CHANGES IN THE CORAL-REEFS OF SAN-BLAS, CARIBBEAN PANAMA - 1983 TO 1990

Between 1983-1990 large changes in abundances of corals and macroalgae occurred on shallow (1-5 m) lagoonal reefs in the San Bias Islands of Panama. In 1983 these reefs were dominated by the vertical plate form s of the coral genera Agaricia and Millepora. By 1990 we observed the following major changes: (1) loss of approximately one-half of the ini tial live coral cover, primarily during 1983-1986, and almost complete ly due to a decline in the abundance of Agaricia. Corals only occupied 12-26% of the reef area by 1990. (2) Macroalgae (mostly Dictyota and Halimeda) increased from similar to 2% cover in 1983 to 28% cover in 1 990. (3) Microalgal cover increased two to ten-fold between 1983 and 1 986, then declined to 50% greater than the initial values by 1990. The re are at least three contributors to these changes in the benthic com munity: (1) a coral bleaching event in 1983, which disproportionately affected Agaricia; (2) the mass mortality of Diadema antillarum in 198 3, which led to decreases in grazing pressure on algae; and (3) possib le increases in sediment and nutrient loads due to runoff from defores ted mountainsides. Temporal patterns and observations of interactions suggest that the decrease in Diadema herbivory is a major factor in th is shift in coral and algal populations.