Environment and development
in coastal regions and in small islands
colbartn.gif (4535 octets)

Coastal region and small island papers 3

Carrie Bow Cay, Belize

Karen H. Koltes1, John J. Tschirky2, and Ilka C. Feller3

1 Biological Resources Division, U.S. Geological Survey, Department of the Interior, MS-301, 12201 Sunrise Valley Drive, Reston VA 20192 USA
2 The Nature Conservancy, Latin American and Caribbean Division, 1815 North Lynn. Arlington VA 22209 USA
3 Smithsonian Environmental Research Center, PO Box 28, Edgewater MD 21037 USA

Carrie Bow Cay is located 18 km offshore in the central province of the Belize Barrier Reef complex. Although the reef-seagrass-mangrove complex near Carrie Bow Cay is representative of the entire barrier reef complex, the central province contains the most spectacular reef development. The climate in this region is subtropical, with a wet season from June to October. The present-day barrier reef seaward of Carrie Bow Cay is characterized by distinctive zones, with biological and geological development controlled primarily by water movement and depth. Coral cover along the CARICOMP transects is currently in the range of about 12-20%, down from an estimated 30-35% in the 1970s. The decline in coral cover appears to have resulted from die-off of Acropora cervicornis, one of the dominant hard corals in the 1970s, and an accompanying increase in macroalgae. The seagrass community is dominated by Thalassia testudinum, with an estimated total biomass at the CARICOMP sites varying seasonally from approximately 3,766 to 4,159 g m-2 dry weight, among the highest reported for the Caribbean. The mangrove community is dominated by Rhizophora mangle L., which forms a monospecific fringe around the peripheries of the islands. In 1995, widespread bleaching was reported in Belize for the first time. At Carrie Bow Cay, bleaching was preceded by unusually calm water conditions, high light transmittance, and high sea surface temperatures. Although the area in the vicinity of Carrie Bow Cay is generally considered pristine at present, increasing pressure from development and the rapidly expanding tourism industry pose significant threats.

Introduction                                                                                             

The Smithsonian Institution’s National Museum of Natural History Field Station on Carrie Bow Cay (16°48'N and 88°05'W) is one of three CARICOMP sites established in Belize (Fig. 1). Carrie Bow Cay is situated 18 km offshore on top of the barrier reef proper. Twin Cays, site of the CARICOMP mangrove and seagrass studies, is located approximately 2.3 km leeward of Carrie Bow Cay in the outer lagoon. The Smithsonian Institution established the research station at Carrie Bow Cay in 1972 and has sponsored extensive multidisciplinary investigations of the coral reef, seagrass, and mangrove communities of the region, resulting in more than 500 publications.

Belize barrier reef
Fig. 1. Map of the central province, Belize Barrier Reef complex, showing
Carrie Bow Cay (coral reef sites) and Twin Cays (seagrass and mangrove
sites).

The Belize Barrier Reef is the largest continuous reef system in the western Atlantic, extending 257 km from Ambergris Cay to the Gulf of Honduras (Burke, 1982). The barrier reef complex varies in width from 10 to 32 km and contains hundreds of mangrove islands, diverse intertidal and subtidal barrier and patch reef zones, three large atolls, and vast lagoonal seagrass beds. The present shape and bathymetry of this system are the outcome principally of faulting along a NNE trend during the Pliocene (Macintyre and Aronson, 1997). The resultant series of multiple fault-block ridges parallel the coastline and, in combination with reef growth and karstification associated with glacio-eustatic fluctuations in Pleistocene sea levels, produce the present-day patterns of parallel shoals, reefs, and mangrove islands (Burke, 1993; Macintyre et al., 1995).

On the basis of biogeomorphic characteristics, Burke (1982, 1993) divided the barrier reef complex into three provinces: northern, central, and southern. The central province, in which Carrie Bow Cay and Twin Cays are located, contains the most spectacular reef development, with 91 km of almost continuous well-developed barrier reef, sand cays on the inner edge of the reef rims, and numerous mangrove islands, patch reefs, and seagrass beds in the center and landward edge of the barrier platform (Burke, 1982; Macintyre and Aronson, 1997). Detailed descriptions of the topography, geology, ecology, and biology of this portion of the Belize Barrier Reef are provided by Rützler and Macintyre (1982a).

Although no data have been collected on water quality, the area around Carrie Bow Cay is generally considered pristine and removed from any potential anthropogenic threats associated with Belize’s small coastal towns and intensive citrus production. Fishing for finfish (primarily snapper and grouper) is conducted by handline and speargun, with little or no trap fishing. Beginning in the 1970s, increased fishing pressure by local fishermen on queen conch and spiny lobster depleted these formerly common species from shallow-water areas around Carrie Bow Cay (Rützler and Macintyre, 1982b). These target species continue to be overfished.

Atmospheric and Oceanographic Climates                                                 

Belize has a subtropical climate, with an average minimum temperature of 20°C in December and January and an average maximum temperature of 31.1°C in August (Stoddart et al., 1982). Temperatures on the barrier reef at Carrie Bow Cay are comparable to those reported on the mainland (Rützler and Ferraris, 1982). Air temperatures recorded from 1993 through 1996 as part of the CARICOMP project at Carrie Bow Cay ranged from a mean monthly maximum of 34.6°C (August 1995) to a minimum of 22.5°C (January 1996; Fig. 2). Northeast tradewinds prevail for about 70% of the year, interrupted during November-February by "northers." These 4-5 day periods of northerly winds and low temperatures are associated with the southerly extension of the North American high pressure system (Stoddart et al., 1982) and are often accompanied by strong swells (Perkins, 1983). Hurricanes occur during the months of July through October. During the 20th century, nine hurricanes and seven tropical storms have passed within a 50-km radius of Carrie Bow Cay. Most of these storms have occurred since 1960.

Air temperature
Fig. 2. Monthly means of the maximum and minimum air temperatures
recorded daily at Carrie Bow Cay, Belize, from January 1993 to December
1996. No data were recorded September - November in 1993 and 1994
while the station was closed.

Rainfall is highly variable in Belize, increasing markedly from north to south (Stoddart et al., 1982). The average annual rainfall in the Dangriga (Stann Creek) area is 2,218 mm, with a maximum of 312 mm in September and a wet season from June to October. The climate on the central barrier reef is drier; the cays get approximately 42% of the mainland rainfall based on measurements at Carrie Bow Cay from 1976 through 1980 (Rützler and Ferraris, 1982; Fig. 3). On average, maximum monthly rainfall for Carrie Bow Cay occurs in November (mean = 336 mm), with a minimum in March (mean = 23 mm). In 1995 and 1996, the only years for which there are complete CARICOMP records, cumulative rainfall at Carrie Bow Cay was 1,691 mm and 1,883 mm, respectively. Maximum monthly rainfall (488 mm) occurred in November 1996, whereas no rain fell in May 1995 (Fig. 3). Humidity ranges from 58% to 96%, with an average of 78% (Rützler and Ferraris, 1982).

Precipitation
Fig. 3. Precipitation at Carrie Bow Cay; monthly mean (in mm) for 1995
and 1996 (CARICOMP) compared to monthly mean from 1976-1980
(
Rützler and Ferraris, 1982). Note precipitation during May 1995 = 0 mm;
no data were collected in February and December 1976-1980

In addition to daily monitoring of precipitation and air temperature as part of the CARICOMP program, weekly monitoring of water temperature (0.5 m below the surface), salinity (0.5 m below the surface), and light attenuation (vertical distance) at the drop-off seaward of the coral reef sites (Carrie Bow Cay) and at the seagrass sites (horizontal distance 0.5 m below the surface; Twin Cays) has been conducted since January 1993 (except during seasonal closure of the field station from September through November in 1993 and 1994). Interstitial salinity measurements have been recorded at the Twin Cays mangrove plots since mid-1995. Bottom water temperatures were recorded from 12 August to 22 December 1994, and continuously since 25 August 1995, using remote data loggers at the coral reef (13 m) and seagrass (1.2 m) monitoring sites. Mean monthly surface water temperatures over the drop-off ranged from 26.2°C (February 1996) to 30.3°C (May 1996; Fig. 4). Daily means of bottom water temperatures at the coral reef site (13 m) ranged from 25.4°C (25 February 1996) to 30.3°C (27 August 1995; Fig. 5). During 1994, bottom water temperatures exhibited a low-frequency oscillation of approximately 1°C, with a cycle of 21 days (Fig. 5). Water temperatures recorded weekly at Seagrass Site II ranged from a monthly mean of 23.6°C (January 1996) to 31.3°C (June 1996; Fig. 4). Salinity ranged from a monthly mean of 33.0‰ (August 1996) to 37.4‰ (March 1996) over the drop-off and 33.3‰ (August 1996) to 37.3‰ (August 1996) over the seagrass sites. Secchi disk readings ranged from 11.0 m (May 1996) to 35.5 m (July 1995) over the drop-off (vertical distance), and 7.0 m (January 1995) to 15.8 m (March 1993) at Seagrass Site II (horizontal distance; Fig. 6).

Surface water temperature
Fig. 4. Surface (0.5 m depth) water temperature over the drop-off seaward
of Carrie Bow Cay and at CARICOMP Seagrass Site II; monthly mean
from January 1993 to December 1996.

Bottom water temperature
Fig. 5. Daily mean of bottom water temperatures (13 m depth) recorded
08/12/94-12/22/94 and 08/25/95-01/22/97 using a remote data logger
deployed at CARICOMP Coral Reef Site I. Note the 21-day temperature
cycle in 1994 and the signatures of Hurricanes Opal and Roxanne in early
October 1995.
Light attenuation
Fig. 6. Light attenuation (monthly mean Secchi disk readings in m)
measured vertically over the drop-off and horizontally at 0.5 m below the
surface over the seagrass beds.

The tide in this region of the Caribbean is microtidal and mixed semi-diurnal; the mean range is 15 cm at Carrie Bow Cay (Kjerfve et al., 1982) and 21 cm at Twin Cays (Wright et al., 1991).

Barrier Reef Environment                                                                          

The western Caribbean is characterized by thick late Holocene shelf and shelf-edge reef accumulations which reach their maximum extent along the Belize Barrier Reef (Macintyre, 1988). Sediments accumulate more rapidly here because lower wave energy (relative to the eastern Caribbean) has allowed fragile branching Acropora cervicornis and other communities to flourish at depths of 20 m or more. Cores taken near Carrie Bow Cay indicate that the barrier reef consists of at least 16 m of unlithified late Holocene sediments in the back reef and more than 18 m of a mixed coral and deeper water coral-head facies in the shallow and deep forereef, with a maximum age of 7,175 ± 100 years BP. A. cervicornis is present but appears to play a minor role in the construction of this outer reef complex (Macintyre, 1988).

On the basis of dominant biological and geological characteristics, Rützler and Macintyre (1982b) divided the present-day barrier reef seaward of Carrie Bow Cay into four major zones: backreef, reef crest, inner forereef, and outer forereef. These zones were further divided into zones, the biological and geological characteristics of which are controlled primarily by water movement and depth (Rützler and Macintyre, 1982b). Site I of the CARICOMP reef transects was established on the study transect set up by Rützler and Macintyre (1982b), approximately 200 m north of Carrie Bow Cay. Site II was established approximately 100 m south of Site I at the same depth (13 m). Both sites are located on the upper edge of the inner reef slope that marks the transition from the inner to the outer forereef habitats (Fig. 7).

Transect lagoon to outer forereef
Fig. 7. Study transect of Rützler and Macintyre (1982b) showing
biogeological zones from lagoon to outer forereef (frs = forereef slope;
hsg = high spur and groove; irs = inner reef slope; lsg = low spur and
groove; or = outer reef; pr = patch reefs; rc = reef crest; rf = reef flat;
sg = seagrass; st = sand trough). Location of CARICOMP Site I reef
transects is indicated by an arrow. CARICOMP Site II is approximately
100 m south at the same water depth (13 m). Note that Acropora
cervicornis,
depicted on the inner reef slope in this diagram from the late
1970s, has disappeared.

The inner forereef zone at Carrie Bow Cay begins seaward of the reef crest with the high-relief spur and groove zone. This zone is characterized by large, well-developed buttresses separated by sand channels (Rützler and Macintyre, 1982b). Percent coral cover ranges from near zero to >60%, and topographic complexity ranges from <0.20 to >0.80 (Aronson and Precht, 1995). In a 1980 survey, algal abundance was low (about 4.5%) and consisted primarily of Porolithon and Halimeda opuntia (Littler et al., 1987). This zone is composed of two overlapping subzones: a shallower set of spurs (0-3 m depth) dominated by Acropora palmata and Millepora spp., and a slightly deeper set of spurs (3-6 m depth) dominated by Agaricia tenuifolia (Rützler and Macintyre, 1982b; Aronson and Precht, 1995). Dominance of the high-relief spur and groove subzone by A. tenuifolia contrasts with many Caribbean reefs in which reef buttresses are constructed primarily by massive corals such as Montastraea annularis. Chornesky (1991) showed that this results from the ability of A. tenuifolia to modify its skeleton so that contiguous, genetically distinct colonies become anchored against each other to form a relatively stable honeycomb of coral skeleton that monopolizes space on the spurs. Other abundant species in this zone include Acropora cervicornis, Porites porites, and P. astreoides. Prior to the major die-off of Diadema antillarum throughout the Caribbean (Lessios et al., 1984), this zone also contained the highest density (4.3 individuals/m-2) of D. antillarum (Lewis and Wainwright, 1985).

The low-relief spur and groove zone (10 m depth) of the inner forereef is a broad shallow slope best described as a hardground that is regularly interrupted by shallow sand channels. The low (about 1 m relief) coral ridges are formed primarily of Montastraea annularis, M. cavernosa and Diploria strigosa (Rützler and Macintyre, 1982b). Much of this area is densely covered with gorgonians dominated by Pseudopterogorgia species (Lasker and Coffroth, 1983). Briareum asbestinum and Muriceopsis flavida are the two most abundant octocorals on the CARICOMP transects. Demospongea and scattered coral heads, primarily M. annularis, also dominate this area (Lewis and Wainwright, 1985). Littler et al. (1987) found that macroalgal abundance in 1980 was extremely low (about 2.5%) in this zone and composed of epilithic forms on the reef rock and scattered rubble. Spatial heterogeneity (3D structure) is high (mean rugosity on CARICOMP Coral Reef Site I 1.71) and herbivorous fish are abundant, reaching their maximum density in this zone (Lewis and Wainwright, 1985).

The transition from the inner forereef to the outer forereef zones occurs at the seaward edge of the low-relief spur and groove where the gradual slope of the inner forereef abruptly changes to a steeply sloping (25°) zone of hard corals and sponges. Columnar colonies of Montastraea annularis at the top of the slope give way to large platy colonies toward the base that are accompanied by Porites astreoides, Siderastrea siderea, and Agaricia spp. (Rützler and Macintyre, 1982b). Until the late 1970s, this zone was dominated by Acropora cervicornis (Rützler and Macintyre, 1982b; Burke, 1993). By 1984, most of the A. cervicornis had died off, apparently from white-band disease (Peters, 1993; Aronson and Precht, 1997; Macintyre and Aronson, 1997). At some Belizean reef sites, A. cervicornis has been replaced in the interim by Agaricia spp., possibly due to high levels of herbivory by the echinoid Echinometra viridis (Aronson and Precht, 1997). In the shallow backreef zone at Carrie Bow Cay, the disappearance of A. cervicornis has been accompanied by a proliferation of Porites porites, an opportunistic species that is starting to overgrow some of the slower growing massive corals (Macintyre and Aronson, 1997). However, in the deeper forereef areas at Carrie Bow Cay, most of the A. cervicornis has been replaced primarily by macroalgae (principally Lobophora variegata, Dictyota spp., and Halimeda spp.). The faunal transition from dominance of hard corals to macroalgae accompanying the demise of A. cervicornis is reflected in the relatively low percent coral cover and high algal (macrophyte) cover presently observed at the CARICOMP sites (Table 1). This is particularly evident at Site I, where macroalgal cover has increased from approximately 3.4% in 1980 (Littler et al., 1987) to about 45% at present, while coral cover has dropped from an estimated 30-35% in the late 1970s (Rützler and Macintyre, 1982b) to around 12% today. A similar decline in coral cover from the 1970s to the 1990s from natural disturbances has been noted in the Gulf of San Blas, Panama (Ogden and Ogden, 1993).

Table 1. Percent cover of benthic organisms at CARICOMP Coral Reef Sites I and II, Carrie Bow Cay, Belize; August 1995.
% Cover
Site Hermatypic Corals Gorgonacea Algae Sponges Abiotic
I 12.3 2.3 65.8 1.4 18.2
II 20.9 1.0 52.7 3.3 22.2
Mean of five 10-m transects per site surveyed as part of semi-annual CARICOMP monitoring (see CARICOMP, 1994, for description of methods). Algae include macrophytes (fleshy and calcareous), turf algae, and cyanophytes.

The inner forereef slope ends at about 24 m in a broad, flat sand trough consisting of poorly-sorted Halimeda-rich sand. From there, the slope rises sharply (45°) to the outer ridge at about 12-14 m below the surface. The narrow outer ridge (about 20 m wide at this point on the Rützler-Macintyre transect) parallels the intertidal reef crest and delineates the continental shelf (Rützler and Macintyre, 1982b). Prior to 1984, this area was also dominated by Acropora cervicornis, but it now consists mainly of A. cervicornis rubble covered by Lobophora spp. (Macintyre and Aronson, 1997), with local accumulations of Montastraea annularis, Agaricia agaricites, A. tenuifolia, and Madracis mirabilis (Rützler and Macintyre, 1982b). The outer forereef slope drops steeply (50-70°) and is dominated in the upper reaches by platy Montastraea cavernosa, M. annularis, Gorgonacea, Agaricia fragilis, Leptoseris cucullata, and Demospongea (Rützler and Macintyre, 1982b).

There was widespread coral bleaching in Belize in 1995, an area from which there had been no previous reports of bleaching (Glynn, 1993; Williams and Bunkley-Williams, 1990). We observed some bleaching of hard corals, especially Montastraea annularis, on the CARICOMP transects during the August 1995 survey. By October 1995, up to 90% of M. annularis colonies were bleached, particularly on the shallow (<10 m depth) forereef off South Water Cay. Extensive bleaching was also reported for Agaricia agaricites, A. tenuifolia, Madracis spp., and Porites porites. Our surveys of the forereef at Carrie Bow Cay in December 1995 showed that bleaching was still widespread. The incidence of bleaching varied among species and appeared, in some species, to be related to water depth. Heavily affected species included Montastraea annularis, Agaricia lamarcki, A. grahamae, Siderastrea siderea, and Diploria labyrinthiformis. Bleaching at Carrie Bow Cay was preceded in June and July 1995 by unusually calm water conditions and high light transmittance (Fig. 6), as well as high sea surface temperatures (SSTs) in the Gulf of Mexico and western Caribbean (Strong et al., in press). As determined by satellite imagery (Gleeson and Strong, 1995), SSTs reached a 12-year high of 29.9°C at Glovers Reef, Belize, in September 1995. Surface water temperatures over the drop-off at Carrie Bow Cay were the highest recorded since CARICOMP monitoring began in January 1993, reaching a peak of 30.4°C during the first two weeks of June 1995 (Fig. 4). Bottom water temperatures at CARICOMP Coral Reef Site I (13 m water depth) averaged 29.8°C (±0.16) during the last week of August (Fig. 5). By December 1995, temperatures had fallen to a monthly average of 27.7°C. The rapid decline in water temperatures was due partly to the passages of Hurricanes Opal and Roxanne across the Yucatan Peninsula in late September and early October 1995, respectively (Figs. 4 and 5). Follow-up surveys of coral bleaching in August 1996 and January 1997 indicated that most of the coral had recovered.

Seagrasses                                                                                               

More than 90% of the lagoon leeward of Carrie Bow Cay is shallow (<6.5 m) soft bottom dominated by Thalassia testudinum; the remainder consists of rubble, reef patches, and large sponges (Rützler and Macintyre, 1982b). As previously noted, overfishing of queen conch, Strombus gigas, has left the lagoon largely devoid of this herbivore. The seagrass beds are crisscrossed by a series of uncolonized sand holes from seismic surveys for petroleum exploration during the 1960s. Recolonization of disturbed areas by Thalassia is slow, often taking years (Zieman and Zieman, 1989; Williams, 1988), possibly due to nutrient accumulation in the sediments (Williams, 1990).

Two permanent monitoring plots were established in the seagrass beds approximately 100 and 150 m east of the CARICOMP mangrove plot at Twin Cays (Fig. 8). Site I was established in what was judged to be a "high productivity" area (100 m offshore), while Site II was established in a "representative" or "average" area (150 m offshore). However, after semi-annual sampling over three years (1993-1995), no difference has been observed between the sites and data on biomass and productivity are pooled in subsequent analyses.

Fig. 8. Twin Cays, Belize, showing the location of CARICOMP Seagrass
Sites I and II (open squares off eastern shoreline) and the three contiguous
mangrove plots along the eastern shoreline.

The seagrass beds adjacent to Twin Cays lie on a shallow shelf that increases gradually in depth from the shoreline to approximately 1.2-1.5 m at the CARICOMP sites, before dropping more steeply to the lagoon bottom (approximately 7 m) about 300 m offshore. Bottom sediments (to at least 1 m) consist primarily of Halimeda sand mixed with fine clay. Thalassia testudinum grows profusely in this area (short shoot density = 960 ± 250 m-2), interspersed with sparse stands of Syringodium filiforme and Halimeda spp. As noted by Young and Young (1982), Thalassia blades in the lagoon are heavily encrusted with coralline algae and are conspicuously devoid of epizoans, except Foraminifera.

Seagrass biomass shows slight seasonal variations (Table 2), somewhat higher in August (4116 681 g m-2 dry wt) than in December (3795 741 g m-2 dry wt). Seagrass biomass estimates for this area are among the highest within the network of sites and among the highest reported in the Caribbean (Zieman and Zieman, 1989). The biomass of these beds appears to be related to greater sediment depth (Tomasko and Lapointe, 1991) or nutrient enrichment from the adjacent mangrove community where turtle grass is more dense and grows three times faster than Thalassia in the nearby open lagoon (Rützler and Feller, 1988). Areal productivity also varies seasonally, with higher productivity during August compared to December (Table 3). The turnover rate for Thalassia leaves does not show significant seasonal variation, averaging about 2.4% per plant per day. Blade turnover rates for this community are comparable to others in the Caribbean and elsewhere in Belize, where they have been observed to remain at near-constant rates (Tomasko and Lapointe, 1991).

Table 2. Biomass (g m-2 dry weight) of seagrass fractions at CARICOMP Seagrass Sites I and II, Twin Cays, Belize; monthly mean and standard deviation of pooled Site I and Site II samples (12/93-07/97).
  Thalassia Other Grass   Calcareous Algae  
Month Above Below Green Non-Green Fleshy Algae Above Below Total
Aug, n = 28
(std dev)
883
(254)
3251
(600)
5
(9)
19
(26)
1.3
(8)
49
(92)
14
(16)
4116*
(681)
Dec, n = 28
(std dev)
731
(211)
2914
(625)
5
(7)
31
(44)
9
(25)
53
(63)
60
(118)
3795*
(741)
*significantly different, p < 0.05; two-tailed t-test with 54 df.
 
Table 3. Thalassia testudinum: areal productivity and turnover (% of the plant present that is replaced each day); mean and standard deviation of pooled Site I and Site II by month (08/93-07/97).
Month Areal Productivity
(g dry weight m-2 d-1)
Turnover
(% per day)
Aug, n = 54
(std dev)
2.86*
(0.96)
2.44**
(0.39)
Dec, n = 42
(std dev)
2.30*
(0.88)
2.36**
(0.59)
*significantly different, p < 0.05; two-tailed t-test with 94 df.; **no significant difference.

Mangrove Forests                                                                                    

Mangroves of the central barrier reef complex occur on both isolated islands and elliptical groups of islands (ranges) several hundred meters leeward of the reef crest. Most of the mangrove communities in this region, including the Twin Cays complex, are built on a limestone platform (fossil patch reef) that was topographically high during the Late Pleistocene (Rützler and Feller, 1988, 1996; Littler et al., 1995; Woodroffe, 1995). Beyond the influence of the terrestrially-derived sediments that underlie mangrove forests of the mainland, the mangrove forests of the central barrier reef are underlain by mangrove-derived peat which accumulated to thicknesses of as much as 10 m during the rising seas of the Holocene transgression (Woodroffe, 1995; Macintyre et al., 1995). Like many mangrove systems of the Caribbean, they are dominated by red mangroves, Rhizophora mangle L., which form a fringe around the periphery of the islands. The interiors have extensive stands of black mangroves, Avicennia germinans (L.) Stearn, which are exposed on the shore in some areas such as Tobacco Range as a result of shoreline erosion (Woodroffe, 1995). There are scattered individuals of white mangroves, Laguncularia racemosa (L.) Gaertn.f., but these do not form the thickets common on other West Indian islands where there has been considerable disturbance to the mangrove vegetation (Woodroffe, 1995).

Twin Cays is a 91.5-ha range of two large and four small intertidal islands (Fig. 8). The two large islands, East Island and West Island, are separated by a 0.5-2.0 m deep, S-shaped navigable channel. Except for sand spits at South Point and just north of West Bay (which flooded prior to being filled for development in the 1980s), Twin Cays lies within the intertidal zone and is overwashed by spring tides. In 1980, Twin Cays became one of the primary study sites for long-term ecological studies by Smithsonian Institution scientists and their collaborators at Carrie Bow Cay.

Although its topographic shoreline is within the intertidal zone, Twin Cays’ vegetation is physiognomically varied and interrupted by tidal creeks, open flats and shallow interior ponds (Fig. 8). Red mangrove forms a dense, monospecific fringe around the island peripheries and along creek banks, and its height generally decreases with distance from the shoreline. Avicennia germinans and L. racemosa occur primarily in the interior of Twin Cays, typically intermixed in a transition zone with R. mangle just landward of the fringe forest. Avicennia germinans also forms monospecific stands in some areas in the interior. However, L. racemosa does not form extensive stands. Ponded areas in the interior are dominated by extensive stands of dwarf R. mangle. (McKee, 1993a). The zonation pattern of mangroves at Twin Cays varies not only with spatial position relative to the shoreline but also with hydro-edaphic conditions (McKee, 1993b). Fertilization studies have shown that phosphorus deficiency is a primary factor limiting the growth of dwarf trees in the interior of Twin Cays (Feller, 1995).

Based on surveys and transects across Twin Cays, Woodroffe (1995) compiled a vegetation map of the occurrence of mangrove species and the structure of mangrove vegetation that includes seven types: R. mangle scrub (<2 m); R. mangle thicket (2-4 m); R. mangle woodland (>4 m); A. germinans woodland; A. germinans open woodland with R. mangle scrub; and dead R. mangle. The mangrove community of Twin Cays is similar to the basin mangrove in Rookery Bay, Florida, and a fringe mangrove in Ceiba, Puerto Rico (Woodroffe, 1995). Woodroffe also noted that the ranges are undergoing disintegration, and there is abundant evidence (extensive unvegetated flats in the interiors and dead mangrove stumps throughout the range) that the pattern of mangrove distribution is changing. Woodroffe suggested that shoreline erosion and the anomalous mangrove distribution patterns on Twin Cays, as well as in other nearby mangrove ranges, may be the result of abiotic stresses in the environment, including hurricane damage, reduced tidal flushing, salinity, and soil chemistry, as well as clearing of mangroves by humans.

In December 1993, five 10 m 10 m (0.1 ha) plots were established 5 m apart along a 70-m transect perpendicular to the shoreline on the eastern (windward) shore of Twin Cays. Plots 1 and 2 (closest to the shoreline), consisted almost entirely of R. mangle, Plots 3 and 4 consisted of A. germinans and a few L. racemosa, while Plot 5 (interior) consisted primarily of dwarf R. mangle. To establish seasonal patterns of productivity, leaf litter was collected monthly in 10 mesh-lined traps deployed within each plot (CARICOMP, 1994). In 1994, productivity showed peaks in spring and fall (Fig. 9). In 1995, the CARICOMP protocol was revised to three 10 m x 10 m plots set up parallel to the shoreline in the red mangrove fringe, with semi-annual sampling of litter fall.

Leaf- litter biomass
Fig. 9. Mean biomass (g m-2 mo-1) of leaf litter from mangrove plots at
Twin Cays, Belize. Plot 1 = shoreline; Plot 5 = interior.

Historically, fishermen used Twin Cays as a base for seasonal fishing camps, for bait collecting, and for storm shelter. Since the early 1980s, local developers have tried repeatedly to establish resorts and vacation homes on Twin Cays. Several areas have been clear-cut and later abandoned. Although Belize’s regulations regarding mangrove protection should protect Twin Cays from future threats of development, attempts by the Forestry Department to enforce these regulations have been only moderately successful. Indeed, in 1995, several hectares along the shoreline immediately north of the CARICOMP site were cleared of mangroves and filled with sediment dredged from the sub-tidal area adjacent to the development site. The developer is proposing to establish a marina and resort on the site. Other threats are also increasing, particularly from the rapidly expanding tourism industry. Substantial quantities of trash have been dumped into several small bays by operators of nearby resorts and sailing yachts that use Twin Cays as an anchorage. Beginning in the late 1980s, boat traffic from tourism began to increase dramatically in the Main Channel. Wakes created from boats driven at high speed have broken mangrove roots and dislodged sessile organisms in the bostrychietum communities. Unfortunately, rapidly rising rates of inadequately managed tourism will likely exert increasing pressure on this and other mangrove communities in Belize.

Acknowledgments                                                                                    

We sincerely thank Dr. Klaus Rützler, Director, Caribbean Coral Reef Ecosystem Program for support of the CARICOMP program at Carrie Bow Cay. We also thank Dr. Ian Macintyre for reviewing the manuscript, Molly Ryan for drafting the artwork, Mike Carpenter for logistical support and assistance in a variety of ways, and the many station managers at Carrie Bow Cay who have helped with data collection and provided field assistance. This is Contribution Number 476 of the Smithsonian Institution’s Caribbean Coral Reef Ecosystem Program.

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