Isabel M. Jimenez, Michael Kühl, A. W. D. Larkum, and P. J. Ralph. Heat budget and thermal microenvironment of shallow-water corals: Do massive corals get warmer than branching corals? Limnol. Oceanogr., 53(4), 2008, 1548–1561

Coral surface temperature was investigated with multiple temperature sensors mounted on hemispherical and branching corals under (a) artificial lighting and controlled flow; (b) natural sunlight and controlled flow; and (c) in situ conditions in a shallow lagoon, under naturally fluctuating irradiance, water flow, and temperature. Under high irradiance and low flow conditions, hemispherical corals were 0.6uC warmer than the surrounding water. Hemispherical corals reached higher temperatures than branching corals, by a measure of 0.2uC to 0.4uC. Microsensor temperature measurements showed the presence of a thermal boundary layer (TBL). The TBL thickness was flow dependent, and under low flow conditions, a TBL up to 3 mm thick limited heat transfer to the ambient water. Combined microsensor measurements of temperature and oxygen showed that the TBL was approximately four times thicker than the diffusive boundary layer, as predicted from heat and mass transfer theory. A simple conceptual model describes coral surface temperature as a function of heat fluxes between coral tissue, skeleton, and surroundings. The slope of the predicted linear relationship between coral temperature and solar irradiance is fixed by the efficiencies of light absorption and the heat losses to the skeleton and the water. Although spectral absorptivity may play a significant role in coral warming, shape-related differences in thermal properties can cause hemispherical corals to reach higher temperatures than branching corals. Shape-related differences in thermal histories may thus help explain differences in susceptibility to coral bleaching between branching and hemispherical coral species. The increasing occurrence of coral bleaching over the last two decades has focused attention on temperature fluctuations on corals reefs (Brown 1997; Berkelmans and Willis 1999; Hoegh-Guldberg 1999). Under mass coral bleaching conditions, small excursions in the ambient water temperature on a coral reef (of just a few degrees Celsius above the normal average temperature maximum) induce the expulsion of the endosymbiotic dinoflagellates (zooxanthellae) and/or the loss of pigments from a wide variety of corals (Glynn 1996; Hoegh-Guldberg 1999). If such high temperature anomalies last for a week or more mass mortality of corals can occur (Glynn 1996; HoeghGuldberg 1999; Coles and Brown 2003). In most of the literature on coral bleaching, temperature of the ambient water is always assumed to be the same as the coral temperature. However, as a result of the shallow nature of many coral reef lagoons, radiant energy reaching the coral surface can increase its temperature relative to the surrounding water. Few studies have considered the 1 Corresponding author ([email protected]). Acknowledgments We thank N. Ralph for constructing the flow-through chamber and R. Bilger and J. Kent for valuable discussion of heat transfer theory. We thank M. Ball for discussions of heat transfer in corals at an early stage of this investigation and two anonymous reviewers for their valuable comments. We acknowledge the Department of Environmental Sciences, University of Technology, Sydney, and all staff at Heron Island Research Station for support. This research was funded by a University of Technology, Sydney, institutional grant to P.J.R.; by the Australian Research Council (P.J.R. and M.K.); and by a grant from the Danish Natural Science Research Council (to M.K.). The research was conducted under Great Barrier Reef Marine Park Authority permits G05/16166.1, G03/12019.1, and G06/178151. This is contribution 219 from the Institute of Water and Environmental Resource Management and contribution 0007 from the Sydney Institute of Marine Science. Limnol. Oceanogr., 53(4), 2008, 1548–1561 E 2008, by the American Society of Limnology and Oceanography, Inc.