Identifying an Evaporative Thermal Refugium for the Preservation of Coral Reefs in a Warming World—The Gulf of Eilat (Aqaba)

Abstract

Coral bleaching events are more frequent and severe as global temperatures rise and marine heat waves are more frequent. However, quantifying the surface energy fluxes in coral reefs at various geoclimatic regions and the mechanisms by which the air-water interactions regulate water temperature is rare. We measure surface energy fluxes over coral reefs using Eddy Covariance towers in two contrasting geo-climatic regions: The typical setup of humid/tropical coral reefs (Heron Reef, Great Barrier Reef, Australia) and the rarer desert coral reef (Golf of Eilat, Israel). We analyze how the surface heat fluxes regulate the temperature of shallow coral reef environments. We show that in the desert reefs, the dry air overlying the shallow coral reef results in extremely high evaporation rates which in turn results in extensive cooling of the water. In humid/tropical reefs, evaporation is suppressed by humidity and is limited in the ability to offset the heating of the water. The extreme difference in evaporative cooling in desert versus tropical reefs is key in the response to marine heat waves. Marine heat waves which impose thermal stress on corals are a result of synoptic-scale circulation variations which suppress evaporation and increase heating. The most severe marine heatwave ever measured at the Gulf of Eilat (August 2021) was found to be caused by low evaporation rates, which resulted from synoptic circulation with weak winds and high humidity. After the onset of water temperature rise and the return of the dry winds, evaporation instantly cooled the water overlying the corals- relieving potential stress. Whereas, at the humid/tropical Heron Reef (Great Barrier Reef, Australia) evaporation solely is unable to reduce the high water temperature and therefore coral heat induce stress events are inevitably longer and more frequent. We conclude that evaporative cooling is a key mechanism protecting coral reefs located in deserts from extreme high-water temperatures, thereby representing possible thermal refugium for corals against background global warming.