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Lights out for muddy water coral reefs as global sea level rises?
Turbid coral reefs are located in shallow coastal waters, often close to urban areas like Singapore. They protect the coast through serving as breakwaters structures and have been considered as potential climate change refugia for marine biodiversity. Global sea level rise (SLR) will present a major threat to turbid coral populations by increasing the depth of water covering them, and reducing the amount of sunlight available on the seafloor. This will change the amount of habitat available for certain corals to grow, shows a new study led by ASE Presidential Postdoctoral Fellow and AXA Research Fellow Dr Kyle Morgan published last week in Proceedings of the Royal Society B.
Although the impacts of climate-related coral bleaching are well documented, knowledge of how SLR will influence reefs is limited. Many coral species get their nutrition from photosynthetic symbiotic algae, which means that these corals depend on sunlight for their growth. Although coral reefs can build vertically over time, studies have shown that they are not likely to be able to keep up with rates of SLR. This is particularly important on coastal turbid reefs (corals that inhabit high turbidity conditions) because light levels are already dramatically depth-limited due to the muddy waters.
Working on the Great Barrier Reef, Dr Morgan and colleagues from the UK and Australia used an extensive dataset of geological reef core records (drilled 4 – 5m down into the reef) and ecological survey data of living coral communities, to develop a model which predicts future changes in reef geomorphology and the spatial extent of reef habitats under different SLR scenarios.
The model shows that modest increases in water depth (low emissions scenario) will reduce coral cover and diversity, particularly of fast-growing branching coral species that provide important 3D structure on reefs. More severe SLR under a “business-as-usual” scenario will completely submerge reefs, and push many of the deeper-water corals below the photic zone, where photosynthesis is not possible.
As such, these findings suggest adverse future trajectories which could threaten the survival of turbid reefs, or lead to major changes in community composition. This is significant because turbid corals may act as a potential climate change refuge for coral reefs, as the shading effect of turbid water can reduce the additive stress of solar irradiance on corals during periods of ocean warming, reducing the risk of coral bleaching.
The response of turbid coral reefs to environmental change is particularly interesting, as organisms living on the edge of their tolerance limit can help researchers understand how other systems may respond to change in the future. In a second paper, also published last week, Morgan and co-authors discuss “Insights from extreme coral reefs in a changing world”. Turbid reefs host some of the most resilient coral species, able to cope with a lack of light and high temperatures. However, high terrestrial run-off and sedimentation from urban centres is also known to have adverse effects on marine life. Thus, the future of turbid reefs depends on both direct (pollution, dredging etc.) and indirect (SLR, climate change) anthropogenic activities.
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