Publication Date:
2016-02-27
Description:
Crustose coralline algae (CCA) cement reefs and create important habitat and settling sites for reef organisms. The susceptibility of CCA to increasing ocean p CO 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 p CO 2 and irradiance. We examined elevated p CO 2 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 p CO 2 and pH were simulated using seawater inflow from a back reef. Although CO 2 enrichment to year 2100 levels resulted in 1087 µatm p CO 2 in the elevated p CO 2 treatment, CaCO 3 saturation states remained high ( cal ≥ 2.7). Under these conditions, elevated p CO 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 p CO 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 p CO 2 interaction. Based on this short-term experiment, tropical macroalgal communities are unlikely to shift at p CO 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.
Print ISSN:
1054-3139
Electronic ISSN:
1095-9289
Topics:
Biology
,
Geosciences
,
Physics
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