Abstract
Ocean acidification is expected to decrease calcification rates of bivalves. Nevertheless, in many coastal areas high p CO 2 variability is encountered already today. K iel F jord ( W estern B altic S ea) is a brackish (12–20 g kg −1 ) and CO 2 enriched habitat, but the blue mussel M ytilus edulis dominates the benthic community. In a coupled field and laboratory study we examined the annual p CO 2 variability in this habitat and the combined effects of elevated p CO 2 and food availability on juvenile M . edulis growth and calcification. In the laboratory experiment, mussel growth and calcification were found to chiefly depend on food supply, with only minor impacts of p CO 2 up to 3350 μatm. Kiel Fjord was characterized by strong seasonal p CO 2 variability. During summer, maximal p CO 2 values of 2500 μatm were observed at the surface and >3000 μatm at the bottom. However, the field growth experiment revealed seven times higher growth and calcification rates of M . edulis at a high p CO 2 inner fjord field station (mean p CO 2 ca. 1000 μatm) in comparison to a low p CO 2 outer fjord station (ca. 600 μatm). In addition, mussels were able to out‐compete the barnacle A mphibalanus improvisus at the high p CO 2 site. High mussel productivity at the inner fjord site was enabled by higher particulate organic carbon concentrations. K iel F jord is highly impacted by eutrophication, which causes bottom water hypoxia and consequently high seawater p CO 2 . At the same time, elevated nutrient concentrations increase the energy availability for filter feeding organisms such as mussels. Thus, M . edulis can dominate over a seemingly more acidification resistant species such as A . improvisus . We conclude that benthic stages of M . edulis tolerate high ambient p CO 2 when food supply is abundant and that important habitat characteristics such as species interactions and energy availability need to be considered to predict species vulnerability to ocean acidification.