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

Coastal region and small island papers 3

Curaçao, Netherlands Antilles

Leendert P. J. J. Pors and Ivan A. Nagelkerken

Carmabi Foundation, Piscaderabaai z/n, PO Box 2090, Curaçao, Netherlands Antilles

Curaçao, an island of the Lesser Antilles has a surface area of 443 km2 and a population of approximately 150,000. The island is completely surrounded by fringing reefs. Seagrass and mangrove communities are found in several drowned coastal valleys; the CARICOMP seagrass and mangrove monitoring sites are situated in two of these inner bays: Spaanse Water and St. Jorisbaai, respectively. Spaanse Water still contains well developed seagrass, algal, and mangrove areas, despite the threat posed by a high level of coastal development. Mean Thalassia biomass ranges from 193 g m-2 to 504 g m-2. The mean areal productivity of Thalassia ranges from 0.14 g m-2 d-1 to 1.2 g m-2 d-1. The mangrove plots are located along the western shore of St. Jorisbaai. This bay is in pristine condition. The maximum tree density per 100 m2 plot is 51, with diameter at breast height > 2.5 cm. The maximum biomass is 12.7 kg m-2. The CARICOMP reef site is located just west of the entrance to Spaanse Water. The total number of hermatypic corals found at this site is 28, and maximum hard coral cover is 36%.


Curaçao, an island of the Lesser Antilles, lies between latitudes 12°2'80" to 12°23'30"N and longitudes 69°10'00" to 68°44'30"W. The island is 61 km long and 14 km wide at its widest point. The surface area measures 443 km2 (Fig. 1). The island is of volcanic origin and was formed 88 million years ago during the formational phase of the evolution of the Caribbean Plate (Sinton et al., 1993). During later phases, sedimentary rocks were deposited in certain areas (Beets, 1972). During the Quartenary, ice-age controlled coral reef development dominated. Limestone terraces resulted; they are especially well developed along the north coast (De Buisonjé, 1974). Reef development, in the form of fringing reefs around the island, continues. Several inner bays were formed as a result of a sea level rise after the last glacial period 16,000 years ago. It is in these drowned erosional valleys that seagrass beds and most of the mangrove stands are found nowadays.

Fig. 1. Location map of Curaçao, Netherlands Antilles, showing

Due to relatively low rainfall (average 570 mm/year) and high evaporation, the climate of the island is classified as semi-arid. Vegetation consists of drought-resistant cacti and thorn scrub.

The population of Curaçao is ~150,000 and is centered around the capital city, Willemstad, which surrounds a natural harbor, the Schottegat inner bay. Although the population has decreased somewhat over the last few years due to increased emigration, pressure on the natural environment has increased dramatically. Massive coastal development, which is linked to tourism, is an increasing threat to marine ecosystems; increased sewage discharge and sedimentation due to deforestation are considered the biggest problems.

The Curaçao Underwater Park was established in 1983, covering 20 km of south-coast reefs, starting at the eastern tip and extending to the west. Some of the best developed reefs are located within the park, which is managed by the Carmabi Foundation (Carmabi: Caribbean Research and Management of Biodiversity). The park includes Spaanse Water, one of the CARICOMP locations.

Two separate areas were selected for CARICOMP monitoring (Fig. 1). Mangrove monitoring takes place in St. Jorisbaai, an inner bay located on the northeastern coast, which has a maximum depth of 5 m and is more or less unspoiled because of the lack of nearby population centers and the absence of coastal development. Heavy grazing by goats of surrounding areas over the past 200 years, however, has resulted in increased erosion and consequently increased sedimentation. The water in the bay is now always somewhat turbid. Chemical data indicate higher levels of heavy metals within the bottom sediments, as compared to pristine areas (Table 1). The source of the heavy metals is unknown. Fringing mangroves are found along the shore, consisting mainly of Rhizophora mangle. Some Thalassia testudinum meadows occur in shallow water areas.

Table 1. Average chemical concentrations January-June 1988 (Djohani and Klok, 1988).
Spaanse Water
Spaanse Water
St. Jorisbaai
St. Jorisbaai

Silicate (µmol l-1) 3.24 2.59 3.67 2.32 1.16
Nitrite (µ/mol l-1) 0.02 0.02 0.03 0.03 0.02
Nitrate (µmol l-1) 0.91 0.56 4.65 1.64 0.37
Phosphate (µmol l-1) 0.93 0.87 0.85 1.14 0.76
Copper (µg g-1)* 4.0    21.0    37.0    28.0      
Lead (µg g-1)* 1.8    6.7   4.9   13.8      
Zinc (µg g-1)* 14.3    30.0    39.2     57.0       
Cadmium (µg g-1)* 0.04 0.07 0.08    
* Copper, lead, zinc, and cadmium levels measured in the upper 2 cm of bottom sediments.

The CARICOMP seagrass site is located in the southwestern part of Spaanse Water, and the reef site is situated just west of the entrance to this inner bay. Spaanse Water is the second largest inner bay, with an area of 3.19 km2 and a mean depth of 5 m. The bay contains the largest seagrass, algal, and mangrove areas of the Curaçao Underwater Park, and it has been identified as a priority area for conservation (Debrot and de Freitas, 1991). However, all ecosystems within Spaanse Water are threatened by increasing urbanization and tourism development. The entire northern coastline of the bay has been developed, and untreated waste is discharged into the bay. The southern section, known as the Caracas Baai Peninsula, used to be a Shell petroleum storage and transshipment facility, and soils are heavily contaminated with petroleum (Fig. 1). A relatively high level of watersports activity takes place in the bay, as well as yacht anchoring. A survey carried out on 11 May 1992 resulted in the following minimum usage numbers for an area within approximately 75 m of the waterline: 188 houses, 57 other buildings larger than 25 m2, 522 small boats without a cabin, 199 small boats with a cabin, 115 motor yachts, and 159 sailing yachts, for a total of 995 boats. Well developed fringing mangroves occur along the eastern and northeastern coastline, but these are threatened by major tourism development planned for the area.

Climate and Oceanography                                                                       

Curaçao is located within an area of low rainfall that extends along the north coast of South America between the Orinoco and Magdalena Rivers. According to the Meterological Service (1990), the average annual rainfall for 1905-1980 was 570 mm. The dry season is March-May, the rainy season is November-December. The mean annual rainfall generally shows some variability between years.

Based on hourly measurements from 1947 through 1980, the mean annual air temperature is 27.5ºC, with mean daily variations ranging from a nocturnal minimum of about 26ºC to a midday peak of about 30ºC. Temperature varies seasonally: January is generally the coolest month, September the hottest. The mean daily relative humidity is 77% (yearly 24-hour mean for the period 1962-1977) and varies slightly in connection with the seasonal rain pattern. Relative humidity is somewhat higher at night than during the day. The mean amount of evaporation is 8.4 mm per day (yearly mean, based on measurements gathered on Bonaire since 1968), which is high compared to the amount of rain the island receives.

Steady trade winds influence sea conditions around the island. The wind direction is primarily (95%) from the east and east-northeast. Mean wind speed is 7.1 m s-1 (yearly mean for the period 1964-1980). The maximum wind speed is 8 m s-1 and occurs in June, decreasing to a minimum of 6 m s-1 in November.

Curaçao is located south of the Atlantic hurricane belt, but tropical storms pass within 200 km of the island every 4-5 years. Thirty tropical cyclones have been reported for 1876-1996, the most recent include Edith (1971, intensity 31 m s-1), Irene (1971, 16 m s-1), Cora (1978, 16 m s-1), Greta (1978, 20 m s-1), Joan (1988, 22 m s-1), Brett (1993, 34 m s-1), and Cesar (1996, 27 m s-1).

Wave action is high along the northeastern coast due to the easterly trade winds; the southern coast is more sheltered. Waves are highest along the southeastern tip of the island and decrease westward. At the CARICOMP reef site, wave heights of 1.5 m are common (Van Duyl, 1985). The ocean current generally flows towards the northwest, with a speed below 0.5 m s-1. However, strong currents and occasionally strong countercurrents and eddies may develop near promontories along the coastline. The mean tidal range is 30 cm, with a maximum of 53 cm and a minimum of 7 cm. The mean range for January-April is consistently lower than the average annual range, whereas the mean range for May-August is higher. As measured in Annabaai Harbor (Schottegat), the tides exhibit a periodic change from diurnal to semidiurnal tides over a 13.7 day period and can be characterized as mixed mainly diurnal (De Haan and Zaneveld, 1959).

Spaanse Water is connected to the sea by a 70 m wide, up to 19 m deep, channel. Water movement in the deeper parts of the bay (up to 12 m in the central portion of the bay) is impeded by a coralline sill at a depth of 6 m at the entrance to the bay. Deeper areas of the bay form a sediment sink, typically with warmer water temperatures and increased turbidity. Annual ranges in physical parameters as given by De Kock and de Wilde (1964) and Djohani and Klok (1988) are: temperatures 26-29ºC; salinity 34-39‰; and Secchi-disk depth in the central area of the bay 2.1-2.8 m. The bay straddles two principal geological formations. The first is a fossil coralline limestone rock formation in the southwestern quadrant of the bay (location of the monitoring site), the second is altered basaltic rock which dominates in the remainder of the bay (Beets, 1972). Because of its varied geology and contrasts between wind-exposed and wind-sheltered areas, a significant range of turbidities and bottom characteristics are found within Spaanse Water.

Kuenen and Debrot (1995) distinguished four principal sediment types for Spaanse Water, based on grain size dominance. Sediments with dominant coarse grain fractions are most common at shallow depths and on exposed shores (such as at the monitoring site). Fine grained sediment are found predominantly in the deeper parts of the channel and central portions of the bay and in sheltered mangrove areas. However, most of the latter sites lie in the eastern basin. Kuenen and Debrot (1995) also measured sedimentation rates and turbidity in Spaanse water and found that the southwestern quadrant has lower sedimentation rates and turbidities. Sedimentation rates averaged 25.5 ± 14.9 g m-2 d-1 for the bay as a whole; the lowest rates were in the western part of the bay (average: 15.1 ± 9.7 g m-2 d-1), the highest rates (average: 31.3 ± 22.0 g m-2 d-1) were in the eastern part. Water column turbidities also differ significantly. Lowest turbidities (light extinction coefficients from 0.332 to 0.421) are from the channel, central basin, and western basin, while highest turbidities are from the eastern basin. By comparison, the average turbidity for Curaçao reef waters as measured by Bosscher (1992) are 0.115 m-1. Kuenen and Debrot (1995) ascribe the reduced turbidity of the western basin to the domination of fossil coralline limestones, which limit terrigenous sediment input, and to a combination of wind exposure and tidal flushing. Detailed information on turbidity, salinity, and temperature for St. Jorisbaai is lacking. As estimated by Frank and Bouma (1990), the turbidity is 2 to 3 on a 1 (clear) to 5 (very turbid) scale.


The mangroves of Curaçao are restricted to a few isolated areas of well-developed intertidal fringe forests in drowned coastal valleys, and in small areas along the coast where a barrier protects the trees from wave action and erosion. Ongoing destruction of mangrove habitat has led to a dramatic decrease in coverage. Curaçao has 55 ha of mangroves remaining (0.12% of the surface area), of which a significant portion is threatened by coastal development (Debrot and de Freitas, 1991). This is less than half of the mangrove coverage of a century ago. Measurements of leaf size of Rhizophora mangle, from stands in the vicinity of Willemstad, indicated that these mangroves, in spite of the high level of eutrophication and pollution in that area, are in good health (Snedaker, 1988).

The St. Jorisbaai mangroves also appear to be in good shape. The forest selected for CARICOMP monitoring reaches a maximum width of 100 m and consists mainly of Rhizophora mangle. Some Laguncularia racemosa trees occur along the landward boundary. Avicennia germinans, as well as Conocarpus erecta, occur in neighboring areas. Within the selected area, five successive 100 m2 quadrants have been set out perpendicular to the coast.

The tallest R. mangle tree in the plots reached a maximum length of 10.2 m and was found in the most landward plot (reference is made to total length, not total height, as a number of the trees in this area grow in a subhorizontal position). Figure 2 summarizes the amount of trees (dbh > 2.5 cm) per plot as well as biomass (kg m-2), according to Golley et al. (1962). Dry weight of litter (leaves, fruits, flowers, wood) has been determined for July 1994-July 1995 (Fig. 3). The production of leaves tends to be somewhat higher during July-August. Fruit production is high in April-September and low in September-April, which mirrors the usual seasonal rain pattern for the island, although the 1994-1995 rain pattern was erratic (Fig. 4). During June 1994-August 1995, the salinity of interstitial water within the plots was measured monthly (Fig. 4), but although mean monthly rainfall for the island is known (Meteorological Service, 1996), there is no obvious relationship. Table 2 lists animal species found in mangrove habitats during the study period.

Mangrove numbers & biomass
Fig. 2. Number of mangrove trees and biomass in five 100 m2 quadrats,
St. Jorisbaai
Productivity, leaves & fruit
Fig. 3. Mean weight of leaves and fruit produced July 1994 through July
Mangroves, rainfall & salinity
Fig. 4. Mean monthly rainfall and salinity of interstitial water in mangrove
soils June 1994 through August 1995.
Table 2. Mangrove habitat species composition: 1994-1995.
Crustacea Mollusca Aves
Aratus pisonii Batillaria minima Coereba flaveola
Cardisoma guanhumi Brachidones cf. recurvus Columba corensis
Chthamalus stellatus Columbella mercatoria Dendroica petechia
Ucides cordatus Crassostrea rhizophorae Tyrannus dominicensis
Goniopsis cruentata Isognomon alatus  
Sesarma curacaoense Littorina angulifera  
Uca pugnax rapax Melampus coffeus  

Seagrass and Algal Beds                                                                          

Several studies have been done on the macrobenthos of Spaanse Water (Van der Horst, 1927; Roos, 1964, 1971; Van den Hoek et al., 1972; Bak, 1975; Fransen, 1986). Also, an assessment of the nursery function of the bay by Briones Sierra (1994) and a study of the seagrass and algal beds by Kuenen (1991) and Kuenen and Debrot (1995) have been carried out.

Of the thirteen different seagrass and algal assemblages described by Kuenen and Debrot (1995) for Spaanse Water, two were found in the vicinity of the CARICOMP seagrass monitoring site. The first was a shallow Thalassia-Halimeda opuntia assemblage from average depths of 0.8 m, average light penetration of 75.2%, and negligible amounts of hard substrate. Average sediment grain size was the highest observed in their study. The dominant species of this assemblage were Thalassia testudinum and Halimeda opuntia. Other species included the algae Acetabularia crenulata, Amphiroa fragilissima, Halimeda incrassata, Hypnea cervicornis, Penicillus capitatus, Spyridia filamentosa, and Valonia ventricosa, the hydrozoan Eudendrium sp., and a polychaete Sabella sp. This cluster showed one of the highest levels of total biotic cover (48.2%). Sessile species richness per (6 m2) station was intermediate (15.6); both diversity (1.0) and evenness (0.4) were low, as compared to other assemblages from the bay.

The second assemblage (Kuenen and Debrot, 1995) was labelled a shallow Halimeda incrassata assemblage, from an average depth of 3.3 m. Light penetration (33.8%) and average sediment grain size (0.43 mm) were intermediate. The dominant alga was Halimeda incrassata. Other species were Acetabularia crenulata, Cladophora sp., Caulerpa sertularioides, Penicillus capitatus, and Spyridia filamentosa, the bryozoan Bugula sp., and the sponges Mycale microsigmatosa and Tedania ignis as well as Halophila decipiens, Thalassia testudinum, the alga Hypnea cervicornis, and the sponge Desmapsamma anchorata. Sessile biotic cover was reported as intermediate (16.2%) and dominated by green algae. Average number of species per station was intermediate (18.1), as were diversity (1.5) and evenness (0.5).

Some community biomass and Thalassia growth data, as measured on three separate occasions at the monitoring site, are shown in Table 3. Total biomass levels (for seagrass and attached macroalgae) ranged from 560 to 670 g m-2. Mean areal productivity for Thalassia varied from 1.20 g m-2 d-1 in March 1994 to 0.14 g m-2 d-1 in September 1995 (Table 4). Mean biomass turnover was less variable, with a high of 3.14% day-1 in March 1995 and a low of 2.44% day-1 in September 1995 (Table 4).

Table 3. Seagrass biomass data (mean of 4 replicates).
(g m-2)
Fleshy Algae
(g m-2)
Calcareous Algae
(g m-2)
March 1994 504.1 20.2  144.6
March 1995 452.6   109.8
September 1995 192.7 0.2 382.6
Table 4. Seagrass growth data (mean of 6 replicates).
  Mean Areal Productivity
(g m-2 day-1)
Mean Turnover per Biomass of Plants
(% day-1)
March 1994 1.2  2.29
March 1995 0.6  3.14
September 1995 0.14 2.44

Scleractinian corals (particularly Siderastrea siderea) which formerly were a conspicuous component of the seagrass and algal beds of the bay have suffered serious mortalities in recent decades (Kuenen, 1991). This can be ascribed largely to pollution associated with the rapid urbanization of the bay and burgeoning uncontrolled recreational use. The seagrass beds of the bay are important nursery areas for various reef fish species, such as snappers and grunts (Briones Sierra, 1994), and also the spiny lobster Panulirus argus (Jalink and Donkersloot, 1985).

Coral Reefs                                                                                             

Curaçao is completely surrounded by fringing reefs, situated at a distance from the coast ranging from 20 m to 250 m (Van Duyl, 1985). Although the reef profile is variable along the coast, a general pattern can be distinguished. From the shore, mostly consisting of steep cliffs and rubble beaches, a submarine terrace gradually slopes to a “drop-off” at 7-12 m depth (Bak, 1975). Here, the reef slopes steeply at 45°-90°, sometimes interrupted by an inclined terrace at 50-60 m, to a second drop-off at 70-80 m, ending in a sandy plain at 80-90 m (Bak, 1975). Two additional common reef profiles are distinguished by Van Duyl (1985): one profile with a broad terrace, a relatively deep drop-off (15-18 m) and a less steep slope (<30°); the second profile with a narrow terrace, bordering on a subsea cliff wall. At the Caricomp site, the reef corresponds to the first mentioned profile and is bordered by a rubble beach that consists mostly of Acropora fragments. The rubble beach has a high abundance and diversity of intertidal mollusks, indicative of pristine conditions (Nagelkerken and Debrot, 1995). The submarine terrace, where the CARICOMP line transects are situated at a depth of 6-8 m, stretches 75 m seaward from the shore to the drop-off, which is located at a depth of 8 m (see Van Duyl, 1985).

The distribution of reef corals in the Netherlands Antilles has been described by Roos (1964, 1971) and Bak (1975, 1977), while Bak (1977) also provided quantitative data on the composition of the coral community in Curaçao. The total number of coral species found in various marine environments in Curaçao is 57, of which 50 species are hermatypic, belonging to 24 genera (Bak, 1975). These numbers indicate that Curaçao belongs to the Caribbean diversity center (Bak, 1977). At the Caricomp reef site, the total number of hermatypic coral species was found to be 28, at a depth of 6-8 m, in an area of 90 m2.

Bak (1975) recognized various zones along the vertical reef profile of Curaçao. The shore zone is a high energy environment that is occasionally subject to partial aerial exposure during low tides. In general, typical organisms of this zone are algae, echinoderms, the coral Diploria clivosa, and other encrusting corals (Bak, 1975). At a depth of 1-4 m, the Acropora palmata zone is characterized by the dominance of the coral A. palmata. Encrusting calcareous red algae, such as Porolithon pachydermum, are important stabilizers of the coral rubble in this zone (Bak, 1975). The barren zone, at 3-4.5 m, is largely devoid of coral growth. The substrate consists largely of sand and coral rubble, or coral rock in more exposed areas. The scouring action of the sand and, before the mass mortality, the grazing of the sea urchin Diadema antillarum, largely inhibit larval settlement of benthic organisms (Bak, 1975). Below 4-5 m, coral cover and diversity increase towards the first drop-off, shifting from a sandy bottom with fields of Acropora cervicornis, towards a substrate of coral rock and living coral dominated by Montastraea annularis, Agaricia agaricites, and Madracis mirabilis (Bak, 1975). Coral cover and diversity remain high over the drop-off, but they decrease rapidly below 35-40 m where the influence of sedimentation is high. Here, calcareous red algae become abundant, although the corals Montastraea annularis and Agaricia undata can still be found down to a depth of 80 m (Bak, 1975).

The algal zonation on the coral reef at the southwestern coast of the island has been described by Van den Hoek et al. (1975) at Klein Piscadera (near the Carmabi lab) along a depth gradient of 0-20 m. Seven different zones were distinguished, and a profusion of fleshy and filamentous algae were found, averaging 54 species per 25 m2. The highest combined coverage and diversity of fleshy and filamentous algae and of crustose corallines were found in shallow (1-4 m) A. palmata-Porolithon-Millepora reefs, while lowest values were found in the deeper (5-13 m) and rich M. annularis and M. cavernosa coral community (Van den Hoek et al., 1975). It is not known, however, to what extent the algal community at the CARICOMP reef site corresponds to these findings.

Few quantitative data are available for the coral reef fish communities of Curaçao. Research on the distribution, abundance, and species diversity of coral reef fishes has been done by Nagelkerken (1974, 1977) and Leloup and Van der Mark (1984), while Briones Sierra (1994) provided quantitative work for the coral reef, mangrove, seagrass, and algal biotopes of Spaanse Water in the vicinity of the CARICOMP seagrass and coral reef sites. The fish community near the latter, at a depth of 8-25 m, is dominated by the four fish species Haemulon chrysargyreum, Acanthurus bahianus, Mulloidichtys martinicus, and Ocyurus chrysurus, accounting for 50% of the total recorded fish abundance (Briones Sierra, 1994). Herbivorous fish of the families Acanthuridae (17%, density 3.9 per 100 m2) and Scaridae (15%, density 3.4 per 100 m2) accounted for about one-third of the total recorded fish abundance, while carnivorous fish accounted for 54%. The coral predator Chaetodon capistratus accounted for 6% (density 1.3 per 100 m2) of total fish abundance.

The shallow-water ecosystems of Spaanse Water appear to play an important role as nursery areas and habitats for coral reef fishes (Briones Sierra, 1994). The reefs of Curaçao are heavily overfished as a result of high fishing pressure and the uncontrolled use of spear guns, fish traps, and gill-nets (Van’t Hof et al., 1995). In 1984, Leloup and Van der Mark found groupers (Serranidae) to be smaller and less abundant on the reefs of Curaçao compared to those in Bonaire, and suggested the higher degree of spearfishing in Curaçao as a likely cause. One species, the balloonfish Diodon holocanthus increased in abundance on the reefs of Curaçao as a result of a mass recruitment of juvenile fish in 1994 (Debrot and Nagelkerken, 1997).

The coral reefs of Curaçao, including the CARICOMP reef site, have recently been impacted by a number of natural and anthropogenic disturbances. Bak and Nieuwland (1995) found that during the last two decades coral cover and number of coral colonies decreased significantly on the shallow forereef (depths 10 and 20 m) in Curaçao. Species richness also decreased at a depth of 30-40 m, and rare species disappeared. Coastal development activities, such as sewage discharge and artificial beach construction, are likely to have caused the decline of the shallow reef (Bak and Nieuwland, 1995).

Curaçao is located outside the hurricane belt. The most recent hurricane passing within 100 nautical miles of the island was Hurricane Joan in 1988 (Meteorological Service, 1990), but its effect on the coral reef was not documented. Tropical storm Brett passed 145 km from Curaçao in 1993 and caused considerable damage to shallow water Acropora palmata and Millepora complanata colonies to a depth of at least 8 m, especially on the exposed eastern side of the southwestern coast of the island (Van Veghel and Hoetjes, 1995). As the CARICOMP reef site is situated in this area, and is located at an exposed promontory, the storm-related damage to the shallow part of this reef was severe as well.

Massive coral bleaching has been documented for Curaçao reefs: in 1987 (Williams and Bunkley-Williams, 1990), in 1990 (Meesters and Bak, 1993), and in 1995 (CARICOMP, 1997). The coral Montastraea annularis is normally most heavily affected. The bleaching-related mortality in M. annularis at the CARICOMP reef site during the 1995 bleaching event was found to be much higher in comparison to other more pristine reef areas (Nagelkerken et al., 1997b). As M. annularis is a main reef builder on Curaçao reefs (Bak, 1975), this event may thus have caused a higher than average disturbance of the coral community at the CARICOMP reef compared to other reef areas.

Several diseases have had detrimental effects on coral reef organisms of Curaçao reefs in the 1980s and 1990s. In 1980, the corals Acropora palmata and A. cervicornis were affected by white-band disease (Bak and Criens, 1981), and in 1983 the population of the sea urchin Diadema antillarum was reduced by 98-100% by an unknown cause (Bak et al., 1984). The latter resulted in a significant increase in cover of fleshy and filamentous algae in combination with a general decrease in coral, crustose coralline, and/or loose sediment cover (De Ruyter van Steveninck and Bak, 1986). In 1995, widespread mortality was observed in Caribbean sea fans (Gorgonia spp.), and in Curaçao the sea fan Gorgonia ventalina was affected (Nagelkerken et al., 1997a). The effects of the disease appeared to be positively related to water depth; the sea fan community at the CARICOMP reef (depth 5 m) showed 62% of the sea fans to be infected, and the mean percentage of tissue surface injured was 7% (Nagelkerken et al., 1997a). The disease was probably caused by a water-borne pathogen likely to have been distributed by sediment particles (Smith et al., 1996). Finally, according to Bak and Nieuwland (1995) black-band disease in corals is rare in Curaçao.

The compound ascidian Trididemnum solidum is a common competitor for space on Curaçao reefs and can easily overgrow corals. Its abundance was first quantified in 1978 by Bak et al. (1981); during a re-survey in 1993, Bak et al. (1996) found that its abundance had increased significantly over 15 years. Eutrophication has been suggested as a possible cause for this increase (Bak and Nieuwland, 1995). So far, T. solidum has not been observed at the CARICOMP reef site, but it is located only 3 km upstream of the nearest reef where the presence of T. solidum has been confirmed (Bak et al., 1996).


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