Effects of recent warming events on coral reef communities of Costa Rica(Central America)

This thesis documents bleaching, mortality and recovery of corals in a time period (1991-2001) when three warming events affected coral reef communities at the Pacific and Caribbean coast of Costa Rica, Central America. During the mass bleaching events, doldrum-like weather (long spells of sunny, dry days together with calm, clear waters) prevailed and was coincidental with the warm water excursions. These conditions are highly conducive to bleaching due to an increase of solar radiation reaching the corals together with the thermal stress. The strongest El Niño episodes not always resulted in the highest tissue mortality, suggesting that selective processes (more thermal tolerant coral genotypes and zooxanthellae clades) and local oceanographic settings may be interacting and decreasing the warming impact. These results indicate that the responses of coral reef communities to warming episodes, such as El Niño, are complex and dynamic, and site and species-specific factors are interacting to a degree that precludes casual generalizations. The stable isotopic (:sup:13:/sup:C and :sup:18:/sup:O) signature of the 1997-98 El Niño in massive corals is currently investigated. Preliminary results indicate that seasonal average amplitude of the :sup:18:/sup:O record, follows the seasonal changes in surface seawater temperature accurately. However, mean :sup:18:/sup:18O, during the peak of the 1997-98 El Niño, was highly inconsistent between colonies, depths, localities and the two studied species, suggesting that very strong events affects coral growth and hence, the isotopic signal is lessened.