Ocean acidification influences the biomass and diversity of reef-associated turf algal communities

Projected ocean acidification will have a detrimental impact on coral reef ecosystems, where fleshy algae are expected to replace corals. Of particular importance to reef ecosystems are fleshy turf algal communities, which have the potential to overgrow corals; few studies have investigated the community structure and diversity of turfs to climate change. Here, we assessed the response of reef-associated turf algal communities from the Great Barrier Reef, Australia to three levels of ocean acidification. Biomass of turf communities was positively affected by increases in carbon dioxide (CO2), where turf communities grown under high CO2 had the greatest biomass. No effect of CO2 was found on mean turf organic content or genus richness. By contrast, turf community evenness and diversity (H') increased under medium and high CO2 treatments. Our results indicate that increased turf growth under high CO2 will aid the overall expansion and growth of fleshy macroalgae in coral reef ecosystems, as opportunistic algae may have an advantage over other reef-associated species. Changes in turf community diversity will help provide insight into how macroalgal communities may be structured in the future, highlighting genera primed to take advantage of the changes in ocean chemistry associated with ocean acidification.