|Environment and development
in coastal regions and in small islands
Coastal region and small island papers 3
Christoph Parker and Hazel A. Oxenford
Bellairs Research Institute, McGill University,
Holetown, St. James, Barbados
Marine Resource and Environmental Management Programme, University of the West Indies, Cave Hill, Barbados
The CARICOMP sites on Barbados include a reef site near Holetown on the west coast and, 25 km away on the south coast, a seagrass site (St. Lawrence Lagoon) and a mangrove site (Graeme Hall Swamp). The 32 ha Graeme Hall Swamp is fringed by red and white mangroves and is greatly impacted by a large population and several channel-dredging projects. The seagrass site is located in a shallow lagoon that is protected from high energy waves by a reef rubble bank; turtle and manatee grasses dominate. Shallow fringing reefs are fairly extensive along the west coast of Barbados. The reef site, the Bellairs fringing reef, extends 300 m seaward from the shoreline and is very shallow, with the base of the reef at a depth of 6 m. Agaricia agaricites is the dominant coral species, but Porites porites was dominant before Hurricane Allen in 1980.
Barbados is a small island (430 km2), with a large population (258,000), located 190 km east and windward of the Lesser Antilles island chain in the southeastern Caribbean (Fig. 1). The island has a relatively low relief (maximum elevation 340 m) and is largely covered (88%) by a Pleistocene coral cap. Average annual rainfall in the coastal regions of the island varies from 1100 mm to 1250 mm (James et al., 1977). There are no locations on the island where a mangrove wetland, a seagrass bed, and a shallow nearshore hard coral reef can been found in close proximity. Therefore, two CARICOMP monitoring sites have been established in Barbados; one on the west coast just north of Holetown at Folkestone Park (13°11'18"N, 59°38'31"W), to monitor a nearshore hard coral reef (Bellairs fringing reef), and one on the southwest coast at Graeme Hall, St. Lawrence (13°04'06"N, 59°34'39"W), to monitor a mangrove wetland (Graeme Hall Swamp) and a seagrass bed (St. Lawrence Lagoon).
|Fig. 1. Map of Barbados with
postulated current patterns and locations of
CARICOMP sites. (redrawn from Younglao, 1988; based on studies by
Murray et al., 1977; Peck, 1978.)
A surface water current with a predominately westward component passes through the Lesser Antilles from the Atlantic Ocean into the Caribbean Sea (Brucks, 1971; Emery, 1972; Kinder, 1983). Barbados receives both South and North Equatorial waters, depending on the time of year. Barbados receives South Equatorial water via the Guiana Current (Ryther et al., 1967; Borstad, 1979; 1982) beginning in January. After August, a change in the tradewind pattern gives the current a stronger westerly component, and the main oceanic water mass arrives via the North Equatorial Current (Borstad, 1982). Analyses of coastal zone color scanner images collected between November 1978 and December 1982 illustrate the existence of ephemeral countercurrent eddies (100 to 250 km diameter) east of Trinidad (Muller-Karger et al., 1989). The generalized surface water flow around Barbados is shown in Fig. 1.
Graeme Hall Swamp covers an area of 32 ha in one of the most densely populated areas of the island (Riven-Ramsey, 1988) (Fig. 2). Residential development surrounds the swamp along the southern, eastern, western and northwestern boundaries, a main coastal road (Hwy 7) runs between the swamp and the sea on the south side, and agricultural lands border the swamp along the northeastern side. The swamp is essentially divided into an eastern and a western quadrant by a wide, man-made footpath/ roadway oriented from north to south through the area.
|Fig. 2. Habitat map of Graeme Hall
Swamp showing main vegetation types
and location of CARICOMP monitoring plots.
In the western quadrant of the swamp, a shallow, roughly rectangular (150 120 m) brackish lake is surrounded by a dense fringe of red (Rhizophora mangle) and white (Avicennia racemosa) mangroves. A detailed survey done in 1987 by Cattaneo et al. (1987) shows that the shores of the lake drop rapidly to a depth of 1 m or more, except along the northeastern shore which remains very shallow (<0.5 m) due to the presence of a deep layer of soft mud. The average depth of the lake is 1.32 m and the maximum depth is 2.71 m. Red mangroves dominate much of the lake shoreline, although white mangroves dominate the northeastern shore and are also found in isolated clusters along the southern boundary of the swamp. A freshwater marsh is located in the eastern quadrant of the swamp, which contains a large stand of mature white mangroves and a network of man-made drainage canals with lotus and water lilies, water lettuce, and filamentous green algae. The banks of the canals support a dense growth of sedges and strips of grassland. The CARICOMP mangrove monitoring site is located in the largest contiguous stand of accessible red mangrove trees, along the eastern shore of the lake.
Graeme Hall Swamp is an important ecosystem to Barbados and has several unique features. It is the largest body of inland water on the island and contains the largest remaining areas of red and white mangrove forest. The swamp is also home to the widest diversity of resident and migratory birds in the island, including locally rare and endangered species such as the red seal coot (Gallinula chloropus barbadensis) and the yellow warbler (Dendroica petechia) (Cattaneo et al., 1987). The oldest nesting site for the cattle egret (Bulbulcus ibis) on the island is also found in the swamp (Riven-Ramsey, 1988) and is presently located in a white mangrove stand in the northeast corner of the lake. Furthermore, over 20 species of fresh and brackish water fish reside in the swamp, including the unique killifish (Rivulus marmoratus), which is the only vertebrate in the world to fertilize its own eggs (Oxenford et al., 1993).
Graeme Hall Swamp has been highly impacted by anthropogenic activities during the last 150 years. The history has been reported in detail by Hutt (1982) and Riven-Ramsey (1993) and is summarized here. A coastal roadway was first built on the sand bar separating the mangrove swamp from the sea in the early 1700s, and the stone bridge over the main exit channel, which remains today, was built in 1871. The swamp itself, which was part of the Graeme Hall Estate, probably remained largely untouched until the mid 1800s when a hunting club was established to shoot water birds in the eastern quadrant. At that time, the freshwater marsh was extensively altered by canalizing the water flow into a series of freshwater trays (dykes) to attract water birds for shooting, and high grass banks from which mule fodder was cut and sold. Peat and mangrove poles were also known to have been cut and sold. Sometime later, a second hunting club was established in the western quadrant, and a number of shallow ponds were cleared and maintained to attract water birds, plus there was an annual cutting of the surrounding mangrove trees. A sluice gate was also installed in the narrow exit channel between the swamp and the sea in the 1930s and was opened only at low tide to control the water level in the shooting pools. Tilapia was introduced to the main lake around this time, and commercial seine harvesting took place. In 1972, the main lake was dredged and the sludge was used to fill in the western ponds and convert the land to pasture. The extensive annual mangrove cutting in the swamp ceased in the 1970s, and shooting in the swamp has been banned since 1981.
The swamp, particularly the eastern quadrant, is now managed by the Ministry of Health for mosquito control (Spielman and Nathan, 1990). The regime consists of periodic clearing of vegetation from the freshwater dykes; cutting down of mangroves that overhang the embankment path/roadway and clog the exit canal to the sea; intensive thermal fogging with Malathion (Spielman and Nathan, 1990), periodic opening of the sluice gate and maintenance of the channel over the beach with heavy digging equipment. The swamp is now mainly used for recreational line fishing and for model yacht racing. Several serious proposals have recently been submitted to the Government and the Inter-American Development Bank (IADB) to establish the swamp as a nature/bird reserve and a protected ecotourism site (Riven-Ramsey, 1993), and to incorporate it into a national marine park (Oxenford et al., 1993). There are also plans to establish a sewage treatment plant for the south coast east of the swamp.
Graeme Hall Swamp drains directly into St. Lawrence Lagoon (Fig. 3). The lagoon covers an area of approximately 18 ha, is protected from high energy waves to seaward by an old reef flat and rubble bank (50-150 m wide), and becomes exposed at spring low tides. Extensive areas of sediment accumulation occur on the southwestern shelf of Barbados (Murray et al., 1977), and the lagoon is bounded on the landward side by an actively accreting, fine, coral sand beach (now 60 m wide). The lagoon is shallow (1-3 m deep at low tide); it has a sand substrate and seagrass beds that are occasionally exposed at spring low tides. The seagrasses are primarily turtle grass (Thalassia testudinum) and manatee grass (Syringodium filiforme), although some sparse, monospecific beds of shoal grass (Halodule beaudettei) occur nearshore in the vicinity of the mangrove swamp drainage canal (Oxenford et al., 1993). Macroalgae, Bryothamnion spp., Caulerpa spp. and Padina spp., are also present in the area directly adjacent to the swamp outfall. The seagrass beds act as adult foraging and/or nursery habitat for sand dollars (Mellita sexiesperforata), a commercially important sea urchin (Tripneustes ventricosus), the endangered green turtle (Chelonia mydas), and several estuarine and reef-associated fishes such as mullet (Mugil spp.), wrasse (Halichoeres spp.), razorfish (Xyrichtys spp.), and grunts (Haemulon spp.) (Oxenford et al., 1993). The CARICOMP monitoring sites are located in the main area of mixed seagrass towards the eastern end of the lagoon.
|Fig. 3. Habitat map of St. Lawrence
Lagoon showing seagrass beds, other
substrate types, and locations of the CARICOMP monitoring plots.
The coral rubble bank is a high energy zone of breaking waves and supports little if any living coral; it is primarily covered by coralline, filamentous, and macroalgae. Beyond the rubble bank is a sandy substratum with a series of soft coral patch reefs found in water depths of 8-15 m. The patch reefs are dominated by gorgonians, but a healthy and diverse coral community is also found, with the hard corals Diploria clivosa, Diploria labyrinthiformis, Diploria strigosa, Meandrina meandrites, Millepora alcicornis, Millepora complanata, Porites astreoides, and Siderastrea siderea dominating (Bellairs Research Institute, 1984). Beyond the patch reefs, the slope is sandy to a depth of 53 m, beyond which a deep hard coral bank reef begins, approximately 1 km from shore.
The St. Lawrence Lagoon is an important ecosystem to Barbados. It is the only area with significant seagrass cover on the west, southwest, and southeast coasts of the island. It also provides an important habitat for the commercially important sea urchin T. ventricosus, the critically endangered green turtle C. mydas, which feeds in the lagoon, and the hawksbill turtle Eretmochelys imbricata, which occasionally nests on the beach. It is also the only location in Barbados where mangrove swamps, seagrass beds, and deep hard coral reefs can be found in close association. A comparison of aerial photographs of the St. Lawrence Lagoon in 1964 and 1991 indicates that the area of seagrass cover has decreased and beach width has increased over the 27 years. In fact, in just the last few years the beach has accreted so rapidly that seagrass in the western end of the lagoon has been smothered by sand and replaced by a beach exposed at low tide. The lagoon is currently used as a mooring basin for small fishing boats and for recreational swimming and windsurfing. A proposal has been submitted to the Government to incorporate the area into a national marine park (Oxenford et al., 1993).
Actively growing, shallow, fringing coral reefs are found along most of the west (leeward) coast of Barbados, extending about 300 m from the shoreline (Lewis, 1960; Oxenford et al., 1993). Monthly mean seawater temperatures off the west coast range from 29.6°C in autumn to 26.0°C in February (Sander and Steven, 1973; Johnson, 1994). Monthly mean salinity varies from 32 to 36 (Johnson, 1994); the lower salinity results from water masses drifting northwards from the Amazon River (Ryther et al., 1967). Tides are mixed semidiurnal, with two low and two high tides every 24.8 hours and with significant diurnal inequality. The maximum tidal range is 1.1 m.
The Bellairs fringing reef is located directly in front of the Folkestone Park beach and the Bellairs Research Institute within the Barbados Marine Reserve (Fig. 4). Net current speed recorded at Porters, just south of Bellairs in 1983, was 0.12 m s-1, with little significant seasonal variation (Cambers, 1984). However, the net current direction observed in the vicinity of the Bellairs fringing reef appears to be highly variable. Murray et al. (1977) observed a flow towards the north in this area, Vezina (1974) recorded a flow towards the southwest, and Ott (1975) postulated the existence of eddies in the area of the bank reef. Cambers (1984) reported that a dominant current flowing towards the northwest was recorded 22 out of 46 times, a southwest current 20 out of 46 times, and an onshore wind-driven current during the remaining four observation periods. These data suggest that the Bellairs fringing reef may be located in an area where major current streams from the south and north of the island meet (Fig. 1; Cambers, 1984).
|Fig. 4. Diagram of the north and south
lobes of the Bellairs fringing
reef showing the ecological zones, dominant substrate types and
the position of the two CARICOMP sites. (Redrawn from Stearn et
A triangular sand channel divides the reef into two unequal sections, with the southern lobe extending 200 m seaward and the northern lobe extending 100 m seaward (Fig. 5). During most of the year, the sand separating the two lobes terminates in a narrow beach. However, the beach is seasonally unstable and is often completely eroded during periodic high swell events, which occur between November and March. The bathymetry of the area and ecological zones of the Bellairs fringing reefs are shown in Figures 4 and 5, and the ensuing description follows that of Stearn et al. (1977). The CARICOMP monitoring plots are located in the spur-and-groove zones of the north and south Bellairs fringing reefs, in the areas of highest live coral cover.
|Fig. 5. Bathymetric chart of the
seafloor in the area of the Bellairs
fringing reef (depth in meters).
The swash zone or backreef area of the fringing reef is narrow (20-30 m) and lies in depths ranging from 0 to 1 m. This area is composed largely of dead coral rubble covered by filamentous algae and mobile sand. The crest zone extends 40 m from the seaward edge of the swash zone, usually in water depths of 1 m, is occasionally exposed during very low tides, and comprises mainly coralline algae coating dead coral. The crest zone is responsible for reducing the wave energy reaching the backreef area by about 75%, calculated from reduction in wave height (Roberts et al., 1975). The coalesced spurs zone supports several mixed species of hard coral in depths ranging from 1 to 2 m. The outermost zone of the reef is the spur-and-groove zone. The spurs range in width from 3 to 10 m, and the tops of the spurs lie in water depths of 1 to 3 m. The largest proportion of living coral is found on the reef spurs. The coral species composition has changed periodically over the years. From examination and radiocarbon dating of the reef framework, Lewis (1984) suggested that Acropora palmata was a major component of the coral community up to the early 1900s, but that it has declined considerably in abundance since then. A dramatic change in the coral community resulted from the passage of Hurricane Allen in 1980. Porites porites was the dominant coral species on the reef prior to Hurricane Allen, according to Stearn et al.. (1977). A re-survey of the reef after the hurricane (1981-1982) indicated that Porites porites coverage had been reduced by 96% and that Agaricia agaricites had become the dominant coral species.
The north and south lobes of the reef lie within the northern limit of the Barbados Marine Reserve, which was established in 1981. Thus, fishing is prohibited in this area and boat use is limited to authorized vessels. However, the area is very popular for recreational swimming, snorkeling, and diving. The area is also periodically exposed to heavy sediment loads originating from surface water runoff out of the nearby Holetown Hole, a narrow brackish water lagoon to the south, and it is also impacted by a general deterioration in nearshore water quality over the last 20 years (Johnson, 1994). Contaminated surface water runoff, ground water seepage, and waste water effluent pipes along the coastline have resulted in elevated nutrient levels (particularly NO3-NO2-N and PO4-P), and signs of eutrophication are obvious in the reef community (Tomascik and Sander, 1985). However the size, shape, and zonation patterns of the reef have not noticeably changed.
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