|Environment and development
in coastal regions and in small islands
Coastal region and small island papers 3
Discovery Bay, Jamaica
Peter M. H. Gayle1 and Jeremy D. Woodley
|1||Discovery Bay Marine Laboratory, PO Box 35, Discovery Bay, St. Ann, Jamaica|
|2||Centre for Marine Sciences, University of the West Indies (Mona), Kingston 7, Jamaica|
The CARICOMP site in Jamaica is located in waters adjacent to the Discovery Bay Marine Laboratory (DBML). A rocky shoreline supports a tangled mass of Rhizophora mangle adjacent to protected euryhaline waters. Mature beds of the seagrass Thalassia testudinum grow offshore in the lagoon behind the reef flat. The fringing coral reef is a spur-and-groove system on a narrow submarine shelf; it shows the effects of natural catastrophes and anthropogenic impacts. Daytime onshore northeasterly trade winds alternate with lighter, southerly, offshore land breezes at night. Human influences on the ecology of the area, which supports a small town, include terrestrial runoff from agricultural and residential developments, the activities of about one hundred fishermen based at two small fishing beaches, and the shipping of bauxite. Current research at DBML seeks to develop community-based management of fishery resources and to define the oceanography, water chemistry, and benthic processes of Discovery Bay.
Introduction: Geology and Geography
Jamaica is the third largest island in the Greater Antilles (235 km long, 99 km wide; Fig. 1), with mountains over 2,000 m high. The CARICOMP site, Discovery Bay (18°2800"N, 77°2430"W; 1.4 km2, maximum depth of 55 m), is located in the west-central portion of the north coast.
On Jamaica, Cretaceous basement rocks are capped by Tertiary limestones and, on the north coast, by the Coastal Formation of Pleistocene reef deposits. Quaternary sea-level changes have created terraces above and below the present sea level, bounded by raised or drowned sea cliffs. On land, subaerial solution has created a karst lithology, with terraces covered by Dry Limestone Woodland. Along the north coast, a narrow submarine shelf (<1 km wide) supports well defined Holocene fringing and sill reefs.
The fringing reef is continuous across the 1.2 km wide mouth of Discovery Bay, and the bay is almost cut off from the open sea. The reef crest breaks the surface in the western half of the bay, but it is just below the surface in the eastern half. Basement levels are lower in the east, due to displacement at a fault that divides the western and eastern halves of the bay (Liddell and Ohlhorst, 1981). The entrance to the bay lies between these two sections; it was 300 m wide and only 5 m deep, but in 1964 a channel 120 m wide and 12 m deep was cut in order to allow the passage of bauxite freighters.
Within the bay, steeply sloping submerged cliffs descend to the muddy floor of a deep central basin where sediments are up to 50 m thick (Hine et al., 1991). On the eastern side of the bay and in the northwestern quadrant, shallow, sandy shelves are covered with seagrass beds. The southern and western sides of the bay are lined by headlands rising from near the waters edge. Most of the shoreline is rocky, but there is a gently sloping, 400 m long stretch of sand in the southwestern corner of the bay known as Puerto Seco Beach. The Discovery Bay Marine Laboratory is located atop the low headland on the northwestern side of the bay. A 13 m deep drowned sink-hole (a blue hole) is nearby (Fig. 1).
The small town of Discovery Bay extends up the southern slopes and over the flat land to the east of the bay. A row of large private houses fronts the sheltered, eastern side of the bay, and the wooded land behind them to the east is the site of proposed housing developments. The southwestern corner of the bay contains Port Rhoades, the loading facility of the Kaiser Jamaica Bauxite Company, which can accommodate ore carrier vessels up to 25,000 dwt. The bauxite, which is excavated from the interior of the Parish of St. Ann, is received and dried at the top of the hill behind Port Rhoades. Most of the rocky, wooded land west of the bay is owned by Kaiser and is undeveloped. Columbus Park, an educational and historical park that contains tourist lookouts, is located on the cliffs extending along the western shore between Port Rhoades and the Marine Laboratory.
The largest employer in Discovery Bay, with over 500 workers, is Kaiser. Fishing, tourism, research, and teaching, as well as the usual social support services characteristic of small towns, are the other main activities in the area. In 1982 the towns population was estimated to be 1,151 (SIJ, 1993), which doubled by 1996. Prior to the establishment of the bauxite company, the main source of employment was artisanal fishing; now many of the fisherman have other jobs to supplement their income. Sary (1995) indicates that about 25% of the fishermen work with Kaiser, while another 25% are also involved with tourism; the remainder are self-employed tradesmen or are retired. Discovery Bay itself boasts a population of 74 resident fishermen (Vatcher, 1995) based at two main beaches within the confines of the bay. The fish populations being exploited are mainly pelagic and reef fishes, lobster, conch, and octopus (traps, 46%; spearfishing, 28%; hook-and-line fishing, 22%; gill nets, 4%).
Climate and Oceanography
Jamaicas climate is subtropical, traditionally marked by two wet and two dry seasons, and irregularly modified by cold fronts from North America in winter and by tropical disturbances from the Atlantic in summer and autumn. Persistent rains, sometimes continuing for a week or more without interruption, occur between October and December; a second period of heavy rainfall often occurs during April and May. However this general pattern can vary significantly from year to year. June and July are normally the driest months of the year; rainfall in Discovery Bay during the dry season of 1983 totaled 260 mm, compared to 875 mm during the wet season of the same year (Fig. 2). Not surprisingly, Ohlhorst (1980) found that rainy days and windy days per month are inversely correlated (p<0.01). Porter (1985) recorded solar radiation at 843 E m-2 yr-1 in February 1983 and 1,384 E m-2 yr-1 in August 1983. During the period 1993-1995, the monthly mean maximum air temperature was lowest in January (28.4°C) and highest in August and September (32.9°C). The monthly mean minimum air temperature was lowest in March (20.9°C) and highest in May and August (25.9°). In Discovery Bay, Porter (1985) recorded an instantaneous high of 34.2°C in August 1983 and a low of 18.1°C in January 1984.
|Fig. 2. Total rainfall (mm) compared
between wet and dry seasons at the
Discovery Bay Marine Laboratory, Jamaica: 1990-1995.
The name given to Discovery Bay by Columbus was Puerto Seco (Dry Harbor) because, unlike the neighboring Rio Bueno, there are no permanent rivers flowing into it. However, groundwater does enter the bay through deep cracks in the basement limestone, especially on the fault line that runs through the ship channel and on the western side. The salinities of the submerged springs are greater than 20, yet they cause a marked stratification in temperature and salinity in the shallow western back reef. They also help to account for differences in these parameters between the three CARICOMP sites. Average monthly water temperatures between 1993 and 1995 (Fig. 3) were cool near the mangroves (25.8-26.7°C), moderate at the coral reef site (27.9-28.7°C), and quite warm over the shallow-water seagrass beds (28.3-29.03°C). Seasonal variability in rainfall was reflected in the monthly means of the salinity measurements at the mangrove site (25.6-27.1), while the shallow seagrass and coral reef sites exhibited a higher range of values (34.1-36.2) indicative of oceanic water influences (Fig. 4). There is no marked stratification in salinity within the main body of Discovery Bay
|Fig. 4. Mean salinity () at
Discovery Bay, Jamaica: 1993-1995.
The weather of the north coast of Jamaica is dominated by northeasterly tradewinds, modified by local sea and land breeze systems. The trades combined with sea breezes usually give rise, by mid-morning (1000 hours local time), to a wind from the northeast that increases from an initial speed of 1-2 m s-1 to 4-9 m s-1 in mid-afternoon (1400 hours) and returns to 1-2 m s-1 in early evening (1800 hours). There is usually no wind between 2100 and 0500 hours because the differences in heat capacities of the land and sea masses generate land breezes that counteract the constant tradewinds (Kjerfve, pers. comm., 1993). Porter (1985) showed that the major changes in wind parameters are not in direction but in speed, with average wind speeds lowest in October and as high as 13 m s-1 in May. During the months December to March, the regular tradewind and land-sea breeze patterns may be interrupted by the passage of cold fronts from North America. These "northers" occur with irregular frequency from year to year; they typically last for three to four days and are accompanied by rain and strong northwesterly winds of 2-15 m s-1 (18-40 km h-1). During summer and early fall, the tradewinds are weaker and less predictable, and there may be extended periods of unusually calm weather with little or no wind during early morning hours.
The north coast of Jamaica is sheltered from oceanic swells by Cuba, which lies 150 km north, and the prevailing tradewinds have limited fetch. The local wave regime is primarily generated by the local wind pattern; when the wind drops in the evening, the sea generally becomes calm and may remain so until the sea breeze returns the next morning, at which time the sea rapidly reaches wave heights of 1.5 m or more. The waves over the shallow back reef within Discovery Bay are capable of resuspending bottom sediments for relatively short periods (days), a factor which may help to restrict coral growth (Dodge et al., 1974). Much higher waves are generated during northers or hurricanes.
Jamaica usually experiences a mixed tidal regime, with primarily diurnal spring tides and low-amplitude semi-diurnal neaps. DBML data for a three-month period in 1984 showed a daily tidal range between 15 and 60 cm; due to seasonal variations in sea level, however, the annual range may be as much as 1 m. Because of the limited tidal range, currents within the bay are primarily wind driven. The fringing reef is generally swept by the slow east-west Caribbean Current, but the prevailing tradewinds generate steep, short period waves (2-3 sec) that approach the Discovery Bay reefs from the northeast and create a distinct bi-directional surge current combined with a slow (0.25 m s-1), unidirectional westerly current on the shallow forereef. Occasional stronger and deeper currents (~0.75 m s-1) may run in opposition to the surface currents on the forereef. This situation is not confined to rough (northwesterly) weather; it has been noted on exceptionally calm days (pers. observation). Behind the reef crest, the tradewinds generate a slow, clockwise surface current as water enters the bay along its eastern margin. Several smaller circulation gyres form inside the bay, before the water exits over the western reef crest. At night, the gentle southerly land breeze creates a general movement of surface water out of the bay through the ship channel and over the entire reef crest. Water enters over the western side of the reef crests during the episodic northwesterlies, again forming gyres inside the bay before exiting to the east of the ship channel (Fitzpatrick, 1993).
Since 1871, Discovery Bay has experienced thirteen hurricanes (Woodley, 1992). Only two of these occurred after 1944, but they were the strongest hurricanes ever recorded in the western Atlantic. Hurricane Allen (1980) and Hurricane Gilbert (1988) generated wind speeds estimated in Discovery Bay at 110 and 180 km h-1, respectively. Although Hurricane Allen weakened as it passed Jamaica, maximum deep-water wave height at 160 km north of Discovery Bay was hindcast at 7.8 m, with a significant wave period of 10.4 sec. After shoaling and refracting as the waves passed over the forereef slope, peak breakers 11.5 m high were calculated to have impacted the eastern forereef (Kjerfve et al., 1986). This analysis is supported by photographs of 12-m-peak waves taken at the site by Woodley et al. (1981). On the western forereef (where the CARICOMP site is located), wave heights were hindcast to have been at least 8 m (Kjerfve et al., 1986). Because of wave refraction, the calculated breaker heights at different points varied by a factor of 2.6. These waves, from the first major storm in 36 years, generated high levels of destruction of corals on much of the completely exposed reefs of the north coast. A secondary effect was the creation of a new substratum for the growth of algae. The deeper reefs (>25 m deep) were only slightly affected by Hurricane Allen (Porter et al., 1981). Woodley and his colleagues documented the cumulative effect of all this destructive power on the reefs of Discovery Bay (Woodley et al., 1981; Woodley, 1989).
In contrast to some other parts of the Caribbean, few fishes are to be found in Jamaican coastal waters. The major cause of their depletion is believed to be overfishing (Koslow et al., 1986). The impact of fishing is especially pronounced along the north coast, where the submarine shelf is very narrow and fishing efforts are concentrated in a small area (Munro, 1983). The fishing methods used are selective and exert the greatest pressure on medium to large herbivores and predators. The large breeding flocks of striped parrotfish (Scarus croicensis) described by Colin (1978) are a tenth of their former size. Roving flocks of surgeon fish are no longer evident in the lagoons, and it is noteworthy when any fish as long as 30 cm is seen. Since the mid-1970s, there has been a marked increase in the number of spearfishers, and spearfishing at night with underwater lamps and scuba gear has also increased in popularity. Pollnac (1995) reports that spearfishers visiting from as far afield as Trelawny Parish, and even some displaced by increased enforcement activities at the Montego Bay Marine Park, are pressuring Discovery Bay fish stocks. In 1996, a group of 15-20 spearfishers was seen cooperating, in a shoulder-to-shoulder line, to sweep sections of the coast. It is possible that the surviving fishes have learned to flee both snorkelers and scuba divers, perhaps making the problem appear to be worse than it really is.
Kaufman (1983) postulates that the loss of spatial heterogeneity caused by Hurricane Allen in 1980, and reinforced by Hurricane Gilbert in 1988, has reduced the carrying capacity of the reef. Hughes (1994) goes further and argues that the combination of anthropogenic and natural factors acting on the reefs has caused a phase shift from high diversity, coral-dominated reefs to a low diversity, algae-dominated system. Changes in the reef community resulting from intensive fishing activity have also been suggested by Woodley (1979). Small territorial damselfish have increased in abundance, and there are still large numbers of midwater plankton feeders such as Creole Wrasse (Clepticus parrai) and Blue Chromis (Chromis cyanea). These possible overfishing indicator species may act as filter screens, decreasing the successful recruitment of other reef fishes, such as happens with fishes and intertidal barnacle populations in temperate waters (Gaines and Roughgarden, 1987).
The problem of overexploited reef fish stocks is the main focus of the Fisheries Improvement Program based at DBML since 1988. The aim of the program is to work with fishermen to help them better manage their fishery resources. One approach used in 1991 was a "two-for-one" exchange program, which involved replacing more than 250 small-mesh pots (3.3-4.1 cm mesh) with large-mesh pots (5.5 cm mesh). Subsequent beach monitoring of fish catches demonstrated an increase in length-at-first-capture and a change in size frequency distribution of the redband parrotfish (S. aurofrenatum; Sary et al., 1997a). By 1994, trap catches had increased in both weight and numbers, and catch composition had shifted toward larger and more economically valuable species (Sary et al., 1997b).
Recreational and residential use by locals and tourists places great pressure on the marine environment of Discovery Bay. Water skiing, jet skiing, and snorkel/scuba diving occur in the shallow backreef waters. Puerto Seco Beach is heavily used on holiday weekends, not only by locals and tourists but also by 15-20 large busloads of people making day trips from surrounding areas. With no central sewage disposal system, there is a proliferation of individual units from residences and businesses surrounding the bay; these range in type from numerous soakaways to a few state-of-the-art tertiary systems. Moreover, blasting for construction of piers and beaches along the eastern shoreline has resulted in cases of erosion.
DElia et al. (1981) demonstrated that groundwaters in this part of Jamaica typically contain 80 µmol of nitrogen per liter, primarily as nitrate, but are essentially devoid of phosphates. Therefore, the submarine springs that feed Discovery Bay are a significant source of nitrate and there is a strong, linear, inverse correlation between salinity and nitrate concentration. These investigators suggested that the nitrates were derived naturally (from forest leaf litter and meteoric water) rather than anthropogenically. More current research indicates that nitrate and phosphate levels in the bay are either undetectable at some sites or are approximately equivalent to accepted critical values established for the health of coral reefs (1 µM nitrogen, 0.1 µM phosphorus). These low nutrient values, comparable to those obtained by DElia et al. (1981), indicate that small, quickly absorbed amounts are entering the bay and/or that there is good mixing and flushing of bay waters. It is also possible that some nutrient-rich urban waste waters flow underground north to the sea rather than into the bay.
Despite the above, there are recent indications that a problem with excess nutrients is developing. Algae such as Chaetomorpha and Dictyosphaeria spp. are more noticeable in the lagoon and on the forereef. Also, black-stained sediments are being found just beneath the surface of sediments near the southern and eastern shores. Spills of fuel, oil, and contaminated bilge water have occurred at Port Rhoades, which, because of the localized circulation gyres within the bay, have affected primarily the eastern side. Kaiser has occasionally used chemical dispersants to break up these spills, but there is no evidence of negative, long-term effects on the flora and fauna of the bay.
Ecosystems and Major Impacts
Jamaicas reefs were the first in the Caribbean to be studied intensively, and more stony coral species have been recorded here than anywhere else in the Caribbean 64 species (Wells and Lang, 1973). The most studied Jamaican reefs are those at Discovery Bay. Figure 5 illustrates the characteristic zonation pattern for these reefs prior to Hurricane Allen in 1980 (from Woodley and Robinson, 1977).
|Fig. 5. Zonation of Discovery Bay reefs (after Kinzie, 1973).|
The reef flat in the western portion of Discovery Bay is founded primarily on erect dead skeletons of Acropora palmata and Millepora complanata. Seaward of the reef flat was a broad thicket of living A. palmata which is now a rubble slope. Farther out on the gently sloping terrace, where massive corals grew among bramble bushes of staghorn (A. cervicornis), the corals now arise from a cemented, eroded mat of staghorn twigs. By 1988, there had been some recruitment by A. agaricites and Porites spp., and colonies of staghorn had become established in some areas and were beginning to thrive; all were smashed by Hurricane Gilbert in 1988. What used to be a chain of islets (the Allen Islands, created by Hurricane Allen in 1980 from A. palmata rubble) united into an almost continuously visible barrier, was remobilized and spread out inside the bay by Hurricane Gilbert in 1988. Although now emergent in only a few places, the barrier effect of the reef crest remains. Montastraea annularis, in the mixed zone at 5-10 m depths (Goreau and Wells, 1967), is still the chief foundation of incipient buttresses, where they occur. The outer part of the terrace is dissected by sand drainage channels to form a spur-and-groove system. The spurs (built by A. cervicornis) terminate at the outer edge of the shallow terrace as a steep intermittent escarpment that slopes steeply from a depth of 17 m to about 25 m. Major sand channels continue down the forereef slope between pinnacles of reef growth before discharging over a submarine cliff at a depth of about 55 m to the talus slope below at a depth of about 130 m. The deep pinnacles (25-40 m depths) are largely composed of the flattened morphs of M. annularis, whereas species of Agaricia are the dominant corals at greater depths, down to 70 m.
It has often been assumed that the primary causes of hurricane-induced mortality on a reef are high wave energy and low salinity due to heavy rain during the storm event. Knowlton et al. (1981) demonstrated substantial tissue and colony mortality in A. cervicornis after Hurricane Allen, an episode that was sustained for 5 months and was an order of magnitude greater than that caused by the immediate effects of the storm. This was a consequence of predation and "white-band" disease, and it resulted in the elimination of more than 98% of the original hurricane survivors. The previously unsuspected but combined effects of disease and predation would go a long way toward explaining widely variable rates of reef recovery previously reported. The coral-eating snail Coralliophila caribbea was particularly effective in killing survivors and recruits of A. cervicornis (Knowlton et al., 1988). This animal might have been unusually abundant due to removal of its predators by overfishing. By 1988, however, the snail was again scarce, and small A. cervicornis colonies were common. Then came Hurricane Gilbert.
By 1983, removal of herbivorous fishes had apparently allowed free-living algae to become more abundant in deeper waters while they remained relatively scarce in shallow waters. There, the grazing sea urchin Diadema antillarum had become more common over the years, due to removal of its predators (Woodley, 1979; Hay, 1984). This urchin had become the single most important herbivore to a depth of about 20 m, and it was maintaining a low level of algal cover that was of benefit to the corals. A Caribbean-wide epidemic disease occurred in 1983 and all but eliminated this important herbivore from Jamaican reefs (99% mortality; Hughes et al., 1985). At Discovery Bay, Liddell and Ohlhorst (1986) found that population density at depths between 0 and 20 m had decreased from 6.6 urchins per square meter to zero; after one year, bottom cover showed a concomitant increase in non-crustose algae from 30.7% to 64.7%. This occurred at the expense of other reef benthos such as crustose coralline algae and sponges. This rich bloom of algae still persists in Jamaican waters because there has been very little recovery in the general Diadema population and herbivorous fishes are still scarce (Hughes et al., 1987). The algae Dictyota and Halimeda spp., Caulerpa racemosa, and Lobophora variegata are particularly abundant. When adjacent to corals, these algae often over-shade them, causing eventual coral death (Liddell and Ohlhorst, 1986). Also, the profuse algal growth may be preventing settlement of coral as well as other invertebrate larvae. From random transects on the Discovery Bay forereef in 1992, coral cover was as low as 2.0%. However, some sessile invertebrates are relatively abundant, such as gorgonians, sea anemones, and zoanthids. It is worth noting that the death of Diadema has not had this catastrophic effect in other Caribbean localities where herbivorous fishes are still abundant, such as the Cayman Islands and Belize. In recent years, the population of urchins at Discovery Bay has been increasing, but only in small groups in shallow water; they are still absent beyond a depth of about 6 m.
In 1987-1988, corals in many parts of the Caribbean, including Discovery Bay, suffered the first mass bleaching on record apparently in response to high sea temperatures (Sandeman, 1988; Gates, 1990). Additional bleaching events occurred in 1989, 1990, and 1995. Some corals recovered when temperatures fell after the first warm spell, but many died in subsequent years. The last major bleaching event (1995) affected 43% of the existing corals on the CARICOMP transect lines. Bleached polyps were seen on ~63% of the total surface area of the corals. Five months later, ~9.5% of the surface area of the corals had died. Interestingly, mortality was not confined to the bleached areas of the coral heads (CARICOMP, 1997).
Coral Reef Site
The actual CARICOMP coral reef site is situated in the mixed/buttress zone of the west forereef in 6-8 m of water (Fig. 6). In 1977, this area was described by Woodley and Robinson as a mixed zone of high coral abundance and diversity; there were a few incipient buttresses, mostly consisting of numerous massive heads of M. annularis in a field of staghorn coral, Porites spp., and castle-like colonies of Dendrogyra cylindrus, with many other coral species and diverse gorgonians. In 1984, Liddell et al. reported that M. annularis had maintained its dominance, with colonies exhibiting great variations in size. Other branching and boulder corals were well represented, including species of Acropora, Porites, and Diploria. Sponge and algal diversity was usually low, whereas gorgonians of Briarium and Plexaurella spp. were reported as being common.
At present after two hurricanes, four bouts of bleaching, and a shortage of herbivores the huge stands of the two Acropora species leveled in 1980 have not recovered. They have also suffered from "white-band" diseases. Meanwhile, the massive heads of Montastraea are being overgrown by the flat clinging leaves of Lobophora as well as other turf algae. Opportunistic corals have again recruited in small numbers on the terrace, but they are having to compete with numerous algae.
Coincidently, the CARICOMP site is only a few meters from a permanent photoquadrat site at a depth of 6-7 m; coral cover here was measured in 1978 and found to be 54% (Porter et al., 1981). Data from the first five CARICOMP coral reef transects in 1993 returned an average hard coral cover of 17.7% (Fig. 7). A further decrease in coral cover over the same five transects was demonstrated in 1994; hard coral cover was found to be 9.5%, with Montastraea spp. being the primary coral species. This level of coral cover was maintained during the 1995 and 1996 reef surveys, which utilized five and ten transects, respectively.
The backreef area on each side of the bay supports a lagoon environment, with generally coarse calcareous sand, scattered coral heads, small patch-reefs, large beds of turtle-grass, and soft mud at its deeper points. Wave action from storms and norwesters scours the shallow area of the bay. At sheltered depths (>10 m), fine sediment accumulates and light levels are relatively low. Conditions on the sides of the deep basin (at 20 m depth) are similar to those on the forereef at 40 m depth. Species and growth forms generally characteristic of greater depths on the forereef are found here in the shallow waters of Red Buoy or Columbus Park reefs. Various forms of suspension feeders dominate the scene; while the diversity of the corals appears to be high in this region, they are not numerous. The shallow lagoon exhibits a transition from coarse sand along its margins to fine silt in its deeper regions, which are hummocked by intermittent burrow-mounds of the ghost shrimp Callianassa spp. The usual assortment of mobile invertebrates is found, such as holothurians and grazing, rock-boring, and burrowing echinoids (Aller and Dodge, 1974), in addition to numerous cnidaria such as Condylactis gigantea and Cassiopea xamachana. Small patch and fringing reefs occur, especially on the southern and western margins. Quantities of coral rubble, mostly old slabs of A. palmata, beneath which is a rich invertebrate fauna, are found within 100 m of the reef flat. Approaching the rear zone, extensive carpets of coelenterates such as Stoichactis and Palythoa spp. are found in the sand as well as covering hard substrate. These mats are interspersed with isolated heads of boulder corals M. annularis and S. siderea (many of them dead and eroded) as well as the more delicate P. porites. The lagoon contains abundant algae, including the calcareous greens Halimeda and Penicillus spp., which are major components of backreef sediment (Liddell et al., 1984). Extensive carpets of Chaetomorpha linum grow in the well-lit shallow waters during warm summer months, covering the bottom sands. During windy weather that creates rougher conditions, these mats slough off the bottom and float around the bay.
For CARICOMP, the important backreef component is the angiosperm Thalassia testudinum, which serves as a habitat for numerous small fishes and mobile or benthic invertebrates. At Discovery Bay, the root system of this particular grass bed overlies a layer of coral rubble and is not particularly thick (0.3 to 0.5 m), nor is there much in the way of fleshy algae or other types of grass in this area. Data for 1995 indicate that mean Thalassia biomass was 1,045.1 g m-2 dry weight, with a calcareous algal fraction of 105.9 g m-2. For the same site, productivity averaged 2.95 g m-2 d-1, with a daily turnover of 3.1% during the summer (Fig. 8). Although not a particularly dense or tall grass bed to the naked eye, the biomass appears to be increasing slowly, and this area serves as an important feeding site for schools of small grunt, silversides, and parrotfish in the bay. Despite the reduced numbers of these fishes, their daily migrations into and out of the site can still be observed at dawn and dusk.
|Fig. 8. Biomass (g/sq m and mean wt.g)
and productivity (g/sq m/day) at
the CARICOMP seagrass site, Discovery Bay, Jamaica: 1993-1995
(1993 productivity is the mean of four numbers).
The shoreline in Discovery Bay is typically highly phytokarsted Pleistocene limestone inhabited in a narrow intertidal range by numerous molluscs and rock-boring echinoids, interspersed with endolithic blue-green and red calcareous algae as well as zoanthids (Fig. 6). This base substrate supports a variable growth of mangrove trees (Rhizophora mangle) showing the expected change in species type as distance from shore increases inland, so that a few white mangroves (Laguncularia racemosa) and black mangroves (Avicennia germinans) are also found in this relatively small stand of trees. In the sheltered westernmost corner of the bay, there is a pocket (~50 m2) of mangrove trees and an adjacent pool of quiet, shaded water known locally as the Blue Maze. Although small in size, this area is ecologically important to the bay because the largest upwellings of freshwater are located in and around this site. As a result, a large fraction of the dissolved nutrients entering the bay does so in this area.
The site contains a stand of largely convoluted red mangrove (Rhizophora mangle) trunks, interspersed with white mangrove (Laguncularia racemosa). It has little or no topsoil; prop roots are supported mainly by a limestone substrate. Despite the lack of peaty substrate, the prop roots bear a healthy community of sessile plants and animals, with many different species of juvenile fishes taking shelter and feeding among them. The R. mangle trunks grow at various angles from the ground, causing them to lie upon one another at times. This creates an almost impenetrable maze of twisting branches, trunks, and roots at varying heights from the substrate. The stand itself is surrounded by brackish water on all sides, with R. mangle occurring mostly at the periphery where prop roots easily reach the water. Data from 1996 indicate that the mean total height of the R. mangle stand is 2.9 m, with a biomass of 7.85 kg m-2 (Golley et al., 1962) and basal area of 10.38 m2 ha-1. Productivity estimates for this site range from 2.10 to 4.09 kg m-2 yr-1 for the months of June to October 1996.
The following persons participated in data collection: A. Downes, K. Fedorka, N. Judd, M. Henderson, and A. Kunze. We are further indebted to K. Fedorka for his work in preparing the illustrations, and to M. Haley and C. Gopaul for editorial comments on the initial draft manuscript.
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