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 (Peter.Ralph@uts.edu.au). AcknowledgmentsWe 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 ...

The thermal microenvironment of corals and the thermal effects of changing flow and radiation are critical to understanding heat-induced coral bleaching, a stress response resulting from the destruction of the symbiosis between corals and their photosynthetic microalgae. Temperature microsensor measurements at the surface of illuminated stony corals with uneven surface topography (Leptastrea purpurea and Platygyra sinensis) revealed millimetre-scale variations in surface temperature and thermal boundary layer (TBL) that may help understand the patchy nature of coral bleaching within single colonies. The effect of water flow on the thermal microenvironment was investigated in hemispherical and branching corals (Porites lobata and Stylophora pistillata, respectively) in a flow chamber experiment. For both coral types, the thickness of the TBL decreased exponentially from 2.5 mm at quasi-stagnant flow (0.3 cm s 21 ), to 1 mm at 5 cm s 21, with an exponent approximately 0.5 consistent with predictions from the heat transfer theory for simple geometrical objects and typical of laminar boundary layer processes. Measurements of mass transfer across the diffusive boundary layer using O 2 microelectrodes revealed a greater exponent for mass transfer when compared with heat transfer, indicating that heat and mass transfer at the surface of corals are not exactly analogous processes.

Reflectance spectroscopy and microscale temperature measurements were used to investigate links between optical and thermal properties of corals. Coral tissue heating showed a species-specific linear correlation to the absorptance of incident irradiance. Heat budgets estimated from absorptance and thermal boundary layer measurements indicated differences in the relative contribution of convection and conduction to heat loss in Porites lobata and Stylophora pistillata, and a higher heat conduction into the skeleton of the thin-tissued branching S. pistillata as compared to the massive thick-tissued P. lobata. Decreasing absorptance associated with bleaching resulted in decreased surface warming of coral tissue. Action spectra of coral tissue heating showed elevated efficiency of heating at wavelengths corresponding to absorption maxima of major zooxanthellae photopigments. Generally, energy-rich radiation (, 500 nm) showed the highest heating efficiency. Speciesspecific relationships between coral tissue heating and absorptance can be strongly affected by differences in the thermal properties of the skeleton and/or tissue arrangement within the skeletal matrix, indicating a yet unresolved potential for coral shape, size, and tissue thickness to affect heat dissipation and especially the conduction of heat into the coral skeleton.

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