Ecological aspects of the distribution of reef corals in the Netherlands Antilles by Rolf P. M. Bak Caribbean Marine Biological Institute, Curaçao, Netherlands Antilles Abstract INTRODUCTION The vertical and horizontal of the distribution patterns The Netherlands Antilles consist of two groups of of corals and coral reefs (to a depth of 90 m) are dis- islands: the Leeward Group of Aruba, Bonaire and cussed in relation the environmental factors: to geomor- Curaçao and the Windward Group of St. Martin, phology of the bottom, available substrate, light, tur- St. Eustatius and Saba inset in fig. 1). Most bidity, sedimentation, water movement and temperature. (see of the research coral has been done in There is a general pattern which is comparable to other on reefs well-developed Caribbean reefs. However, as in other the Curaçao, with some attention being paid to areas variations are found, e.g. the depth and growth reefs of Aruba and Bonaire. The marine habitats form of Acropora palmata will depend on the degree of of the Windward are unstudied. There correla- Group relatively exposure to water movement. are strong show Leeward tions between the environmental variables and the oc- The few explorations made, the currence of coral species and their growth form, the Group of islands to be much richer in reef develop- species composition of coral communities and the charac- ment (and in speoies diversity) than St. Martin, St. ter of the coral reef. In some cases the relationship is not Eustatius and Saba. that obvious. The absence of Agaricia species at certain The factors coral distribution are points along the coast of Aruba and the dominance of governing Sargassum on the deep bottom at some places along the well known and include: light, sedimentation, wa- windward coast of Curaçao is not yet explained. ter movement, substrate, bottom morphology and The relative importance of the different factors in an temperature. I will attempt to show how these environmental setting is shown by a comparison of reef factors influence the actual distribution of reef cor- communities and reef habitats with a coral community of als and coral reefs, and a muddy, shallow inland bay. The community of the bay vertically horizontally, from consists, apart the hardier coral species, of corals in the Netherlands Antilles. which are characteristic of the deep reef: Scolymia lacera, S. cubensis and Helioseris cucullata. These corals are adapted to sedimentation and low light intensities and I. THE REEFS OF THE LEEWARD GROUP to are apparently able withstand a temperature and salin- much broader than that of their ity range deep reef habitat. Vertical distribution The paucity of corals and coral reef development Goreau & Wells noted that the around the islands of the Windward Group (deeper habi- (1967) geomorphol- of influences the tat included) can generally be explained by the morphol- ogy a coast strongly morphology of the the lack of suitable substrate and the ogy sea floor, of the coral reef. Although the profiles of the coast effect of hurricanes. The exposed coasts of Saba and St. there are variable, are a number of general fea- Eustatius, being virtually unexplored, may have richer tures of the S.W. coasts of Curaçao and Bonaire: coral growth. a cliff or a A new list of species of the Scleractinia of the Leeward steep shingle beach, gradually sloping and Windward groups, consisting of 57 species, is in- terrace in front, a drop-off at 7 to 12 m with a cluded. seaward slope varying from 45° to vertical. This either slope may be continuous or interrupted by an inclined terrace at a depth of 50 to 60 m. There is generally a second vertical drop-off at !) Paper presented at the Seventh Caribbean Geological 70 to 80 m coming to an end at a sandy plain at Conference, Section 5: Caribbean Reef Systems, Gua- 80 90 this deloupe, 1-5 July 1974. to m (fig. 1). Over profile there is a 182 R. P. M. BAK - ECOLOGY OF REEF CORALS Fig. 1. General profile of the S.W. coasts of Curaçao and Bonaire. The inset shows the geographical position of the Netherlands Antilles in the Caribbean. The reef at the first agaricites Fig. 2. drop-off (12 m) on the N.W. coast of Bonaire. Dominant corals are Agaricia and Montastrea annularis. The latter species occurs in several different growth forms in the same place. BIJDRAGEN TOT DE DIERKUNDE, 45 (2) - 1975 183 definite series of coral communities, each one rep- waves. The relatively rapid growth of the A. palm- above-men- increase in to 10 resenting a different response to the ata branches, mean length 6 cm a tioned environmental factors. Of these, tempera- year in situ (Bak, unpublished observations), ac- ture is probably not of major influence. Tempera- comodates greatly the establishment of broken- off branches Whole ture measurements in situ from 10 to 50 m have as new colonies. colonies, being never shown a difference over the reef in excess thrown over, cement themselves to the substrate branches. In of 2°C. As the mean monthly temperature at 18 and start forming new upward this in varies from of will be m depth Curaçao waters 26° to zone, where settlement young colonies 28 °C throughout the year, temperature never dif- hindered by the scouring of sand over the rocky fers significantly from optimum values. substrate and the abundance of the grazing sea The reef be A urchin Diadema antillarum Bak & Van following zones may recognized: (cf. Eys, shore zone, either in front of a shingle beach or in 1975), this mode of "vegetaltive" reproduction is of Also in the front of a cliff. In the first case, the most con- particular adaptive value. barren zone, and the of sand carried spicuous organisms are algae echinoderms, scouring the course by oscil- few corals will be In will be while a encrusting present. lating water movement over the bottom, of the main factors of the latter case, the rock directly under the water one checking settlement surface will be covered with algae, the zoanthid sessile organisms. In more exposed areas the bot- Palythoa mammillosa and encrusting Diploria tom consists of coral rock and no loose debris is In this the of corals and clivosa. These are areas of heavy stress, subject to present. case, density strong water movement and occasionally, to par- gorgonians is higher. At 4to 5 the number of coral colonies in- tial emergence during low tides. Diploria clivosa m and in is a typical coral of these high energy environ- creases, reaching a great density diversity ments. the area before the first drop-off (the blue edge: 6 in shal- From 1 to 4 m Acropora palmata is by far the Roos, 1964) at to 12 m. The bottom the most abundant coral and the most important cal- lower part is very sandy with isolated coral col- cium carbonate producer. The growth form of the onies and gorgonians. Here Acropora cervicornis colonies varies in relation to water movement sometimes forms extensive fields. Towards the decreases and (Shinn, 1963; Roos, 1964, 1971) from a variety drop-off the sandy surface most of of branched forms to encrusting colonies. In this the substrate is living coral and coral rock, covered of reef zone and in the barren zone (the next one sea- with a variety organisms {fig. 2). Among the Montastrea ici wards) encrusting calcareous red algae (Corallina- corals, annularis, Agar a agarici- ceae), especially Porolithon pachydermum (cf. Van tes and Madracis mirabilis are most abundant. Calcareous red become den Hoek et al., in press) are of consequence. algae more prominent dead is in is Their mode of growing over coral rubble again. Except very rough weather, this area influenced important in stabilizing the substrate. In some lo- not by the destructive force of the calities, these waves. algae or Millepora complanata may dominate reef formers. Over the the as drop-off, on steep slope, high coral and The barren zone (3 to 4.5 m) is devoid of great density diversity continues, only to decrease builders. substrate consists below 35-40 quantities of reef The rapidly m. Calcareous red algae are of sand and coral fragments, the latter being abundant, often preferring the darker crevices of mostly dead Acropora palmata branches, partly the reef, e.g. the important species Hydrolithon cemented together by the crustose corallines. On- boergesenii occurs on the undersides of flattened coral ly a few living corals and gorgonians are present. colonies. They are important reef frame and often coral surfaces The Acropora palmata zone and the barren zone cementers protect against both far above the normal are 1/2 wave length. penetration by boring sponges (Clionidae) (Bak, The Acropora zone in particular takes the full unpublished observations). On the deeper part of of the After of the force waves. a stormy period (maxi- the slope, with diminuation light intensity, Corallinaceae mum winds up to 36 knots, wind direction paral- become less cryptic and grow on the examined broken branch- more Other sessile lel to coast) I many exposed outcrops. organisms A. Jan Thiel of show similar in Madra- es of palmata at (S.W. coast a change microhabitat, e.g. cis Their Curaçao). The largest fracture measured 4 by 30 pharensis. place in the holes and crev- section. of the reef is taken cm in None fractures showed any ices of the, deep over mainly by signs of boring organisms, which shows that the sponges. This area is rarely influenced by strong only agent of destruction was the force of the water movement, light diminishes rapidly down 184 R.