Species composition, habitat, and water quality influence coral bleaching in southern Florida

The present study examines coral bleaching along the Florida Keys reef tract (USA) during a major thermal-stress event in the summer of 2005, and during the summers of 2006 and 2007, which were mild thermal stress years. The primary objectives were to (1) examine the spatial pattern of coral bleaching and its relationship to coral community composition and habitat and (2) determine the relationship between environmental parameters and coral-bleaching prevalence (i.e. the proportion of colonies that bleached within each population). Over 50% of all coral species bleached along much of the reef tract in 2005. The Lower Keys supported more colonies per unit area than elsewhere and showed the highest number of bleached coral colonies; Biscayne and the Middle Keys showed the highest coral-bleaching prevalence. The most thermally sensitive corals were Porites furcata, Millepora complanata, Siderastrea radians, Porites divaricata, Agaricia agaricites, Millepora alcicornis, and Porites porites. The most extensive bleaching was recorded for large colonies, >= 30 cm, except in 2005 when small branching Porites colonies (< 30 cm) showed higher bleaching prevalence. Shallow-water coral colonies and corals at localities with high productivity, in the form of chlorophyll a and dissolved inorganic nitrogen concentration, showed higher bleaching prevalence than both deep coral colonies and corals at localities with low productivity. By locally regulating waste-water discharge from the land, and thereby reducing local primary productivity, the severity of coral bleaching may be reduced when subjected to high regional water temperatures.