had large amounts of coarse Halimeda sand, which gave way into large expanses of Acropora palmata and Porites furcata rubble that comprised most of the bottom substrate. Living among the rubble were Porites astreoides, Porites furcata, Diploria strigosa, all species and growth forms of Millepora, the zoanthid Palythoa caribaeorum, and the gorgonians Plexaura, Briareum, Gorgonia ventalina, and Plexaurella. Small numbers of Acropora cervicornis, and Acropora palmata with small living sections on dead skeletons, as well as Montastrea annularis, Diploria clivosa , Dichocoenia stokesii, Favia fragum, Agaricia agaricites, Stylaster roseus, and Zoanthus sociatus were found. Coral cover was very low, less than 10 percent, and many of these corals showed early signs of bleaching. Among the species affected were Palythoa caribaeorum, Porites astreoides, Porites furcata, Millepora alcicornis, Millepora complanata, and the three palmate/fenestrate forms of Millepora. Algae were abundant at this site only in the area behind the reef crest, overgrowing coral rubble and live corals. In sharp contrast the back reef area was composed of clean sand and rubble, while the fore reef rubble was almost devoid of all fleshy algae. The encrusting corallines Neogoniolithon spectabile, Porolithon pachydermum, and Hydrolithon bergensenii formed thin encrustations over the dead fragments of elkhorn rubble. In the algae-dominated zone the algae Dilophus alternans, Padina sanctae-crucis, and Galaxaura oblongata were most abundant, though Ceramium sp, Dictyota divaricata, Dictyota cervicornis, Acanthophora spicifera, Digenia simplex, a finely branched Amphiroa sp., Laurencia obtusa, Caulerpa verticillata, Caulerpa sertularioides, Turbinaria tricostata, and Ventricaria ventricosa were also observed. Also present were the weedy algae Dictyosphaera cavernosa and Lobophora variegata, which overgrew many corals. In sandier areas, Udotea flabellum, Halimeda monile, Halimeda opuntia, Penicillus capitatus, Ripocephalus phoenix , and small amounts of Halimeda tuna and Halimeda incrassata could be found. However, the calcareous algae were very few, which was surprising, given the vast amounts of Halimeda sand.
10. Sunken Rock
This areaís main substrate was a thin limestone crust made by the calcareous red algae Porolithon growing over a pinnacle of volcano-clastic formations, reaching from 110 feet to within 10 feet of the surface, with many crevices and dropoffs. Encrusting red algae blanketed 80 percent of the bottom. The gorgonians Plexaura, Eunicea, and Gorgonia, and sponge and hydroid colonies were very abundant on upper surfaces. Stony corals were very scarce and small, comprising 5-10 percent of the bottom on upper surfaces, and far less on vertical walls. These were primarily Diploria strigosa, Porites astreoides, and Millepora squarrosa, with smaller numbers of Millepora alcicornis, Millepora complanata, layered fenestrate Millepora, Montastrea annularis, Montastrea cavernosa, Diploria clivosa, Siderastrea siderea, Meandrina meandrites, and Agaricia agaricites. The soft zoanthid Palythoa caribaeorum encrusted the botom, while Palythoa swiftii grew on sponges, primarily Iotrochata birotulata, and lavender Zoanthus pulchellus. was also abundant. Bleaching had affected the Montastrea species, especially Montastrea cavernosa, and had turned many Millepora alcicornis and Palythoa caribaeorum white. However Siderastrea and Agaricia were not affected. Fleshy and branching calcareous algae were very scarce, limited to extremely small branches of Dictyota bartayresii, and comprised less that 5 percent of the bottom. At this site, the encrusting algae was being ravaged by CLD (Caribbean Coralline Lethal Disease) a disease resembling the photosynthetic bacterium CLOD (Coralline Lethal Orange Disease), a phenomenon first discovered in the Pacific in 1994, and only recently found in Caribbean waters since early 1996 by T.J. Goreau. The dead algae, instead of turning orange as it does in the Pacific, turned green.
11. Cades Reef
This area ranged from barrenness to 20 percent live coral in depths from 10 to 30 feet. Large mounds of Acropora palmata and Acropora cervicornis rubble covered by crustose coralline algae made up 60 percent of the bottom. Up to half of the encrusting red algae had been killed by CLD, causing a mottled pink, purple, white, and green appearance. Coral species diversity was relatively good, with many large heads of Montastrea annularis, Montastrea cavernosa, Porites astreoides, and Diploria strigosa. Also common were Millepora alcicornis, Millepora complanata, Millepora squarrosa, and a layered fenestrate Millepora, Favia fragum, several reasonably large Acropora palmata, and a few small, but healthy colonies of Acropora cervicornis. At this site, we also found Siderastrea siderea, Diploria clivosa, Dendrogyra cylindrus, Porites furcata, Porites porites, Porites branneri, Agaricia agaricites, Agaricia humilis, Isophyllia sinuosa, Eusmilia fastigiata, Mycetophyllia ferox, and the most Isophylastrea rigida seen at any site encountered in all of Antigua and Barbuda or elsewhere in the Caribbean. In a temperature of 29.7 degrees, Palythoa caribaeorum, Millepora alcicornis, Porites porites, Favia fragum, Agaricia agaricites and Montastrea annularis had developed completely white bleached areas, while the Diploria species, Porites furcata, and Millepora complanata were all pale. The Montastrea cavernosa were all bleached, and normally brownSiderastrea siderea had turned green or blue, showing they were in full stages of bleaching. The encrusting algae species were various coralline reds, including Mesophyllum growing in crevices, Neogoniolithon spectabile, Peyssonelia sp., Porolithon pachydermum, and another Porolithon species. Both of the latter were covered in parrotfish bites. Relatively large numbers of Diadema sea urchins were found in less than 15 feet of water, living in densely populated patches. They had no visible herbivory impact on fleshy algae, which made up less than 10 percent of the bottom cover both inside and outside areas of high sea urchin abundance. This was the only site where they were seen in abundance. Calcareous algae such as Halimeda opuntia, Galaxaura oblongata, Jania adherens, and cup-shaped Penicillus pyriformis (on sand) were present but very scarce. The only specimens of Martensia pavonia and Bryopsis plumosa found in any of the sites we surveyed were found here in small bunches. The abundant non-coralline algae were of the weedier variety, namely encrusting Lobophora variegata, Dictyota divaricata, Dictyota bartayresii, and the blue-green algae Lyngbya sp., Schizothrix calciola, and long strands of another blue-green algae species. Surprisingly, however, this area had the lowest levels of chlorophyll measured.
12. Soft Willy- Ariadne shoals
The bottom substrate at this site was about 30 percent bare rubble and rock covered by encrusting Porolithon, 30 percent fleshy algae, 20 percent hard coral, and 20 percent Palythoa caribaeorum. This area lacked a constructional reef framework, and the only corals that could be seen were small and growing on a non-coral limestone shelf which was raised above an expanse of limestone rubble by about two feet. Since there was no vertical coral reef structure, most of the fish observed congregated in undercuts below the edge of the limestone shelf. Fish were surprisingly large and plentiful at this site, given the seeming lack of suitable habitat. Many branching gorgonians made up the bulk of invertebrate biomass, followed by sponges and corals. Very small, individual heads of Diploria strigosa, Diploria labyrinthiformis, Siderastrea siderea, yellow and brown Porites astreoides, and Meandrina meandrites made up most of the coral cover, along with Millepora alcicornis, Millepora squarrosa, and Porites furcata. Other species included Montastrea annularis, Montastrea cavernosa, Dichocoenia stokesii, Stephanocoenia michelinii, Dendrogyra cylindrus, and Agaricia agaricites. Temperatures around 29.5 degrees had caused cases of bleaching in all the coral species, with the exception of the more resistant Dendrogyra cylindrus and Millepora squarrosa. Around three quarters of all corals at this site showed partial or early bleaching. Very little algae was present, mainly of the species Dilophus alternans. Not a single Halimeda was found and sand made up of this species was absent.
13. BJís Cave- Ariadne shoals
This region of the Ariadne shoals was similar to the previous site but had even less live coral (ranging from 10 to 20 percent), and consisted mostly of limestone (not coral) rubble. The zoanthid Palythoa caribaeorum was a significant 10 percent of the bottom. Algae ranged from 0 to 50 percent in patches, but consisted only of Dilophus alternans, Dictyota divaricata, and red blue-green algae species. The coral species found were generally the same as in Soft Wily, but had an even greater number of gorgonian species, including Briareum abestenum, Plexaura sp., Eunicea sp., Pseudoplexaura sp., Plexaurella sp., Pterogorgia sp., Pseudopterogorgia sp., Gorgonia ventalina, Gorgonia flabellum, and Gorgonia mariae. Many small corals less that 5 centimeters in diameter were found, possibly colonies which were recruited after the most recent hurricanes.
14. St. Johnís Harbour mouth- mid-channel
A water sample taken in turbid water at the mouth of the channel showed measurements characteristic of ocean values. Although the harbour is normally in the lee of the island, at the time of sampling winds and waves out of the northwest were entering the harbour, the result of winds driven by the approach of Hurricane Edouard.
15. South Tanner Street bridge- St. Johnís Harbour
Water samples were taken inside the harbour, close to where two gullies containing sewage runoff enter the water. Despite the presence of nutrient indicating algae such as Ulva lactuca on rocks and seawalls, measured nutrient values were very low, perhaps due to low flow and high wind driven water exchange beteen the harbour and ocean due to the unusual wind direction.
16. Outflow pipe at The Cove- St. Johnís Harbour
Water samples taken here also showed low nutrient values, despite the abundance of high nutrient indicating algae species such as Ulva lactuca and Enteromorpha along the shore. The water was extremely brown due to resuspension of fine mud from the piles of dredged material banked up around the bay.
17. Dickenson Bay off Sandals beach
Water samples taken at this site had characteristic ocean values. The water was turbid due to waves from the northwest due to long distance airflow towards the eye of Hurricane Luis, which was then east of the island, veering in a northwestward direction. The algae Cymoplia barbata and Gracilaria sp. were abundant in the surf zone, having been torn off deeper offshore rocky bottom by the high waves (according to Ashton Williams and Tom John).
18. Codrington Lagoon
Water in the lagoon had a yellowish green colour. The bottom consisted of sand with patches of turtle grass, Thalassia testudinum, and manatee grass, Syringodium filiforme and was free of corals or algae. Temperatures were high, 30.2, and salinity was 37.7 ppt, above ocean values. The higher salt content is due to evaporation in the large shallow lagoon, which has only a few narrow entrances allowing exchange with the ocean. Most of the stunted mangrove (about 5 feet tall) around the lagoon had been killed or severely damaged by Hurricane Luis, and most showed no recovery or only a few green leaves respouting.
19. Frigate bird breeding sanctuary, N. Codrington Lagoon
The waters here were of an extremely yellow-green color, fairly turbid, and devoid of corals. The temperature was extremely high, 32.4 degrees celsius. Due to the stagnant state of the water, and evaporation of seawater, the salinity concentration was high, 41.2 ppt Both values were distinctly higher than the centre of the lagoon, probably due to restricted circulation and high evaporation in this dead end branch of the lagoon. The red mangroves (Rhizophora mangle), black mangroves (Avicenia germinans) and aquatic plants nearby were far greener and healthier than those around the rest of the lagoon, and the water and tree roots abounded with algae, probably due to nutrients generated from the excretion by the many frigate birds nesting there. This area is one of the largest frigate bird colonies in the Atlantic, and the birds eat fish elsewhere and defecate in the nesting area, concentrating nutrients from a much larger area. Large mats of blue-green algae and manatee grass, Syringodium filiforme covered the bottom, with smaller areas of Thalassia testudinum. Living among the mats were purple photosynthetic sulphur bacteria. Large floating mats of Chaetomorpha linum , Chaetomorpha crassa, and Ulva fasciata were found only here, suggesting greatly excessive nutrient levels. Other plentiful algae were Acetabularia sp., Avrainvillea sp., Halimeda monile, and a finely branched Hypnea cervicornis. There were vast numbers of the upside-down jellyfish, Cassiopea xamaychana on the bottom sediments.
20. beach- North Barbuda
This impressive area averaged around 50 percent live coral, 35-40 percent of which was Acropora palmata with minor amounts of a knobby variant strongly resembling Acropora prolifera. Acropora palmata colonies were large and healtthy, forming large stands of a type which has virtually vanished throughout the Caribbean. The other most abundant species were Porites astreoides, Montastrea annularis, Palythoa caribaeorum, Zoanthus sociatus, and Zoanthus pulchellus. Several extremely large heads of Montastrea annularis, Montastrea cavernosa, Colpophyllia natans, Diploria strigosa, and Siderastrea siderea had survived the recent hurricanes, as well as several Porites furcata, Isophyllastrea rigida, and Eusmilia fastigiata. Bleaching was observed in Montastrea cavernosa and in Palythoa caribaeorum. Algae were virtually non-existent, limited to encrustations of Porolithon and other corallines, and one piece of Liagora mucosa. However large amounts of Dilophus alternans and Sargassum polyceratium fragments were suspended in the strong currents, derived from areas upcurrent since no living attached specimens of either species were seen. Rocks and dead corals were completely clean and free of sediments, possibly due to the extremely strong currents both above and below the surface. This area clearly was free of algae due to the low nutrient levels, since not a single sea urchin was found. Vast numbers of large, healthy fish, including vast schools of Bermuda chub, the Black Durgeon triggerfish, and blue tangs were observed, though fishing is heavy in this area (according to Ashton Williams). Since the constructional coral reef provided excellent habitat for fish, some were always able to hide, grow, and live to reproduce, thus restocking the population to keep up with the fishermen. The high live coral cover, large coral size, exceptional abundance of elkhorn coral, lack of algae, and unusually high fish abundance and sizes made this the healthiest reef seen.
21. Sand-mining dock-South Barbuda
This area had very low cover by live coral, less than 5 percent. The visibility was very low, and sedimentation was high. Individual colonies of Porites astreoides, Porites porites, Porites branneri, Diploria strigosa, Diploria clivosa, Diploria labyrinthiformis, Millepora alcicornis, Millepora complanata, Millepora squarrosa, 3 palmate/fenestrate Millepora sp., Siderastrea siderea, Siderastrea radians, Montastrea annularis, Montastrea cavernosa, Agaricia agaricites, Palythoa caribaeorum, Zoanthus sociatus, and Zoanthus pulchellus were found, frequently overgrown with eutrophic, weedy algae. The 90 percent algae cover was mainly Lyngbya sp., Liagora mucosa, Dilophus alternans, Cladophoropsis macromeres, and Chaetomorpha linum. Large amounts were also found of Dictyota divaricata, Dictyota cervicornis, Digenia simplex, Ceramium nitens, Centroceras clavulatum, Neomeris annulata, Turbinaria tricostata, Caulerpa racemosa, Laurencia intricata, Laurencia poitei, Laurencia papillosa, and Padina sanctae-crucis, the problem-alga Dictyosphaera cavernaosa, and the calcareous algae Halimeda opuntia, Halimeda incrassata, Avrainvillea sp., Udotea sp., Penicillus capitatus, Galaxaura oblongata, and a finely-branched Amphiroa sp.
The coral reef structure had clearly once been exceptionally healthy, and had strong vertical structure, rising 20 feet above the sand. Nevertheless most corals were dead and algae covered. High numbers of large, standing dead corals suggest that this reef was was destroyed by something other than hurricanes. The quantity and species of weedy algae present suggest that there is a very significant nutrient source affecting this area, though the nutrient levels we measured were only slightly higher than those in healthy reefs, and chlorophyll was only moderately high. The exact opposite of the site examined in the north, this was the most unhealthy site we observed in Antigua and Barbuda. The abundance of algae cannot be due to absence of sea urchins, as large amounts of black and red sea urchins, Echinometra lucunter, were a unique feature of this reef. The types of algae seen are suggestive of very high nutrient levels, and the site was similar to extremely eutrophic reefs in Jamaica subjected to high levels of land-derived sewage nutrients. Since the area has very few people nearby, the source of nutrients is a mystery and requires further investigation. High water turbidity is probably due to the proximity to the dock from which sand is exported, but high levels of sediment cannot be the cause of algae growth since sediment deppresses both coral and algae growth, and reefs subject to stress from excessive sediments alone wihout nutrient inputs, such as in Panama, show dead sediment covered coral without any proliferation of algae.
CONCLUSIONS AND ENVIRONMENTAL MANAGEMENT IMPLICATIONS
Overall reef conditions, in terms of live coral cover, were generally low, 20% or less at all sites examined except the north of Barbuda. All sites were dominated by dead coral rubble, primarily of elkhorn coral, but with patches of staghorn and finger coral rubble. These areas should be classified as coral communities, marine ecosystems containing isolated corals which are not currently building up a vertically growing wave resistant structure. Healthy coral reefs contain over 90% live coral cover, with corals competing with each other for space rather than being separated from each other by areas of hard bottom or algae.
These fairly poor reef conditions could not be explained in terms of water quality. Most water quality values measured were typical of open ocean conditions, with the exception of saltier conditions seen in Codrington Lagoon. There was no measureable freshwater runoff at any site. Nutrient levels were exceptionally low at all sites, with nitrate and phosphate being essentially undetectable by the methods used. These results, which include the most polluted areas of Antigua, suggest that the dirtiest waters in Antigua and Barbuda are lower in nutrients than the cleanest sites measured in Jamaica, where almost all reefs are severely eutrophic. Nevertheless several sites showed high levels of high-nutrient indicating algae, and fairly high chlorophyll levels, suggesting that either nutrient inputs are episodic or that more sensitive analytical methods need to be used.
Most areas had very low levels of fleshy algae and a marked predominance of sand-producing calcareous algae, but the deeper offshore sites on Ariadne Shoal, Sunken Rock, and the reef on north Barbuda were marked by a virtually complete absence of the sand-producing branching red or platy green calcareous algae despite abundance of encrusting calcareous red algae. This may be due to the abundance of large parrotfish, which are generally overfished in more accessible coastal waters. Only one coral reef site, south Barbuda, was clearly eutrophic, but the source of nutrients was undetermined and needs further study. No obvious local source exists, but there clearly must be one whose origin is at present a mystery. Other sites which were clearly dominated by eutrophic algae, in the Codrington Lagoon frigate bird sanctuary and in St. Johns Harbour were marked by lack of corals due to excessive temperature caused by poor circulation and excessive sediments caused by dredging respectively.
Two sites provided excellent coral cover in shallow water, but one, north Barbuda, is too dangerous to be used for snorkeling due to extremely strong currents. The other site, south of Bird Island, is ideal for snorkeling but needs strong protection because of the large number of extremely fragile staghorn and finger corals which can be broken by snorkelers fins, hands, or anchors. Boat mooring buoys should be provided as part of development of a protected area, along with signs instructing snorkellers not to touch or stand on corals. The extreme rarity of living branching corals at all other sites in Antigua indicates the very high conservation priority which should be assigned to protecting this and all other similar reef areas in good condition.
The tremendous amounts of coral rubble seen at all shallow sites indicates that branching corals, especially elkhorn coral, once had magnificent stands in shallow waters all around Antigua, but only a tiny fraction of them have survived. In many areas the dead coral were standing unbroken. The degree of coverage of rubble by encrusting calcareous red algae and alteration of rubble by biorerosion and physical abrasion indicate that the rubble is not due to recent hurricanes. The percentages of live coral cover compared to sites where this has been previously estimated by transects are very similar. The first sets of measurements were taken just before Hurricane Hugo in 1989, and the second set just before Hurricanes Luis and Marilyn. This suggests that the degree of damage caused by the hurricanes was very minor, and that most of the mortality preceded them, with the hurricanes basically acting to redistribute dead coral skeleton. Prior to Hugo, no major hurricane had struck for several decades. Highly experienced long term divers and fishermen all insisted that the severe decline in the reef corals preceded the hurricanes. Dredging is not a likely cause, as most of the major dredging around Antigua killed only reefs immediately down-current from dredging activity, and reef decline is not consistent in either space or time with dredging, since undeveloped areas on the windward shore which are upcurrent of all dredging were also affected. Overfishing and sea urchin abundance appear to have had only a very minor effect on algae abundances, except possibly for the erect calcareous algae. The presence of vast amounts of Halimeda sand all around both islands suggests that this algae was present in shallow water in great abundance long before overfishing of parrotfish took place. Remote sources of pollution are very unlikely as these reefs are upcurrent of all other islands and exposed to open Atlantic waters. Any sources of pollution from Europe or Africa are too remote, and would be diluted out by the long transport distance. Ship borne sources of pollution would also have affected the reefs of northern and eastern Barbuda. As neither physical, chemical, or overfishing stresses can account for this decline the only possibilities are biological stresses. The major cause of decline of the branching corals is inferred to be due to the prevalence of coral diseases such as white band disease, which has wiped out most of the Acropora corals over large areas of the Caribbean. These have produced vast amounts of standing dead corals in areas such as the US Virgin Islands, the Grenadines, the Bahamas, and the offshore islands and banks of Colombia. White band disease was noted affecting live elkhorn corals at low incidence.
Coral bleaching was prevalent, with up to three fourths of all corals partially bleached at some sites. The most susceptible species were the soft coral, Palythoa carbbaeorum, which was exceptionally abundant and almost all bleached, and the hard corals Montastrea cavernosa and Siderastrea siderea, which were bleached in most colonies. Almost all of the Diploria brain coral species were pale. About half a dozen species also had bleached colonies. Water temperatures were close to 30 degrees celsius at almost all sites, which is at least one degree above the warmest month average in this part of the Caribbean, and above the bleaching temperature threshold for the northeastern Caribbean. Even though bleaching was relatively mild, with most corals only partially bleached to pale colours rather than bleached to whiteness, this yearís event, which was also observed in St. Kitts during the preceding week, in Jamaica earlier in the month, and in Panama in the previous month, may be the most serious yet experienced in the area. The northeast Caribbean has had little bleaching reported because it has been fortunate not to have suffered the high frequency of excessively hot water temperatures which have affected the central and western Caribbean since the 1980s. The most severe Caribbean bleaching episode to date took place in 1995, striking all reefs west of Puerto Rico. However the Lesser Antilles were not affected because the unusual hurricane activity which hit the area in 1995 acted to cool down the water by mixing of cool deep water upwards by strong hurricane winds and waves.
Regular monitoring of reef conditions is clearly needed to assess ongoing and future changes in Antiguan and Barbudan reefs. It is too soon after Hurricane Luis to have much new coral settlement, and the smaller corals are primarily those which settled after Hurricane Hugo. The high levels of algae at some sites need to be monitored to determine if they are spreading, covering the hard bottom and preventing settlement of young corals and reef recovery. It is recommended that existing transect sites set up by L. Bunce et al. continue to be monitored on an annual basis, but that the extent of monitoring be greatly expanded by use of video transects covering much larger areas. During this study such experimental video transects were taken by Foster Derrick. Improvements in technique and coverage to include wide angle views for determining bottom cover along with close-ups for species identification will allow quantitative assessments to be done over large areas. Despite the low concentrations of nutrients reported here, more detailed work using more sensitive methods will be valuable to identify the sources of nutrients supplying some of the large algae patches seen, especially in the severely eutrophic reefs in southwestern Barbuda.
Artificial reefs need to be more fully developed for reef coral restoration, reef fisheries rehabilitation, and shore protection, due to the poor ecological state of most reefs seen. The experimental work by Foster Derrick and by Dennis and Raymond Compton have developed local expertise that could be combined with windmill and photovoltaic power sources to build Hilbertz-Goreau limestone artificial reefs, which provide the most favorable substrate for coral growth and recruitment of young corals, reef organisms and fish.
Our observations have many implications for reef protection policies. The most severe stresses affecting reefs in Antigua and Barbuda appear to be due to wide spread regional and global stresses caused by diseases and climate change, rather than due to local sources of stresses which could be controlled or mitigated by local environmental management policies. While little can be done about diseases short of supporting fundamental research into the unknown causes and spread of these problems, the threat posed by global warming requires strong action in international climate change negotiations. The threat to Antiguan beaches posed by lack of recovery of the branching corals which provide the barrier to shore erosion will be greatly accentuated if sea level rise continues at the unprecedented levels of 4 millimetres per year which has taken place over the last few years, or if sea level rise continues to increase. In the long run only strong international action to halt global climate change will be able to protect Antiguaís beaches, tourism, fisheries, and coastal infrastructure from the twin threats of rising temperatures and rising sea levels. Antigua and Barbuda could provide a position of leadership among small island nations in international legal agreements.
This work would not have been possible without the kind help, information, and knowledge provided by Antiguan and Barbudan divers, fishermen, and environmentalists. Extra special thanks go to Foster Derrick, who accompanied us to all sites, unselfishly sacrificing all his other commitments to transport us by boat and land, and providing generous hospitality during our stay, along with invaluable knowledge and insights into local conditions. We also thank Ashton Williams, Tom John, John and Era Birk, for valuable information and discussions on their long experience of changes in reef conditions observed by local divers. Brian Nunes, John Fuller, and Eli Fuller provided additional information into changes in reef conditions observed from their long experience fishing in these waters. We thank young Bari for accompanying us on several boat trips. Dr. Nicholas Fuller kindly provided access to his photographs of Antiguan reefs from the air and underwater. Kim Derrick, President of the Environmental Awareness Group dived with us and arranged a public lecture through courtesy of the EAG. Dennis Compton provided useful information on availability of renewable power sources and local expertise in mineral accretion technology for artificial reef structures. Cheryl Jeffreys of the Fisheries Division kindly arranged for us to join them on their trip to Barbuda. We thank the Coast Guard of the Antigua and Barbuda Defense Forces for transportation to Barbuda. Vernon Joseph and Foster Hopkins took us to field sites in Barbuda and provided information on local environmental changes. John and Albertine Jurgensen provided helpful background information on environmental problems in Antigua. Dwayna, Ariel, and Alyssa Foster were kind hosts. To all of these go our thanks, and hopes for future cooperation.