Unexpected role of communities colonizing dead coral substrate in the calcification of coral reefs

Global and local anthropogenic stressors such as climate change, acidification, overfishing, and pollution are expected to shift the benthic community composition of coral reefs from dominance by calcifying organisms to dominance by non-calcifying algae. These changes could reduce the ability of coral reef ecosystems to maintain positive net calcium carbonate accretion. However, relationships between community composition and calcification rates remain unclear. We performed field experiments to quantify the metabolic rates of the two most dominant coral reef substrate types, live coral and dead coral substrate colonized by a mixed algal assemblage, using a novel underwater respirometer. Our results revealed that calcification rates in the daytime were similar for the live coral and dead coral substrate communities. However, in the dark, while live corals continued to calcify at slower rates, the dead coral substrate communities exhibited carbonate dissolution. Daytime net photosynthesis of the dead coral substrate communities was up to five times as much as for live corals, which we hypothesize may have created favorable conditions for the precipitation of carbonate minerals. We conclude that: (1) calcification from dead coral substrate communities can contribute to coral reef community calcification during the day, and (2) dead coral substrate communities can also contribute to carbonate mineral dissolution at night, decreasing ecosystem calcification over a diel cycle. This provides evidence that reefs could shift from slow, long-term accretion of calcium carbonate to a state where large daily cycling of calcium carbonate occurs, but with little or no long-term accumulation of the carbonate minerals needed to sustain the reef against erosional forces.

Automated summary

Global and local anthropogenic stressors are expected to shift coral reef benthic community composition from calcifying organisms to non-calcifying algae, affecting coral reef ecosystem calcification. Field experiments showed that live coral and dead coral substrate communities have similar calcification rates in the daytime, but the dead coral substrate communities experienced carbonate dissolution at night. This could lead to large daily cycling of calcium carbonate without long-term accumulation needed to sustain the reef.