Competition between calcifying and noncalcifying temperate marine macroalgae under elevated CO₂ levels

Since pre-industrial times, uptake of anthropogenic CO₂ by surface ocean waters has caused a documented change of 0.1 pH units. Calcifying organisms are sensitive to elevated CO₂ concentrations due to their calcium carbonate skeletons. In temperate rocky intertidal environments, calcifying and noncalcifying macroalgae make up diverse benthic photoautotrophic communities. These communities may change as calcifiers and noncalcifiers respond differently to rising CO₂ concentrations. In order to test this hypothesis, we conducted an 86 d mesocosm ex periment to investigate the physiological and competitive responses of calcifying and noncalcifying temperate marine macroalgae to 385, 665, and 1486 μatm CO₂. We focused on comparing 2 abundant red algae in the Northeast Atlantic:Corallina officinalis(calcifying) andChondrus crispus(noncalcifying). We found an interactive effect of CO₂ concentration and exposure time on growth rates ofC. officinalis, and total protein and carbohydrate concentrations in both species. Photosynthetic rates did not show a strong response. Calcification inC. officinalisshowed a parabolic response, while skeletal inorganic carbon decreased with increasing CO₂. Community structure changed, asChondrus crispuscover increased in all treatments, whileC. officinaliscover decreased in both elevated-CO₂ treatments. Photochemical parameters of other species are also presented. Our results suggest that CO₂ will alter the competitive strengths of calcifying and noncalcifying temperate benthic macroalgae, resulting in different community structures, unless these species are able to adapt at a rate similar to or faster than the current rate of increasing sea-surface CO₂ concentrations.