Coral communities on the central Pacific coast of Costa Rica were affected during the 1991-92 El Niño warming event. More than 57% of all observed colonies at three localities (Parque Nacional Manuel Antonio, Punta Cambutal, and Parque Marino Ballena) were bleached. Mortality during this El Niño was much lower (~9%) than in previous events. Psammocora spp. accounted for ~66% of dead corals, while massive (Porites lobata, Pavona spp.) and branching (Pocillopora spp.) for ~34%. Our results suggest that the observed bleaching in P. lobata was related to zooxanthellar densities and not to changes in pigment concentrations: only chlorophyll a varied between normally pigmented and bleached colonies at one locality (Ballena). Site differences in zooxanthellar densities or their pigment concentrations, may not be the result of the bleaching event itself, because a percentage of dead corals and zooxanthellar densities of bleached colonies seems to follow a trend with the exposure to tidal regimes and currents at each site. Local oceanographic conditions can be influencing the zooxanthellar densities and their response to the warming, together with intrinsic differences between colonies as well. The impact of this event can be considered serious given the short period of time that elapsed between El Niño related mortalities and the slow reefs recovery, the mode of reproduction of reef building species, and the anthropogenic-originated disturbances which affect the coral communities and reefs of the Costa Rican central Pacific coast.
Keywords: El Niño, ENSO, bleaching, corals, Costa Rica, zooxanthella, eastern Pacific