Long-term impacts of rising sea temperature and sea level on shallow water coral communities over a ~40 year period.

Effects of combined rising sea temperature and increasing sea level on coral reefs, both factors associated with global warming, have rarely been addressed. In this ~40 y study of shallow reefs in the eastern Indian Ocean, we show that a rising relative sea level, currently estimated at ~11 mm y-1, has not only promoted coral cover but also has potential to limit damaging effects of thermally-induced bleaching. In 2010 the region experienced the most severe bleaching on record with corals subject to sea temperatures of >31 °C for 7 weeks. While the reef flats studied have a common aspect and are dominated by a similar suite of coral species, there was considerable spatial variation in their bleaching response which corresponded with reef-flat depth. Greatest loss of coral cover and community structure disruption occurred on the shallowest reef flats. Damage was less severe on the deepest reef flat where corals were subject to less aerial exposure, rapid flushing and longer submergence in turbid waters. Recovery of the most damaged sites took only ~8 y. While future trajectories of these resilient reefs will depend on sea-level anomalies, and frequency of extreme bleaching the positive role of rising sea level should not be under-estimated.

Large-amplitude internal waves benefit corals during thermal stress.

Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SSTs) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proved a reliable predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale that are poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. Despite a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large-amplitude internal waves (LAIW) mitigated coral bleaching and mortality in shallow waters. In LAIW-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals. LAIW are ubiquitous in tropical stratified waters and their swash zones may thus be important conservation areas for the maintenance of coral diversity in a warming climate. Taking LAIW into account can significantly improve coral bleaching predictions and provide a valuable tool for coral reef conservation and management.