Abstract
Coral bleaching caused by rising sea temperature is a primary cause of coral reef degradation. However, bleaching patterns often show significant spatial variability, therefore identifying locations where local conditions may provide thermal refuges is a high conservation priority. Coral bleaching mortality often diminishes with increasing depth, but clear depth zonation of coral communities and putative limited overlap in species composition between deep and shallow reef habitats has led to the conclusion that deeper reef habitats will provide limited refuge from bleaching for most species. Here, we show that coral mortality following a severe bleaching event diminished sharply with depth.Bleaching-induced mortality ofAcroporawas approximately 90% at 0-2m, 60% at 3-4 m, yet at 6-8m there was negligible mortality. Importantly, at least two-thirds of the shallow-water (2-3 m)Acroporaassemblage had a depth range that straddled the transition from high to low mortality. Cold-water upwelling may have contributed to the lower mortality observed in all but the shallowest depths. Our results demonstrate that, in this instance, depth provided a refuge for individuals from a high proportion of species in thisAcropora-dominated assemblage. The persistence of deeper populations may provide a critical source of propagules to assist recovery of adjacent shallow-water reefs.
Highlights
Mass bleaching events causing extensive mortality of reef-building corals have become more frequent and widespread in recent decades and have affected almost all coral reef regions[1,2,3]
They are often amongst the most susceptible taxa to bleaching-induced mortality, and bleaching events often result in shifts from Acropora – dominated communities towards communities dominated by more bleaching resistant taxa (e.g. Porites and the family Merulinidae)[26,30]
A total of 40 Acropora species were observed during the study, confirming the high diversity previously reported on Acehnese reefs[31]
Summary
See referee reports the most severe and widespread bleaching event on record[26]. This transition often occurred across a fairly sharp depth boundary at intermediate depths of 10–15 m26, species with depth ranges that straddle this transition from high to low bleaching mortality will have a refuge from bleaching in deeper water. Recent studies of deep-water reefs have indicated that many corals may occur over a wider depth range than currently thought[27,28]
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