Field-based experimental acidification alters fouling community structure and reduces diversity

1. Increasing levels of CO₂ in the atmosphere are affecting ocean chemistry, leading to increased acidification (i.e. decreased pH) and reductions in calcium carbonate saturation state. 2. Many species are likely to respond to acidification, but the direction and magnitude of these responses will be based on interspecific and ontogenetic variation in physiology and the relative importance of calcification. Differential responses to ocean acidification (OA) among species will likely result in important changes in community structure and diversity. 3. To characterize the potential impacts of OA on community composition and structure, we examined the response of a marine fouling community to experimental CO₂ enrichment in field-deployed flow-through mesocosm systems. 4. Acidification significantly altered the community structure by altering the relative abundance of species and reduced community variability, resulting in more homogenous biofouling communities from one experimental tile to the next both among and within the acidified mesocosms. Mussel (Mytilus trossulus) recruitment was reduced by over 30% in the elevated CO₂ treatment compared to the ambient treatment by the end of the experiment. Strong differences in mussel cover (up to 40% lower in acidified conditions) developed over the second half of the 10-week experiment. Acidification did not appear to affect the mussel growth, as average mussel sizes were similar between treatments at the end of the experiment. Hydroid (Obelia dichotoma) cover was significantly reduced in the elevated CO₂ treatment after 8 weeks. Conversely, the percentage cover of bryozoan colonies (Mebranipora membranacea) was higher under acidified conditions with differences becoming apparent after 6 weeks. Neither recruitment nor final size of barnacles (Balanus crenatus) was affected by acidification. By the end of the experiment, diversity was 41% lower in the acidified treatment relative to ambient conditions. 5. Overall, our findings support the general expectation that OA will simplify marine communities by acting on important ecological processes that ultimately determine the community structure and diversity.