Abstract
Worldwide coral reefs face catastrophic damage due to a series of anthropogenic stressors. Investigating how coral reefs ecosystems are connected, in particular across depth, will help us understand if deeper reefs harbour distinct communities. Here, we explore changes in benthic community structure across 15–300 m depths using technical divers and submersibles around Bermuda. We report high levels of floral and faunal differentiation across depth, with distinct assemblages occupying each depth surveyed, except 200–300 m, corresponding to the lower rariphotic zone. Community turnover was highest at the boundary depths of mesophotic coral ecosystems (30–150 m) driven largely by taxonomic turnover and to a lesser degree by ordered species loss (nestedness). Our work highlights the biologically unique nature of benthic communities in the mesophotic and rariphotic zones, and their limited connectivity to shallow reefs, thus emphasizing the need to manage and protect deeper reefs as distinct entities.
Highlights
Shallow coral reef habitats are degrading rapidly as a result of escalating anthropogenic pressures including royalsocietypublishing.org/journal/rsos R
We found scleractinian corals decreased rapidly greater than 30 m depth
This is similar to the 40 m depth limit that has been reported in previous surveys from Bermuda [30], but in general, much shallower compared to the dense aggregations of hard corals described at greater than 65 m elsewhere [31,32]
Summary
Shallow (less than 30 m) coral reef habitats are degrading rapidly as a result of escalating anthropogenic pressures including royalsocietypublishing.org/journal/rsos R. Overfishing, pollution, coastal development, non-native species invasions and the effects of climate 2 change (i.e. rising seawater temperatures and ocean acidification) [1,2]. Adjacent, deeper mesophotic (low light) coral ecosystems MCEs have been proposed to provide spatial refuge for species common in perturbed shallow-water reefs, which could provide coral propagules to recolonize shallow reefs following disturbance events (a.k.a. the deep reef refuge hypothesis—DRRH; [4,5]), scientific consensus has yet to be reached Even in cases where high faunal connectivity occurs, reduced reproductive performance of MCE inhabitants casts doubt on their ability to replenish shallow reefs [7,8]. Much more work is required in this sparsely studied environment before attempting any regional or global-scale extrapolations
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