|Environment and development
in coastal regions and in small islands
Coastal region and small island papers 3
Bonaire, Netherlands Antilles
Kalli De Meyer
Bonaire Marine Park, PO Box 368, Bonaire, Netherlands Antilles
The island of Bonaire, one of the five islands that form the Netherlands Antilles, is one of the most southerly sites within the CARICOMP Program and is characterized by a hot, dry climate, hypersaline conditions, low population density, and lack of industrialization. Bonaires economy is firmly based on dive tourism, and the health of the islands marine resources is therefore of paramount importance to its economic wellbeing. An open lagoonal area on the windward shore, Lac Bay, was selected as a study site because it is the only area where seagrass and mangrove ecosystems are significantly present on Bonaire. Because reef development is relatively poor in waters shallower than 12 m along the windward shore, a coral reef monitoring site was selected on the leeward shore, in an area where disturbance was low and where reefs are characteristically dominated by massive colonies of Montastraea annularis.
Bonaire is situated in the southern Caribbean (12°10'N, 68°15'W) approximately 100 km north of Venezuela (Fig. 1). It is one of the five islands that form the Netherlands Antilles: Bonaire, Curaçao, and the Windward Islands of St. Maarten, Saba, and St. Eustatius (Statia). Bonaire is a crescent shaped island, oriented NW-SE, approximately 40 km long by 11 km at its widest point, with a land area of 288 km². The small uninhabited satellite island of Klein Bonaire ("small" Bonaire), which is located some 750 m off the western shore of Bonaire, is a RAMSAR site; it is privately owned and, to date, is entirely undeveloped.
In 1990, the resident population was 10,791 (CBS, 1991). With a population density of only 37 persons/km², Bonaire is the least densely populated island of the Netherlands Antilles. The main center of population, Kralendijk (locally called "Playa") is located in the center of the island; a second and older center, Rincon, is located in the north. There are only five other "villages," and the rest of the island is uninhabited and undeveloped. The coastal zone adjacent to Kralendijk has been extensively developed to provide tourist accommodation and, more recently, for private residential housing to accommodate an influx of wealthy retirees taking advantage of tax concessions. The economy of Bonaire is remarkably undiversified (Scura and vant Hof, 1993) consisting of an oil transshipment facility located on the northwestern shore, a solar salt extraction plant whose "condensers" (evaporation ponds) take up most of the southern end of the island, and tourism which is firmly based on the dive travel segment. Tourism is considered the mainstay of the island economy, with gross revenues from dive tourism in 1994 estimated at US$34 million (Scura and vant Hof, 1993).
Geography and Geology
Both Bonaire and Klein Bonaire are very flat. Little of the southern land area of Bonaire is more than 2 m above sea level, with higher elevations found only in the north and reaching a maximum of 238 m. The geology of Bonaire is complex, with the core of the island consisting of strongly folded and faulted rocks of volcanic origin, silica rich sediments and turbidites formed during the Cretaceous era some 120 million years BP (Beets, 1972a,1972b). Overlying this are later fossil reef and reef-generated calcareous deposits. It is these limestone formations which make up the coastline in the form of coral-rubble beaches (coral shingle and calcareous sand) or iron shore, except in the north where low limestone cliffs are found (Zonneveld et al., 1972). Klein Bonaire consists entirely of limestone formations (Buisonje, 1974). Substantial changes in sea level have left up to four stranded terraces above the present mean sea level, and one below, most of which can be distinguished by "solution notches" (undercutting caused by bioerosion and/or wave action) in the elevated seaward facing limestone cliffs.
Water retention of the soil is poor so that most rainfall quickly runs off into permanently or temporarily flooded "salinas" (hypersaline lakes separated from the sea by a coral rubble barrier), or directly into the sea (Roos, 1971). Together with low rainfall, this means that less than 10% of the land area of Bonaire is suitable for rain-fed agriculture. Natural vegetation is xerotrophic, comprising some 340 species of flora; Cactaceae, Acacia, Prosopis, Capparis, Haematoxylon, Lantana and Croton are characteristic species.
The climate is semi-arid tropical, with little seasonal variation and almost constant easterly trade winds. Average monthly air temperatures range from 26.6°C (February) to 28.4°C (October), and average rainfall is just 490.5 mm/year. Rainfall is unequally distributed geographically, with approximately four times as much rain falling in the northern portion of the island as in the south. The rainy season begins at the end of October and lasts until around the beginning of January; a second, shorter rainy season occurs in July. Commonly, no rainfall is recorded during the dry months. For 97.2% of the time, Bonaire experiences constant easterly trade winds (from 70°-110°) with an average velocity of 6.7 m s-1. Wind speeds are generally highest in June and July and lowest in November at the start of the rainy season. Bonaire lies outside the hurricane belt (the last recorded hurricane occurred in 1877), although tropical storms and hurricanes passing north of Bonaire can cause the wind to swing around (locally known as a "wind reversal"), which may cause extensive damage to the reefs and coastal zone of the leeward shore. Damaging wind reversals were recorded in 1976, 1981, 1985, 1990, and 1996.
The exposed eastern (windward) and protected western (leeward) shorelines are strikingly different. The windward shore is a very high wave energy environment characterized by rough water conditions and constant waves breaking against low limestone cliffs or onto the iron-shore coast. Water conditions on the leeward shore rarely exceed Beaufort Force 4, with only moderate swells affecting the northern and southern extremes of the island.
The maximum annual tidal range is approximately 1 m, with an average range of 0.30 m during a lunar cycle (Bak, 1977). Currents are unpredictable but slight, rarely exceeding 0.5 m s-1. The predominant current movement is toward the north along the leeward shore, but this pattern is complicated by local eddies and upwelling. Water conditions are stable, with a constant 34-36 ppt salinity and mean water temperatures varying from 26°C to 28°C.
Conservation and Human Impact
Bonaire has always taken the conservation of its resources, both terrestrial and marine, very seriously. Since 1969, nearly 20% of the total land area of Bonaire has been protected as a national park. Since 1979, the waters around Bonaire, from the high water mark to the 200 foot depth contour, have been designated a marine park and are protected by law (Verordening Marien Milieu, 1991). Activities within the marine park are restricted in order to ensure the continued sustainability of the coral reef, sea grass, and mangrove systems. Destructive practices such as anchoring, coral collection, and spearfishing are banned within the marine park.
Bonaires industrial base is confined to the transshipment of oil, the production and transportation of salt, and the refining of rice. Since both fishing and agriculture are essentially small-scale artisanal activities, major impacts on the marine ecosystem are direct and indirect results of tourism.
Most of the tourist activity associated with Bonaires reefs is confined to scuba diving, although snorkelling is increasing. Lac Bay is visited by tourists mainly for windsurfing, with some kayaking and snorkelling. Dive tourism began in 1963 and, by 1994, visitation had risen to approximately 57,000, of whom 25,000 were divers; tourist activity continues to rise by 7-10% annually. The direct impact of tourism includes occasional illegal anchoring, groundings, and dragging of mooring barrels through misuse, as well as direct contact damage by divers and snorkellers. Preliminary work carried out by vant Hof in 1991 suggests that the estimated carrying capacity of 250,000 dives per annum has already been exceeded at some of the more popular sites. This is evident through the loss of coral cover and changed species diversity when comparing dived and undived "reserve" sites (Scura and van't Hof, 1993). However, levels of physical damage to the reef are low, with only 2.7% of corals showing signs of damage, even at the most heavily utilized sites (Roberts and Hawkins, 1994), probably as a result of intensive educational efforts aimed at making divers aware of reef fragility. The more insidious and indirect results of increased tourism and coastal zone development are more difficult to quantify; they include increased nutrient loading through leaching of septic waste into the sea, particularly from hotels in the coastal zone, and also poor landscaping practices and increased sedimentation through land clearance and poor construction practices. The effects of these have resulted in increased algal cover (Roberts and Hawkins, 1994) and reduced visibility, especially after rain.
Both Bonaire and Klein Bonaire are surrounded by continuous, fringing coral reefs from the shoreline seaward to depths in excess of 70 m, covering an area of some 2,600 ha. Reef formation begins at the shoreline with a gradually shelving submarine terrace extending seaward by up to 250 m. Beyond this, at depths of 10-12 m, the terrace drops off and the reef slope commences. The dropoff zone exhibits maximum diversity of benthos and maximum coral cover (Bak, 1977). The reef slope drops down steeply at a 20-50° angle to depths of 25-55 m where it flattens out into a shelf. A second dropoff occurs beyond this (van Duyl, 1985).
Vant Hof (1982) recognized six distinct coral zones from the shoreline to 50 m. These are: a shore zone (0-1 m), characterized by Diploria clivosa; an elkhorn zone (1-4 m), dominated by Acropora palmata, Millepora sp., and crustose coralline algae; a staghorn zone (4-7 m), characterized by Acropora cervicornis interspersed with Madracis mirabilis, Colpophyllia natans, and Montastraea annularis and bounded by gorgonians; a dropoff zone (7-12 m), characterized by gorgonians, Montastrea annularis, Madracis mirabilis, and Eusmilia fastigiata; an upper reef slope (12-25 m), characterized by massive Montastrea annularis and Agaricia sp.; and finally a lower reef slope (25 m+), dominated by Agaricia sp. and some flattened forms of Montastraea annularis, Montastraea cavernosa, and Stephanocoenia michelinii but with few other abundant corals.
A conspicuous feature of the reef slope, especially along the northwestern shore, are coral tongues (spurs) separated by sediment channels (grooves) which may form as a result of the inherent instability of corals at the top of the reef slope causing the reef to collapse locally. True spur-and-groove coral formations occur in shallow water at only two sites on Bonaire on the northwestern shore (Boca Bartol and Playa Benge). Along the windward shore, coral development is virtually absent in water shallower than 12 m, where there is an abundance of crustose coralline algae and dense stands of Sargassum platycarpum that may extend to 40 m water depth. Bonaires reefs were mapped in 1983, and detailed maps were produced of the shallow coral communities to a depth of 10-12 m along the leeward shore and Klein Bonaire (van Duyl, 1985).
In addition to storm damage, significant natural impacts on Bonaires reefs during the 1970s and 1980s include an outbreak of white band disease (1980-1982), which caused the death of 90% of the standing stock of Acropora cervicornis, and the mass mortality of Diadema antillarum (1985) thought to be caused by a water-borne pathogen. Both events affected the entire Caribbean Basin to a greater or lesser extent. Bonaire supports some of the best remaining coral reefs in the Caribbean, with exceptionally high coral cover and exceptionally large populations of predatory fish (Roberts and Hawkins, 1994). The importance of maintaining healthy reefs both for their own intrinsic value and to ensure a sustainable economy for the island of Bonaire cannot be overstressed.
The CARICOMP coral reef monitoring site, known locally as Barcadera, is situated at 12°11'45"N, 68°18'00"W. This site is on the leeward shore of Bonaire, north of the town of Kralendijk, and consists of typical Montastraea annularis dominated fringing reefs at the 10-12m depth range (Fig. 2). The stony coral cover ranges from 20 to 40%. The site is located approximately 400 m from the nearest dive mooring and gives only limited shore access via an open rung ladder from a low limestone cliff. Land use at this area has changed dramatically over the past five years, particularly with the building of up-scale residential housing in a previously undisturbed natural area. Increased sedimentation through land clearance and landscaping is affecting the reefs, as is increased nutrient loading from sewage. As the coastal zone has now been developed, perhaps land use has stabilized for the foreseeable future.
Because the prescribed 10-12 m depth range corresponds to the dropoff zone, transects were laid out oblique to the coast so as to fall within the prescribed depth range in two adjacent groups of five transects. Within the groups, transects are approximately 2 m apart. Data collected at this site show a reef benthos dominated by massive stony corals (29.4 % mean cover) and algae (33.0% mean cover), with few soft corals (2.4% mean cover) or sponges present.
Mangroves and Seagrass Beds
A sheltered shallow inland bay, Lac Bay, occurs on the windward shore in southeastern Bonaire. It is the largest inland bay in the Netherlands Antilles, with a flooded area of approximately 7.5 km², and is internationally protected as a RAMSAR site. The maximum water depth within the bay is 4.5 m; tidal range is limited to approximately 0.3 m (van Moorsel and Meijer, 1993). The bay is bounded seaward by exposed fringing coral reefs that protect the bay from wave action. Waves break over the reef, flood the bay, and flow out though a deep water channel at the northernmost tip of the bay, creating a rip current. Lac Bay supports Bonaires only significant mangrove and seagrass ecosystems. The open water area of the bay is covered by seagrass beds dominated by Thalassia testudinum (varying in cover from 5 to 100%) together with Syringodium filiforme, and banks of the calcareous alga Halimeda sp. Along the landward edge of the bay is an actively growing fringe of Rhizophora mangle (with an average height of 8 m and an average dbh of 17 cm) that is systematically encroaching on the bay. The geomorphological nature of the mangrove stands is relatively rare, as classified by Thom (1984); there is no riverine input into the system, making them particularly vulnerable. Within the mangrove system are a number of permanently dry, ribbon-like cays as well as several important feeder channels that supply water to the back of the mangrove fringe. The mangroves are dominated by Rhizophora mangle along the landward and seaward edges and Avicennia germinans within the mangroves and around the drier ground associated with the islands. Within the Avicennia zone, average tree height is 5 m and average dbh is 8.1 cm. Conocarpus erectus is also not uncommon. One of the dominant features of the mangrove system is a significant die-back of Rhizophora mangle at its northwestern extreme, thought to be due to hypersaline conditions created by the landward damming of freshwater and choking of feeder channels on the seaward side resulting in water temperatures of 40°C and salinities of up to 100 ppt (van Moorsel and Meijer, 1993). The mangrove stands are particularly important as nesting and roosting areas for birds, and the seagrass beds form nursery grounds for some important reef fish as well as an important foraging area for Chelonia mydas. The bay is possibly also a breeding ground for Caretta caretta and Eretmochelys imbricata (van Moorsel and Meijer, 1993).
The mangrove monitoring site is situated on the windward coast in Lac Bay at 12°7'40"N, 68°13' 70"W (Fig. 3). This site was chosen as it is one of the very few places where the mangroves can be accessed relatively easily; the landward and shoreward stands of Rhizophora mangle are effectively impenetrable due to high tree density. The site is completely undisturbed, as is the adjacent mangrove area. The five mangrove plots are set up on the seaward side of Isla di Chico. From the westernmost tip of Isla di Chico, the first three plots (A, B, and C) are laid out at intervals of approximately 100 m and are adjacent to the cay at a distance of approximately 35 m. The final two plots (D and E) are located along a transect line approximately 500 m from the eastern end of the cay. Plots A, B, and C consist entirely of Rhizophora mangle (mean tree density 0.22 m-2; mean height 7.7 m). Plots D and E, in contrast, consist entirely of Avicennia germinans (mean tree density 0.50 m-2; mean height 4.6 m).
The seagrass monitoring site is situated on the windward coast in Lac Bay at 12°7'00"N, 68°13' 70"W. It is located in an area of dense seagrass cover, made up predominately of Thalassia testudinum (Fig. 3). Halimeda sp. is also present within the benthic community, and occasional Meoma ventricosa, Oreaster reticulatus, Strombus gigas, and Petochirus diogenes can be found. The area is also an important foraging area for Chelonia mydas as witnessed by the fact that the seagrass within the study site was twice leveled by their activity during productivity experiments.
I am sincerely indebted to everyone who has helped make CARICOMP Level 1 happen on Bonaire, not least to Leanne Miller, Gerard Alleng, and Kurt Bonair for their unflagging support with fieldwork, the Meteorological Office, Bonaire Marine Park staff, Kenny Buchan, Deborah Diggons, and finally Eric Newton for casting his incisive eye over this manuscript.
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