HEAT EXPOSURE OF CORALS: INVESTIGATING THE "OTHER" DIFFUSIVE BOUNDARY LAYER

Radiant energy reaching shallow water corals can cause their temperature to increase above that of the surrounding water, an effect which is reduced as flow increases. In order to better understand the thermal exposure of corals under bleaching conditions, we used temperature microsensors to investigate the thermal boundary layer (TBL) of a branching and a hemispherical coral species (Stylophora pistillata and Porites lobata). The TBL thickness for both species was 2 mm at quasi stagnant flow (0.3 cm/s), and declined exponentially at increasing flow. Dimensionless analysis of heat transfer (Nusselt-Reynolds number plots) resulted in a heat exponent of approx. 0.5, indicative of a laminar boundary layer and consistent with predictions from engineering theory for simple geometrical objects. However, additional measurements of oxygen transfer across the diffusive boundary layer (DBL) and a dimensionless analysis (Sherwood-Reynolds number plots) resulted in a greater exponent for mass transfer (~0.7) thus suggesting that heat and mass transfer at the surface of corals should not be treated as analogous processes.