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Dutra, E., Koch, M., Peach, K., & Manfrino, C. (2016). Tropical crustose coralline algal individual and community responses to elevated pCO(2) under high and low irradiance. ICES J. Mar. Sci., 73(3), 803–813.
Abstract: Crustose coralline algae (CCA) cement reefs and create important habitat and settling sites for reef organisms. The susceptibility of CCA to increasing ocean pCO(2) and declining pH or ocean acidification (OA) is a growing concern. Although CCA are autotrophs, there has been little focus on the interaction of elevated pCO(2) and irradiance. We examined elevated pCO(2) effects on individual CCA and macroalgal benthic communities at high and low irradiance (205 13 mol photons m(-2) s(-1)) in an aquaria experiment (35 d, June August 2014) on Little Cayman Island, Caribbean. A dominant Cayman reef wall CCA (Peyssonnelia sp.) in its adult lobed form and individual CCA recruits were used as experimental units. Changes in CCA, fleshy macroalgae (branching and turfs), and microalgae (including microbial biofilm) per cent cover and frequency were examined on macroalgal communities that settled onto plates from the reef. Reef diel cycles of pCO(2) and pH were simulated using seawater inflow from a back reef. Although CO2 enrichment to year 2100 levels resulted in 1087 mu atm pCO(2) in the elevated pCO(2) treatment, CaCO3 saturation states remained high (Omega(cal) >= 2.7). Under these conditions, elevated pCO(2) had no effect on Peyssonnelia sp. calcification rates or survival regardless of irradiance. Individual CCA surface area on the bottom of settling plates was lower under elevated pCO(2), but per cent cover or frequency within the community was unchanged. In contrast, there was a strong and consistent community assemblage response to irradiance. Microalgae increased at high irradiance and CCA increased under low irradiance with no significant pCO(2) interaction. Based on this short-term experiment, tropical macroalgal communities are unlikely to shift at pCO(2) levels predicted for year 2100 under high or low irradiance. Rather, irradiance and other factors that promote microalgae are likely to be strong drivers of tropical benthic algal community structure under climate change.