LONG-TERM CHANGE IN CORAL COMMUNITIES ALONG DEPTH GRADIENTS OVER LEEWARD REEFS IN THE NETHERLANDS-ANTILLES

Reef slope coral communities were surveyed for long-term (20 years) ch anges in scleractinian coral cover, numbers of coral colonies and spec ies richness, over the time intervals between the years 1973, 1983 and 1992. We compare such long-term structural changes in the communities at depths of 10, 20, 30 and 40 m. Our data are based on series of pho tographic records of permanent quadrats, a total of 36 m(2) reef botto m at each depth, along Four transects on the leeward coasts of the isl ands of Curacao and Bonaire. We summarize the changes in the permanent quadrats over time to demonstrate the main trends in the data set and , to understand the significance of the data for the reef community, t est the results as effects of time and depth using mixed model ANOVA's . Changes in numbers of coral colonies and coral cover were a function of depth. Number of coral colonies decreased significantly at depths of 10, 20 and 30 m, but not at 40 m. Coral cover decreased significant ly at 10 and 20 m, but not at 30 and 40 m. Diversity (species richness ) decreased through the years independent of depth. There were no cons istent differences between the two 10-year time-intervals. These resul ts confirm earlier observations of coral mortality and spatial mobilit y which showed the deep reef (30, 40 m) as a much more constant enviro nment than the relatively disturbed shallower reef (10, 20 m).