Changes in the coral reefs of San Blas, Caribbean Panama: 1983-1990

Abstract

Abstract: Between 1983-1990 large changes in abundances of corals and macroalgae occurred on shallow (1-5 m) lagoonal reefs in the San Blas Islands of Panama. In 1983 these reefs were dominated by the vertical plate forms of the coral genera Agaricia and Millepora. By 1990 we observed the following major changes: (1) loss of approximately one-half of the initial live coral cover, primarily during 1983-1986, and almost completely 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 apprx 2% cover in 1983 to 28% cover in 1990. (3) Microalgal cover increased two to ten-fold between 1983 and 1986, then declined to 50% greater than the initial values by 1990. There are at least three contributors to these changes in the benthic community: (1) a coral bleaching event in 1983, which disproportionately affected Agaricia; (2) the mass mortality of Diadema antillarum in 1983, which led to decreases in grazing pressure on algae; and (3) possible increases in sediment and nutrient loads due to runoff from deforested mountainsides. Temporal patterns and observations of interactions suggest that the decrease in Diadema herbivory is a major factor in this shift in coral and algal populations.