Influence of ocean warming and acidification on habitat-forming coralline algae and their associated molluscan assemblages

When ocean warming and acidification impact habitat-forming species, substantial alterations to the supported ecological communities and associated ecosystems are likely to follow. Here, we used experimental manipulations to test the hypotheses that ocean warming and acidification would negatively affect habitat-forming coralline algal turfs and the diverse molluscan assemblages they support. Boulders covered in a turf of Amphiroa anceps with intact faunal assemblages were subjected to an orthogonal combination of current (~ 23 °C) and future (~ 26 °C) ocean temperatures, and current (~ 430 µatm) and future (~ 880 µatm) seawater pCO2. Ocean warming negatively impacted the cover and photosynthetic efficiency of Amphiroa fronds, whereas ocean acidification caused the biomass per unit area and the frond density of Amphiroa turf to be 56% and 59% less than current ocean conditions, respectively. Ocean acidification also caused a significant change in the structure of molluscan assemblages associated with Amphiroa turf, which included a 43% and a 61% reduction in the species richness and overall abundance of molluscs, respectively. The results demonstrate that coralline algal turfs are particularly vulnerable to ocean climate change, which has implications for the biodiversity and ecosystem functions supported by these globally distributed foundation species.