Environment and development
in coastal regions and in small islands
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Coastal region and small island papers 3

Calabash Caye, Turneffe Islands Atoll, Belize

Eden Garcia and Karie Holtermann

University College of Belize Marine Research Centre, Calabash Caye, Turneff Islands Atoll, Belize

Turneffe Islands Atoll is located 51 km off the coast of Belize. Calabash Caye, on the eastern rim of the atoll, is a CARICOMP site maintained by the University College of Belize Marine Research Centre. There are several subtidal mangrove cayes on the western edge of the atoll, sandy cayes on the eastern edge, and an extensive fringing reef surrounding the atoll. The chain of islands forming the atoll partially encloses two lagoons, the North Lagoon and the South Lagoon, which are dominated by seagrass beds. The mangroves, seagrass beds, and coral reefs enhance the diversity of the marine and terrestrial organisms at Calabash Caye. Over the past decade, only a few studies concerning marine resources have been carried out on the atoll.

Introduction                                                                                             

Belize is a small (46,620 km2) English-speaking country in Central America, bordered by Mexico to the north, Guatemala to the west and south, and the Caribbean to the east (Fig. 1; 17°09'00" to 17°38' 00"N, 87°44'30" to 87°57'30"W). It is a tropical country with two principal ecosystems, rain forest and barrier reef, linked by rivers, coastal waters, and seagrass and mangrove communities. Off the eastern coast of Belize is the second longest barrier reef in the world (more than 27 km long), which extends from the Mexican border to the Gulf of Honduras. Three of the four existing atolls in the western hemisphere are located east of the barrier reef: Glover’s Reef, Lighthouse Reef, and the Turneffe Islands Atoll (Fig. 1). The University College of Belize Marine Research Centre (UCBMRC), which was established in 1994, is located on a 2.025 ha plot on Calabash Caye, a sandy caye on the eastern rim of the Turneffe Islands Atoll. Vegetation consists of dense cocal, caye, and mangrove forests. The CARICOMP protocols have been implemented at the Calabash Caye Field Station.

Map of Belize
Fig. 1. Map of Belize, showing the barrier reef and the three atolls:
Glover's Reef, Lighthouse Reef, and the Turneffe Islands Atoll.

Geology of Belize                                                                                     

Belize is located on the Yucatan continental block, one of two Paleozoic blocks forming northern Central America. The splitting of the Yucatan Block from the Nicaragua-Honduras Block to the south occurred along an east-west fracture zone, what is now the Cayman Trench (Wantland and Pusey, 1971). Plate movement reconstruction shows that, by the Late Eocene, both blocks had rotated from their pre- drift location on Pangaea during the Early Triassic to their position. From Cretaceous to Pliocene time, the Yucatan Block subsided and tilted northward; the submerged part is now the Campeche Bank (Wantland and Pusey, 1971). A prominent series of five parallel submarine ridges and scarps formed along the eastern edge of the Yucatan Block. These north-northeast oriented ridges characterize the present continental shelf margin of Belize. They have been modified by erosion, sedimentation, and reef growth during Pleistocene sea-level changes. Evidence suggests that the continental margin is still in the process of tectonic development (James and Ginsburg, 1979).

The five submarine ridges are the most distinctive features of the Belizean continental margin; they parallel the major rivers in northern Belize and are thought to be fault controlled (James and Ginsburg, 1979). The best-developed ridge forms the southern edge of the continental shelf and extends north, becoming the foundation for Glover’s Reef and Lighthouse Reef; the depth reaches 3,000 m on the east side. The most poorly defined ridge makes up the northern part of the barrier reef in Belize and Ambergris Caye. The bases of the ridges are beleived to be of continental origin as they are thought to consist of material very similar to that of the Maya Mountains. Drilling has shown that much of the relief on these ridges is due to coral growth (Dillon and Vedder, 1973). Three of the five ridges form the foundation for the barrier reef platform.

The Belize barrier reef consists of three biogeomorphic provinces: the northern province has 46 km of shallow-water reefs, the central has 91 km of shallow-water reefs, and the southern has 10 km of shallow-water reefs (Burke, 1993). The central province is the most well developed, with continuous wide reefs. The northern and southern reefs are discontinuous, except along Ambergris Caye. Researchers believe the limited reef development in the south is due to rapid sea-level rise, high-energy waves, and high turbidity. The development of the northern reef was hindered by heavy terrestrial run-off, high turbidity, variable salinity, and prevalence of winds (Burke, 1993). The central province was protected from wave energy by the outer atolls and had a slower sea-level rise (Burke, 1993).

The shelf in Belize extends east from the mainland to a seaward boundary at a depth of 180 m just beyond the barrier reef, where the slope dips 40° into abyssal waters. The shelf is 195 km long and lies between 15°50'N and 17°55'N. The shelf has two main hydrographic provinces: the barrier platform, which consists of the barrier reef, backreef lagoon, and caye complex, and the shelf lagoon, which at the latitude of Belize City consists of bathymetrically different northern and southern sections. The northern part of the shelf is bordered by a low karsted surface of flat-lying Cenozoic and Cretaceous carbonates with a few small rivers draining onto the shelf (James and Ginsburg, 1979). The inner shelf is generally flat, extending to a depth of only 8 m. The southern continental shelf is bordered by the Maya Mountains, through which many rivers flow onto the shelf. The depth of the shelf lagoon ranges from 25 m near Belize City to 200 m near Honduras (James and Ginsburg, 1979).

Geography of the Turneffe Islands Atoll and Calabash Caye                      

The Turneffe Islands Atoll is roughly lens-shaped, with a maximum length of 50 km and a width of 16 km at its widest point (Fig. 2). The atoll developed along one of the fault-controlled submarine ridges that occur off the coast of Belize along with the Cinchorro Bank Atoll in Mexico. Turneffe is separated from the barrier reef by a 10-16 km wide, 275-300 m deep channel (Gibson, 1990). It is bounded by a drying reef to the north and by Bokel Caye to the south. There is a well-defined narrow reef on the windward, eastern side. The reef crest is narrow and fringes the outer edge of a reef-flat less than 400 m wide. The reef is highly segmented, with about 23 gaps, or channels, most of which are less than 50 m wide and 6 m deep. Small sandy cayes are located between the channels on the inner edge of the eastern side of the reef flat; one of these is Calabash Caye.

Turneffe Islands Atoll
Fig. 2. Map of the Turneffe Islands Atoll
showing Calabash Caye.

Calabash Caye, home to the UCB Marine Research Centre, is a sandy caye located on the eastern rim of the atoll, aligned NNE-SSW, parallel to the reef (Fig. 3). In 1962, Stoddart reported Calabash Caye (which is now the eastern portion of the island) to be 43 m long and 32-50 m wide; the caye has grown considerably since then.

Calabash Caye
Fig. 3. Map of Calabash Caye, showing the location
of the mangrove, seagrass and coral reef sites.

The caye has a dense cover of cocal, caye, mangrove forests, and other low cover vegetation. A 500-m long interior lagoon, referred to as the "back lagoon," is located in the middle of the caye and has been isolated for a long time. The back lagoon has an average salinity of 36‰ and there is little flushing. The back lagoon supports a unique ecosystem of sponges, upside-down jellyfish (Cassiopea fondosa), and tunicates (Catterall, 1996). A sandy beach and an extensive fringing reef with diverse species of fish, coral, algae, and other marine invertebrates extend along the entire east coast of Calabash Caye. Turtle grass (Thalassia testidinum), which is the dominant species, and manatee grass (Syringodium filiforme) are found in the shallow waters surrounding the caye.

Calabash Caye has been inhabited since the beginning of the 20th century, based first on the sponge industry and later the coconut industry, but was completely devastated by Hurricane Hattie in 1962. Currently, there is a camp south of the Marine Research Centre, which houses persons engaged in the coconut business.

Climate and Oceanographic Conditions                                                     

Few climatic and oceanographic measurements have been recorded in the Turneffe Islands Atoll; data are available primarily from coastal stations and pertain to the mainland. The rainy season in Belize occurs from June through December, with variable patterns of rainfall in the north and north-central areas. There is a wide range of rainfall, from less than 2,000 mm per year in the north, more than 4,500 mm per year in the south, and over 10,000 mm per year in the Maya Mountains (Perkins, 1995). Turneffe lies between the latitudes of the Belize and Stann Creek districts, but rainfall is significantly lower than on the mainland (Stoddart, 1962). Over the shelf, rainfall is less than at the corresponding mainland sites (Perkins, 1995). The only rainfall figures for Turneffe were recorded during the summer months of 1937-1939 and show considerable variation: 558 mm in 1937, 379 mm in 1938, and 121 mm in 1939 (Stoddart, 1962). Occasional heavy rains are associated with winter storms (northers).

Along the coast of Belize, winds are steady from the northeast, east, and southeast at 3-8 m s-1 (Stoddart, 1962), with easterly winds in the atoll area for most of the year. From November through March, cold fronts moving through from southern North America bring northers, with winds gusting to 31 m s-1. The Turneffe Islands Atoll is also affected by hurricanes moving through the western Caribbean, with strong easterly winds and high meteorological tides. The normal tidal range on the coast of Belize is 0.5 m but may reach 0.8 m during northers.

Air temperature data are sparse for Turneffe and surrounding environment. During northers, temperatures sharply fall 5-7°C and remain cooler than normal for several days. Surface water temperatures were recorded in the shelf lagoon near the barrier reef in July and August 1991 (29°C and 30°C), and the bottom temperature was 0.5-1.0° colder. Average oceanic surface temperature off the coast of Belize ranges from 25.5°C in February to 28.5°C in August. No cold-water upwellings, which may alter seawater temperature, are known to occur in Belizean waters, although there are some farther north, off the Yucatan Peninsula (Stoddart, 1962). Normal seawater salinities of 35-37‰ have been recorded offshore, approaching the barrier platform (Perkins, 1995).

Along the coast of Belize, wind-generated currents are more influential than tidal currents. Inside the barrier reef, the prevailing currents are southerly. Seaward of the Turneffe Islands Atoll, winds are northerly (1 m s-1), but part of the current flows westward around the northern part of the atoll, creating a southerly drift along the leeward side. Inside the atoll lagoons, water currents flow west, creating a westward drift. Stoddart (1962) observed a large N/S-oriented drift on the eastern side of Calabash Caye, between the sandy ridge shoreline and the fringing reef at the front.

CARICOMP climate data (air temperature, minimum and maximum temperatures, and cumulative rainfall) are now being recorded daily at the Marine Research Centre Field Station; oceanographic data are collected weekly at the secondary seagrass site and at the mangrove site east of the inner lagoon (Fig. 3).

Vegetation and Soil Type Classification at Calabash Caye                          

Three major land systems (defined as an area, or group of areas, throughout which there is a recurring pattern of topography, soils, and vegetation) have been identified in Turneffe (Minty et al., 1995). The two most dominant types at Calabash Caye are the strand-plain land system and the saline-swamp land system. The strand-plain system consists of wave- and storm-built ridges of sand, coral rubble, a mixture of both materials. The characteristic natural vegetation in this system on Calabash Caye is caye forest and cocal forest on an organic sand soil (Minty et al., 1995). The saline-swamp system occupies the low tidal flat, with characteristic mangrove vegetation on waterlogged saline peat soil (Minty et al., 1995), which may extend to a depth of 10 m (Stevely and Sweat, 1994).

Minty et al. (1995) described caye forests as the climax association of the higher cayes, where saline influence is minimal or absent and is restricted to sand ridge areas on organic sands. This type of forest consists of broadleaf trees generally 7-15 m tall: Bursera simaruba, Metopium brownei, Cardia sebestena, Ponteria campechiana, Bumelia retusa, and occasionally Coccoloba uvifera. The understory is dominated by Ficus sp. and young trees of the above species. The shrub layer is mostly Pithecellobium keyense. There is no information on the herb layer in this type of forest at Calabash Caye.

The cocal forest at Calabash Caye consists of Cocos nucifera, the canopy of which ranges from 7 to 15 m and is usually continuous. Little vegetation is present in the understory and shrub layer, and there is either bare ground or palm frond litter at the base. A herb layer of Stachytarpheta jamaicensis, Ambrosia hispida, Cakile lanceolata, and Wedelia is present under moderately discontinuous canopies (Minty et al., 1995).

Mangrove forests, predominantly Rhizophora mangle, dominate the western side and northern tip of Calabash Caye, and R. mangle forms monospecific stands on the periphery of the inner lagoon. Mangrove zonation patterns are observed in certain areas of the caye. The canopy consists of mixed stands of R. mangle, Avicennia germinans, and Laguncularia racemosa, with occasional Conocarpus erectus. Young trees of these species, plus Batis maritima, consititute the shrub and herb layers of the cocal forest. The ground layer consists of leaf litter and twigs.

Mangrove Ecosystems                                                                             

Mangroves border most of the coastline of Belize and extend upstream from numerous river mouths; they fringe or cover most lagoon cayes. Red mangrove stilt roots line all channels, creeks, and lagoons; below tide level, they support spectacularly colored clusters of algae, sponges, tunicates, anemones, and associated species. They also provide hiding or feeding places for fish and shellfish (Rützler and Feller, 1987/1988). According to Stevely and Sweat (1994), the mangrove ecosystems in the Turneffe Islands Atoll have kept pace with seamount subsidence and sea-level rise.

The red mangrove (R. mangle) typically forms monospecific stands along shorelines, creeks, and lagoon banks where tidal inundation is frequent. At more interior, less frequently inundated sites, the black mangrove (A. germinans) dominates. The white mangrove (L. racemosa) forms extensive stands at high elevations where flooding and salinity stresses are minimal, but also occurs occasionally at lower elevations (McKee, 1995). On cayes with slightly higher elevations, additional woody and herbaceous (soft-stemmed) halophytes are associated with the mangove ecosystem, such as buttonwood (C. erectus), salt wort (B. maritima), and sea pulsane (Sesuvium sp.) (Rützler and Feller, 1987/1988).

A perpendicular transect from the eastern shoreline of the inner lagoon traverses three zones: a shore zone dominated by R. mangle, a transition zone with a mixed canopy of all three species, and a landward zone dominated by A. germinans. A few L. racemosa and C. erectus can also be found in the landward zone that is dominated by A. germinans (McKee, 1995).

The CARICOMP mangrove site at Calabash Caye was inventoried by August 1996. It consists of three 10 ´ 10 m study plots located on the eastern shoreline of the inner lagoon (17°16'52.6"N, 87°48' 50.9"W; Fig. 3). All three plots contain a few A. germinans and L. racemosa, but R. mangle is dominant. Total tree height for R. mangle ranges from 1.2 m to 7.2 m; A. germinans 2.4-10.7 m, and L. racemosa 4.5-9.4 m. Interstitial soil-water salinity tests are carried out on each study plot on a monthly basis; average salinity in the area is 37‰. Seedlings, leaf litter, and algal mats cover the ground surface of the study plots. Leaf litter has been collected but the data are insufficient to determine growth or biomass production patterns in the area.

Seagrass Ecosystems                                                                               

The seagrass ecosystem is frequently connected to mangrove forests on the landward side and to the coral reefs on the seaward side. These marine grasses bind sediments and provide a stable substrate for benthic organisms. Seagrass beds also provide food and shelter to commercially important species. Shrimp, lobster (Panulirus argus), and conch (Strombus gigas) depend on the seagrass areas as a foraging area for food (Hain, 1987/1988). Seagrass beds within the Turneffe Islands Atoll are intact and are not greatly disturbed except by dolphins (Tursiops turncatus) and crocodiles (Corcodylus acutus) that inhabit the lagoons, and by fishermen harvesting conch, lobster, and fish in the area.

The CARICOMP seagrass sites are located on the eastern side of Calabash Caye. The primary site is on the northeastern side (17°16'54.0"N, 87°48'31.2"W; Fig. 3), in a red mangrove-fringed channel between the northeastern edge of Calabash Caye and a small island called Little Calabash. The primary site (in water depth 0.5-1.0 m) is densely covered with Thalassia testudinum, with leaves up to 30 cm long, along with a few Syringodium filiforme and Halimeda algae. The bottom substrate consists of soft muddy sediments with dead Halimeda. The secondary site is located 40 m to the south, in water depth 2-3 m, and has a compact sandy bottom substrate mixed with some dead Halimeda and shells. The secondary site contains the same species as the primary site but with less dense Thalassia cover.

Coral Reef Ecosystem                                                                              

There are no data as yet from the newly set up CARICOMP reef site at Calabash Reef. Previous researchers established two transects in the area, but these are no longer in use. The previous findings will be compared with CARICOMP surveys now being carried out.

Stoddart (1962) established a reef transect from Little Calabash Island into deeper water, which showed reef zonation east of Calabash Caye after Hurricane Hattie (Table 1). A more recent study of coral distribution was conducted by the Planetary Coral Reef Foundation in 1992, using a chain transect and video. The study site was on Calabash Reef 2 km south of the channel separating Calabash Caye from Blackbird Caye, an area representative of the most well-developed region on the windward reef. Their 900 m transects extended from behind the reef crest seaward to a depth of 45 m. The results of these two surveys found Calabash Reefs having a clear zonation pattern of a windward West Indian coral reef with a backreef, reef crest, spur-and-groove zone, mixed zone, forereef terrace, escarpment, forereef, and deep forereef. The coral coverage varied, with the highest coverage in the spur-and-groove zone. Approximately 29 sceractinian species were found, not including separation of Montastraea annularis into subgroups; this was 69% of the hermatypic species in the area. Algal cover exhibited a pattern of increasing abundance with increasing depth. The cover was 30% in the spur-and-groove zone and increased with depth. Sea urchin abundance decreased with depth, with a mean of 83 urchins per search period in the spur-and-groove zone to a low of 3 urchins on the forereef slope. Urchins larger than 10 cm on the reef crest were Diadema antillarum; the dominant species in the spur-and-groove zone was Echinometra viridis (Dustan et al., 1992).

Table 1. Reef transect seaward from Little Calabash Island just after Hurricane Hattie.
Zone Description from Stoddart (1962)
Sand and rubble platform adjacent to the caye There is about 12 inches of water on this platform and it is about 200 feet wide. It is carpeted with Thalassia and other green algae, including Halimedia. Near the outer edge of the platform Porites rubble becomes prominent.
Reef flat A sandy area under 18 inches of water 20-30 feet wide. Small corals (Favia fragusm and Porites divaricata) and sea-urchins (Diadema) are scattered in the turtle grass.
Inner reef zone A sandy area under 2 feet of water, with scattered colonies of Montastraea annularis, Porites asteroides, and Dendrogyra cylindrus about a foot high, with small colonies of Siderastrea siderea. Also, sea fans and sea whips are found here.
Cervicornis zone The ground is covered by Acropora cervicornis, much of which is dead. The Montastraea and Porites colonies are larger.
Annularis zone Large blocks of Montastraea annularis, Porites asteroides, Dendrogyra cylindrus, and Siderastrea siderea. Also present are encrusting and foliaceous Agaricia with Acropora cervicornis and Acropora palmata. This zone is approximately 8 feet deep and 15 yards wide.
Reef crest (Agaricia) This is made up of massive blocks of mostly dead corals covered with Agaricia agaricites. Channels between the blocks are 10 feet deep. This area is only a few yards wide.
Outer slope A platform from 10 to 15 feet deep and deepening toward the sea. Colonies of Montastraea, Porites, and Siderastra are 2 feet in average. Low colonies of Acropora palmata are also found in this area.

The results of both these previous studies show that Calabash Reef is a thriving windward West Indian coral reef community with signs of hurricane damage on the populations of some coral species. In the early 1990s, coral bleaching was found throughout the Caribbean but Belize was seemingly spared. However, from August to October of 1995, NOAA satellites detected elevated sea temperatures spanning much of the Gulf of Mexico and the western Caribbean basin from Belize to Jamaica, Honduras, and Venezuela. It was at this time that coral bleaching was reported for the first time in Belize (Stout, 1995).

Conclusions                                                                                             

Tourism is now Belize’s largest industry. The Tourist Board estimates Bz$150 million (US$7.5 million) were generated by tourism in 1994 (Gibson et al., 1995). Tourism has traditionally been based on visits to the cayes, with 78% of hotels located in the coastal zone. Additionally, 1995 fishing exports totalled an estimated Bz$20 million (Gibson et al., 1995). Two resorts focusing on dive tourism already exist on the Turneffe Islands Atoll, and fishing pressures are not well documented but seem to have increased in recent years; the atoll remains an unprotected area. The Marine Research Centre is focused on collecting baseline data to contribute to the understanding of these pressures and marine processes in the area as well as throughout the Caribbean. The MRC joined CARICOMP in November 1996.

Acknowledgements                                                                                  

We would like to acknowledge support, assistance, and encouragement from the University College of Belize; Mr. Hugo Matus, GIS specialist at the Coastal Zone Management Unit in Belize; Mr. K. Mustafa Toure, Director of the Marine Research Centre; and Dr. Ilka C. Feller, Smithsonian Research Institute.

References                                                                                               

Burke, R. B. 1993. How Have Holocene Sea Level Rise and Antecedent Topography Influenced Belize Barrier Reef Development? Proceedings of the Colloquium on Global Aspects of Coral Reef Health, Hazards, and History. Rosenstiel School of Marine and Atmospheric Science, University of Miami (with the cooperation of the International Society for Reef Studies), Miami FL, USA.

Catterall, D. P. 1996. A Strategic Environmental Assessment of Tourism Development on Turneffe Atoll, Belize. DIC dissertation, Imperial College of Science, Technology and Medicine, London, UK.

Chamberlain, T. L. 1995. Coastal and Reef Dynamics of Belize: Field Manual. College of Natural Resources, Colorado State University, Fort Collins CO, USA, 16 pp.

Dillon, W. P., J. G. Vedder. 1973. Structure and development of the continental margin of British Honduras. Bulletin of the Geological Society (London), 84:2713-2731.

Dustan, P., A. King, E. Treml, C. Handte. 1992. Hard Coral Species Population Survey, Turneffe Atoll, Belize. Planetary Coral Reef Foundation, 12 p.

Gibson, J. 1990. Status Report on Turneffe Islands. Coastal Zone Management Project, Belize, 7 pp.

Gibson, J., M. McField, S. Well. 1995. State of the Coastal Zone Report, Belize, 254 pp.

Hain, J. S. 1987/1988. Belize. Oceanus, 30(4):74-76.

James, N. P., R. N. Ginsburg. 1979. The Seaward Margin of Belize Barrier and Atoll Reefs: Morphology, Sedimentology, Organism Distribution and Late Quaternary History. Publication 3, International Association of Sedimentologists, 191 pp.

McKee, K. L. 1995. Seedling recruitment patterns in a Belize mangrove forest: Effects of establishment ability and physico-chemical factors. Oecologia, 101:448-460.

Minty, C., M. Murray, S. Zisman. 1995. Turneffe Terrestrial Resource Reconnaissance Report to Coral Caye Conservation Ltd. Marine Research Centre, University College of Belize, Technical Report Series No. 2, 35 pp.

Ogden, J. C. 1987/88. Cooperative coastal ecology at Caribbean marine laboratories. Oceanus, 30(4): 10-14.

Platt, S. G. 1995. Preliminary Assessment of the Status of the American crocodile (Crocodylus acutus) in the Turneffe Atoll, Belize. Marine Research Centre, University College of Belize, Technical Report Series No. 3, 14 pp.

Perkins, J. S. 1995. Belize Barrier Reef Ecosystem: An Assessment of Resources. The New York Zoological Society, New York NY, USA, 215 pp.

Raines, P., A. Gill. 1994. The Belize Marine Research Centre, Turneffe Atoll, Belize. Coral Caye Conservation Ltd., London, UK, 24 pp.

Rützler, K., I. C. Feller. 1987/1988. Mangrove swamp communities. Oceanus, 30(4):17-21.

Rützler, K., I. C. Feller. 1996. Caribbean mangrove swamps. Scientific American, 274(3):94-99.

Stevely, J. M., D. E. Sweat. 1994. A Preliminary Evaluation of the Commercial Sponge Resources of Belize, with Reference to the Location of the Turneffe Islands Sponge Farm. Atoll Research Bulletin, 424:14.

Stoddart, D. R. 1962. Three Caribbean atolls: Turneffe Islands, Lighthouse Reef and Glover’s Reef, British Honduras. Atoll Research Bulletin, 87:31-49.

Stout, M. 1995. Coral Reef Bleaching Found in Belize for the First Time. US National Oceanic and Atmospheric Administration, Internet homepage.

Wantland, K. F., W. C. Pusey III. 1971. A Guidebook for the Field: The Southern Shelf of British Honduras. 21st Annual Meeting of the Gulf Coast Association of Geological Societies, 87 pp.

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