Water-flow rates and passive diffusion partially explain differential survival of corals during the 1998 bleaching event

In the western Pacific during 1998, coral bleaching, or the paling of corals through loss of pigmentation or loss of symbiotic algae (zooxanthellae), coincided with some of the warmest sea-surface temperatures (SSTs) on record. However, there was considerable spatial variation in coral survivorship; for example, corals of the same species at different locations around the Ryukyu Islands (Japan), within kilometers of each other, showed vastly different responses. Some locations experienced 100% coral mortality while other locations, nearby, suffered little coral mortality. Here we show experimental evidence for high survivorship of Acropora digitifera coral colonies that were subjected to both high SSTs (ranging from 26.22 to 33.65 degreesC) and high-water flow (50 to 70 cm s(-1)), while corals that were subjected to both high SSTs and low-water now (2 to 3 cm s(-1)) showed low survivorship. All experiments were conducted under high irradiance (similar to 95% photosynthetically active radiation). We also empirically show that no coral mortality occurred when SSTs were below 30 degreesC (ranging from 26.64 to 29.74 degreesC) under similar flow regimes. The spatial differences in coral mortality during the 1998 bleaching event may have been, in part, a result of differences in water-flow rates that induced differential rates of passive diffusion, which varied among habitats.