Requirement of water-flow for sustainable growth of Pocilloporid corals during high temperature periods.

Reef-building corals are threatened worldwide by mass-scale coral bleaching episodes that are pronounced in high sea surface temperature (SST) conditions. Although water-flow has been suggested to be a mitigating factor for bleaching, long-term effects of flow-mediated bleaching suppression are as yet not fully understood. In order to investigate flow effects, we monitored the corals Pocillopora damicornis and Stylophora pistillata grown for 20 months in experimental outdoor flumes with the flow rates of 20 cms(-1) (flow) and <3 cms(-1) (still). Although bleaching was observed under high SST conditions, both species showed a shorter period or entirely no visible bleaching under the flow conditions. Better colony growth was found in the flow conditions whereas significant growth suppression and higher mortality were observed in still conditions. We conclude that water-flow is an essential environmental factor for the corals P. damicornis and S. pistillata, especially under high SST conditions.