Warming and acidification promote cyanobacterial dominance in turf algal assemblages

As marine ecosystems are predicted to be facing increases in both temperature and CO2 levels, resulting in increased acidity, scientific research is attempting to predict the effect that the altered environmental conditions will have on species, communities and, ultimately, ecosystems. This study focused on elevated temperature and ocean acidification effects on the ubiquitous, yet often overlooked, turf algal assemblages on the Great Barrier Reef, Australia. In order to assess possible changes in relative abundance of species, biomass and productivity, 3 different levels of pH and 2 different levels of temperature treatments were applied to shallow water turf algal assemblages in a multifactorial (orthogonal) experiment. The eukaryotic component of the algal assemblages showed differential responses to combinations of pH and temperature treatments. The response was however dominated by a strong increase in the relative abundance of Lyngbya, a cyanobacterium, under acidification and higher temperature treatment levels, while other cyanobacteria, such as the Rivulariaceae, did not respond to the altered environment. Possible explanations for this observation may be differences in nitrogen fixation capacity and/or temperature optima. The biomass of the algal assemblages remained stable under all treatment levels, while changes in productivity associated with an interaction of the factors were observed. The findings of this experimental study highlight the complexity of turf algal assemblages in their composition and in their response to altered environmental conditions. However, they also support the dominant theoretical physiological predictions for eukaryotic and prokaryotic turf algae that suggest a positive or neutral response to future environmental conditions.