title: Depth-dependent mortality of reef corals following a severe bleaching event: implications for thermal refuges and population recovery.
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 of Acropora was 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) Acropora assemblage 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 this Acropora-dominated assemblage. The persistence of deeper populations may provide a critical source of propagules to assist recovery of adjacent shallow-water reefs.
authors.affiliation: ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Queensland 4811, Australia ; Australian Institute of Marine Science, Townsville, Queensland 4810, Australia.
authors.forename: Tom C L
authors.initials: TC
authors.lastname: Bridge
authors.affiliation: ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Queensland 4811, Australia.
authors.forename: Andrew S
authors.initials: AS
authors.lastname: Hoey
authors.affiliation: Wildlife Conservation Society, Indonesia Marine Program, Bogor, Indonesia.
authors.forename: Stuart J
authors.initials: SJ
authors.lastname: Campbell
authors.affiliation: Wildlife Conservation Society, Indonesia Marine Program, Bogor, Indonesia.
authors.forename: Efin
authors.initials: E
authors.lastname: Muttaqin
authors.affiliation: Centre for Marine and Fisheries Studies, Syiah Kuala University, Banda Aceh, Indonesia.
authors.forename: Edi
authors.initials: E
authors.lastname: Rudi
authors.affiliation: Centre for Marine and Fisheries Studies, Syiah Kuala University, Banda Aceh, Indonesia.
authors.forename: Nur
authors.initials: N
authors.lastname: Fadli
authors.affiliation: ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Queensland 4811, Australia.
authors.forename: Andrew H
authors.initials: AH
authors.lastname: Baird