Tropical Crustose coralline algal individual and community responses to elevated pCO2 Under high and low irradiance

Crustose coralline algae (CCA) cement reefs and create important habitat and settling sites for reef organisms. The susceptibility of CCA to increasing ocean pCO2 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 pCO2 and irradiance. We examined elevated pCO2 effects on individual CCA and macroalgal benthic communities at high and low irradiance (205-13 μmol 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 pCO2 and pH were simulated using seawater inflowfrom a back reef. Although CO2 enrichment to year 2100 levels resulted in 1087 μatm pCO2 in the elevated pCO2 treatment, CaCO3 saturation states remained high (Ωcal ≥ 2.7). Under these conditions, elevated pCO2 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 pCO2, 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 pCO2 interaction. Based on this short-term experiment, tropical macroalgal communities are unlikely to shift at pCO2 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