Turf Algae on Dead Corals: Daytime Calcification and Nighttime Dissolution

Global and local anthropogenic stressors are expected to shift the benthic community composition of coral reefs from dominance by calcifying organisms to dominance by turf algae and fleshy macroalgae. In conjunction with decreases in calcification due to ocean acidification and warming, these changes in community composition could reduce the ability of coral reef ecosystems to maintain positive calcium carbonate accretion. However, any relationships between shifting community composition and calcification rates are still unclear. We performed field experiments on Lizard Island, a severely degraded reef in the northern Great Barrier Reef, to quantify the effects of benthic community shifts on net ecosystem calcification rates. We measured in-situ net calcification rates of live corals and turf algal communities growing over recently dead corals. Our results revealed that calcification rates in the light (daytime) were similar in both the live coral and algal turf communities. However, in the dark (nighttime) live corals maintained positive calcification rates, while the algal turf communities were net dissolving. We conclude that: 1) substantial calcification in algal turf communities can contribute to high coral reef community calcification rates measured during daytime, and 2) algal turf communities may contribute to nighttime carbonate mineral dissolution, decreasing net ecosystem calcification over a diel cycle. This provides evidence that non-coral calcifiers in the benthic community play important roles in coral reef accretion and dissolution pathways.