No refuge in the deep for shallow reef ecosystems

Deep water coral reefs are not the places of refuge for shallow reef organisms that some scientists have considered them to be, a new report suggests. Instead, concludes this report – based on one of the largest-ever datasets of deep coral reef visual surveys in the world – deep water coral reefs are distinct habitats, susceptible to the same natural and human impacts, and in as much need of protection as their critically endangered shallow-water counterparts. In fact, measures to protect deeper ecosystems should be prioritized in environmental policy for global marine conservation, the authors say. Coral reefs are amongst the most endangered ecosystems on the planet, suffering catastrophic damage due to human-driven climate change and habitat destruction. Deeper, mesophotic coral reefs, lying at depths between 50 and 150 meters, have been widely thought of as less vulnerable to these impacts, and further, as being capable of providing potential refuge for threatened reef species in shallower waters. This understanding is based largely on the analysis of reported depth ranges for species that could inhabit both shallow and deep reefs. However, this method offers a limited and perhaps misleading view of the overlap and connectivity between shallow and deep populations, according to Luiz Rocha et al. Rocha and colleagues re-evaluated the potential refuge role of deep water reefs through in-situ underwater observations of fish made during dozens of technical and deep-water dives on reefs in the Atlantic and Pacific Oceans. Their results show that mesophotic reefs are home to their own independent communities, with very little species overlap between depth zones. Furthermore, the effects of natural and human impacts throughout the deep reefs were documented – from storm and fishing damage to coral bleaching and plastic pollution. Rocha et al.'s study suggests that the potential for deep reefs to act as refuges is far less than thought, and reef conservation efforts should not rely on them as such. Due to their unique biodiversity and susceptibility, deep reefs should be included in marine conservation efforts, say the authors.

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       http://dx.doi.org/10.1126/science.aaq1614 
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