Coral-algal interactions are one of the most fundamental aspects of coral reef ecology, as their relative abundance is critical to the structure and function of entire ecosystems. As coral reefs degrade under global anthropogenic stressors, the interspecific interactions between coral and macroalgae are of particular importance because altered competitive dynamics are likely to drive ecosystem shifts. Even so, species interactions are generally ignored in assessments of climate change impacts. In order to gain a more complete understanding of how coral reef communities are influenced by global anthropogenic stressors, we investigated how the ecophysiology of a common coral-algal interaction is influenced by anticipated end-of-the-century ocean warming and acidification conditions. Specifically, we examine the effects of ocean warming and acidification on the survivorship, calcification, and productivity of the common coral-algal interaction between Acropora intermedia and Halimeda heteromorpha over 8 weeks in two austral seasons.
Our results reveal no evidence of contact-induced physiological inhibition, even under the combined stressors of ocean warming and acidification. Both A. intermedia and H. heteromorpha calcification rates were significantly higher under present day conditions in the summer. Peak H. heteromorpha growth occurs when A. intermedia needs the most respite from high light and temperatures, with the increase in H. heteromorpha biomass likely influencing light conditions through shading and resulting in a 37% increase in A. intermedia productivity during the summer. The combined stressors of ocean warming and acidification, however, acted synergistically in the summer, causing decalcification and a significant decline in A. intermedia and H. heteromorpha calcification rates, respectively. Furthermore, A. intermedia experienced significant coral bleaching and mortality due to a reduction in endosymbiotic dinoflagellate densities, a decline in productivity, and a depletion of available host protein content. Although the interactions between coral and macroalgae are often seen to involve negative competitive interactions, the results of our study provide novel insights into how species interactions can modify local environmental conditions through indirect yet positive mechanisms. These positive effects, however, are not enough to mitigate the effects of ocean warming and acidification projected to occur by mid- to late- century.