Eddy covariance measurements of the surface energy balance associated with a localised coral bleaching event, Heron Reef, Great Barrier Reef, Australia

While their ecological, economical and societal values are irrefutable, an uncertain future exists for coral reefs and numerous studies suggest that the threat of coral bleaching will worsen under global warming. Despite the association between global warming and coral reef health, there is a paucity of data relating to the exchanges of heat, moisture and momentum between the atmosphere and the reef surface during bleaching episodes. Understanding these key reef-atmosphere interactions is essential for identifying the processes which control the thermal environment of water on the reef. In this paper we present new direct in situ measurements of radiation transfers and sensible and latent heat fluxes associated with an observed localised coral bleaching event at Heron Reef, in the southern Great Barrier Reef.

Measurements were made during a three day period in February 2009 using the Eddy Covariance method. Under the influence of a weak barometric pressure gradient, clear skies and light north easterly winds, daily net radiation peaked at more than 800 W m², with up to 95% of the net radiation during the morning being partitioned into heating the water column, substrate and benthic cover. Heating was exacerbated by a midafternoon low tide, and low wind speeds resulted in reduced evaporative cooling of the water surface. As a result, daily maximum water surface temperatures exceeded 34°C and near-bottom temperatures 33°C. Such conditions in conjunction with limited horizontal heat transfer due to the pooling of reef waters at the site meant that the water surface and near-bottom water temperatures exceeded 30°C for 27% and 38% of the duration of the observation period respectively resulting in widespread coral bleaching.