Coral-macroalgal competition under ocean warming and acidification

Abstract

Competition between corals and macroalgae is frequently observed on reefs with the outcome of these interactions affecting the relative abundance of reef organisms and therefore reef health. Anthropogenic activities have resulted in increased atmospheric CO2 levels and a subsequent rise in ocean temperatures. In addition to increasing water temperature, elevated CO2 levels are leading to a decrease in oceanic pH (ocean acidification). These two changes have the potential to alter ecological processes within the oceans, including the outcome of competitive coral-macroalgal interactions. In our study, we explored the combined effect of temperature increase and ocean acidification on the competition between the coral Porites lobata and on the Great Barrier Reef abundant macroalga Chlorodesmis fastigiata. A temperature increase of +1 °C above present temperatures and CO2 increase of +85 ppm were used to simulate a low end emission scenario for the mid- to late 21st century, according to the Representative Concentration Pathway 2.6 (RCP2.6). Our results revealed that the net photosynthesis of P. lobata decreased when it was in contact with C. fastigiata under ambient conditions, and that dark respiration increased under RCP2.6 conditions. The Photosynthesis to Respiration (P:R) ratios of corals as they interacted with macroalgal competitors were not significantly different between scenarios. Dark calcification rates of corals under RCP2.6 conditions, however, were negative and significantly decreased compared to ambient conditions. Light calcification rates were negatively affected by the interaction of macroalgal contact in the RCP2.6 scenario, compared to algal mimics and to coral under ambient conditions. Chlorophyll a, and protein content increased in the RCP2.6 scenario, but were not influenced by contact with the macroalga. We conclude that the coral host was negatively affected by RCP2.6 conditions, whereas the productivity of its symbionts (zooxanthellae) was enhanced. While a negative effect of the macroalga (C. fastigiata) on the coral (P. lobata) was observed for the P:R ratio under control conditions, it was not enhanced under RCP2.6 conditions.