Vertical mixing and coral death in the Red Sea following the eruption of Mount Pinatubo

Abstract

THE eruption of Mount Pinatubo in the Philippines led to a cold air-temperature anomaly throughout the Middle East during the winter of 1992¹. Here we report that the vertical mixing in the Gulf of Eilat (Aqaba) that winter was unusually deep―extending to >850 m―resulting in increased supply of nutrients to surface waters, which fuelled extraordinarily large algal and phytoplank-ton blooms. By spring, a thick mat of filamentous algae covered broad sections of the underlying reef causing extensive coral death. Branching colonies and solitary mushroom corals were most severely affected. This sequence of events, in which a short-term atmospheric cooling leads to a remarkable ecological response, is made possible by the unusually weak water-column stratification of the Gulf of Eilat. The depth of local vertical mixing during winter is determined by the net heat loss across the seaá¤-air interface, so that anomalously cold winters drive the deeper mixing that can lead to increased phytoplankton blooms. Records of such events in fossil reefs may provide useful indicators of past variations in regional air temperatures.