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Species identity and depth predict bleaching severity in reef-building corals: shall the deep inherit the reef?

ABSTRACT: Mass bleaching associated with unusually high sea temperatures represents one of the greatest threats to corals and coral reef ecosystems. Deeper reef areas are hypothesized as potential refugia, but the susceptibility of Scleractinian species over depth has not been quantified. During the most severe bleaching event on record, we found up to 83% of coral cover severely affected on Maldivian reefs at a depth of 3-5 m, but significantly reduced effects at 24-30 m. Analysis of 153 species' responses showed depth, shading and species identity had strong, significant effects on susceptibility. Overall, 73.3% of the shallow-reef assemblage had individuals at a depth of 24-30 m with reduced effects, potentially mitigating local extinction and providing a source of recruits for population recovery. Although susceptibility was phylogenetically constrained, species-level effects caused most lineages to contain some partially resistant species. Many genera showed wide variation between species, including Acropora, previously considered highly susceptible. Extinction risk estimates showed species and lineages of concern and those likely to dominate following repeated events. Our results show that deeper reef areas provide refuge for a large proportion of Scleractinian species during severe bleaching events and that the deepest occurring individuals of each population have the greatest potential to survive and drive reef recovery.

SUBMITTER: Muir PR

PROVIDER: S-EPMC5647302 | biostudies-literature | 2017 Oct

REPOSITORIES: biostudies-literature

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Species identity and depth predict bleaching severity in reef-building corals: shall the deep inherit the reef?

Muir Paul R PR Marshall Paul A PA Abdulla Ameer A Aguirre J David JD

Proceedings. Biological sciences 20171001 1864

Mass bleaching associated with unusually high sea temperatures represents one of the greatest threats to corals and coral reef ecosystems. Deeper reef areas are hypothesized as potential refugia, but the susceptibility of Scleractinian species over depth has not been quantified. During the most severe bleaching event on record, we found up to 83% of coral cover severely affected on Maldivian reefs at a depth of 3-5 m, but significantly reduced effects at 24-30 m. Analysis of 153 species' respons ...[more]

PMID: 29021175

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Project description:BACKGROUND:At the forefront of ecosystems adversely affected by climate change, coral reefs are sensitive to anomalously high temperatures which disassociate (bleaching) photosynthetic symbionts (Symbiodinium) from coral hosts and cause increasingly frequent and severe mass mortality events. Susceptibility to bleaching and mortality is variable among corals, and is determined by unknown proportions of environmental history and the synergy of Symbiodinium- and coral-specific properties. Symbiodinium live within host tissues overlaying the coral skeleton, which increases light availability through multiple light-scattering, forming one of the most efficient biological collectors of solar radiation. Light-transport in the upper ~200 ?m layer of corals skeletons (measured as 'microscopic' reduced-scattering coefficient, ?'(S,m)), has been identified as a determinant of excess light increase during bleaching and is therefore a potential determinant of the differential rate and severity of bleaching response among coral species. RESULTS:Here we experimentally demonstrate (in ten coral species) that, under thermal stress alone or combined thermal and light stress, low-?'(S,m) corals bleach at higher rate and severity than high-?'(S,m) corals and the Symbiodinium associated with low-?'(S,m) corals experience twice the decrease in photochemical efficiency. We further modelled the light absorbed by Symbiodinium due to skeletal-scattering and show that the estimated skeleton-dependent light absorbed by Symbiodinium (per unit of photosynthetic pigment) and the temporal rate of increase in absorbed light during bleaching are several fold higher in low-?'(S,m) corals. CONCLUSIONS:While symbionts associated with low-[Formula: see text] corals receive less total light from the skeleton, they experience a higher rate of light increase once bleaching is initiated and absorbing bodies are lost; further precipitating the bleaching response. Because microscopic skeletal light-scattering is a robust predictor of light-dependent bleaching among the corals assessed here, this work establishes ?'(S,m) as one of the key determinants of differential bleaching response.

Project description:In 2016, global temperatures were the highest on record, and mass coral bleaching occurred world-wide. However, around Sesoko Island, Okinawa, southwestern Japan, the heat stress assessed by degree heating week (DHW) based on local temperature measurements was moderate in 2016; in 1998, DHW was three times higher than in 2016 (10.6 vs. 3.3 in September in respective years). On a reef flat of Sesoko Island where the effect of severe coral bleaching on coral assemblage was monitored in 1998, significant coral bleaching occurred in 2016. Bleaching of the heat stress sensitive Acropora corals began in July 2016 on the reef flat as seawater temperature rose. We observed the bleaching and post-bleaching mortality status of individual colonies of Acropora spp. in 2016 in fixed plots on the reef flat. In total, 123 Acropora colonies were followed for six months after seawater temperature became normal by multiple surveys. At the beginning of September 2016, 99.2% of colonies, were either completely (92.7%) or partially (6.5%) bleached. Of those, the dominant species or species groups were A. gemmifera (Ag), A. digitifera (Ad), and tabular Acropora (tA). For all Acropora colonies, the overall whole and partial mortality was 41.5% and 11.4%, respectively. Whole mortality rate differed significantly among species; 72.5%, 17.9%, and 27.8% in Ag, Ad, and tA, respectively. Mortality rates at the end of the surveys were similar in smaller (?10 cm in diameter) and larger Ag, but the former suffered mortality earlier than the latter. Higher survival of smaller colonies was observed only in tA (100%), which may be associated with large morphological differences between smaller and larger colonies. Some of the dominant Acropora colonies had survived without partial mortality including 15.0% survival of the most vulnerable Ag at the end of the surveys. These results suggest that moderate heat stress may have a potential for selecting heat-tolerant genotypes. A longer period of mortality lasting for six months, was observed in Ag in addition to immediate whole mortality after bleaching, due to the continuous loss of living tissue by partial mortality. This highlights the need for multiple surveys at least during several months to accurately assess the impact of thermal stress event to corals. In contrast to DHW based on local measurements, DHW obtained from satellite data were similar between 1998 and 2016. Although satellite-based measurement of sea surface temperature is very useful to reveal variations in heat stress at a large spatial scale, temperature should be measured on site when variations at smaller spatial scales are of interest.

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Species identity and depth predict bleaching severity in reef-building corals: shall the deep inherit the reef?

Publications

Species identity and depth predict bleaching severity in reef-building corals: shall the deep inherit the reef?

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