Thirty-six m² of reef slope communities were analysed at each of four depths; 10, 20 30 and 40 m, for changes in the time intervals between the years 1973, 1983 and 1992. Comparisons over the twenty year period are based on photographic records of large, 3 x 3 quadrats along four transects on the leeward coasts of Cura~ao and Bonaire. We compared coral cover numbers of coral colonies and species richness at each data point. There was a significant decrease in coral cover and colony number in both periods in the upper part of the reef (10-20 m). In the deep reef (30-40 m) this decrease was not observed and coral cover was maintained or increased. Species richness decreased at all depths in both ten-year periods.
Curiosity and interest in the linkage between the biology of individual coral species, expressed in their respective ecological strategies, and the development of benthic reef communities, prompted us to start monotonng permanent quadrats on the reef bottom in 1973 (13ak and Luckhurst 1980). Our first analysis of the changes occurring in the quadrats after five years indicated that ecological strategies of coral species, which show an enormous range (e.g. Bak and Engel 1979, Bak 1983, Porter 1987), are clearly reflected by the changes in the coral community (Bak and Luckhurst 1980). A second result was that we found processes and phenomena on the deep reef (30, 40 m) to differ significantly from those in the shallower parts of the reef (10, 20 m).
Because we saw changes in the development of the coral communities and observed that these changes differed significantly in magnitude between depths, we became intrigued by the long-term development of our communities. At that time the only long-term monitoring we were aware of was the Connell study of quadrats in very shallow reef habitats on the Great Barrier Reef (Connell 1976, 1978) and it seemed expedient to continue our surveys. Consequently our quadrats were photographed at least yearly between 1973 and 1992 (except 1984/1985). In this case study we compare the communities in 1973, 1983 and 1992 in view of the subject of the Colloquim: Health, Hazards and History. We focus on health and history and report on changes in species number, coral cover and numbers of colonies over a twenty-year time interval.