Decline in Carbonate Production Threatens Coral Reef Growth

Corals provide complex three-dimensional habitats that sustain some of the planet’s most biologically diverse ecosystems and act as protective barriers for adjacent shorelines. However, coral reefs have been declining globally over the last few decades. Since the mid 1970s, coral cover has decreased nearly 80 percent in the Caribbean. Perry et al. (2013) show that current carbonate production rates are below historical (Holocene) values—current production rates are 50 percent lower than historical values and 37 percent of reefs surveyed were net erosional. Perry and colleagues suggest that there is an ecological threshold of about 10 percent live coral that is critical to maintain positive carbonate production rates. If a reef falls below 10 percent coral cover, it is likely that its production rate will become negative and the reef will start to degrade.—Kelsey Waite
Chris T. Perry, Gary N. Murphy, Paul S. Kench, Scott G. Smithers, Evan N. Edinger, Robert S. Steneck, Peter J. Mumby. Caribbean-wide decline in carbonate production threatens coral reef growth. Nature Communications, 2013; 4: 1402, 1–7.

                  Perry and colleagues measured current rates of reef carbonate production and bioerosion in 101 transects spanning 19 individual reefs and four countries (Bahamas, Belize, Bonaire, and Grand Cayman) in the Caribbean—Data was collected within common habitats including nearshore hardgrounds, Acropora palmatehabitats, Montastraea spur-and-groove zones, fore-reef slopes, and deep (18–20m) shelf-edge Montastraea reefs. The majority of the reefs studied had a live coral cover of around 25–30 percent, a high macroalgal abundance, and a low abundance of substrate grazing taxa (parrotfish and urchins). At each site, the ReefBudget census-based methodology was used to measure the rate of biologically driven carbonate production and erosion.
                  Perry and colleagues show that net carbonate production rates vary between different habitats studied, as well as within the same habitat. Overall, 21 percent of reefs studied had net negative budgets and 26 percent had net positive budgets, but had net rates lower than 5G (kg CaCO3m-2year-1). The highest producing reef (+3.63G at 5m and +9.53G at 10m) was located in the ‘no dive reserve’ in Bonaire. Historical (mid and late-Holocene periods) carbonate production rates are reported to be in the range of 10–17G, suggesting that current rates of production are less than half of what they used to be. Furthermore, Perry and colleagues suggest that their data may be underestimating the rate of erosion (endolithic bioerosion is challenging to quantify accurately), so the actual net production rates may be lower than calculated.
There was a significant correlation between live coral coverage and net production rates—suggesting that declines in coral cover and changes in benthic community composition are compromising coral reef accretion and carbonate production rates. Perry et al. suggest a threshold of roughly 10 percent live coral cover in order to maintain a positive reef production budget. 

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