Effects of seagrass presence on Porites spp. physiology under ocean acidification conditions

Abstract

Ocean Acidification (OA) refers to the unprecedented rate of ocean pH decrease resulting from anthropogenic CO2 emission. This lowers the saturation state of minerals utilized by calcifying organisms, making their skeletons tend to dissolve rather than form. This phenomenon threatens the viability of valuable ecosystems such as coral reefs, which are in a state of global decline. Seagrass meadows often found adjacent to coral reefs are mostly net autotrophic, utilizing the forms of inorganic carbon created during OA. This lends the possibility of locally mediating OA effects on corals downstream of seagrass meadows. This study tested the experimental impacts of the presence of seagrass Halodule wrightii on carbonate chemistry and Porites spp. physiology at ambient pH 8.1 and acidified pH 7.7. Results show the potential for increase in pH up to 0.116 units, and Ωarag increases up to 1.02 with the presence of seagrass. After 2 weeks, Porites spp. respiration rates were significantly higher with seagrass presence (p<0.05). Photosynthesis rates were not significantly different. Light calcification was significantly lower at at pH 7.7 vs. pH 8.1 (p<0.05). Dark calcification was also significantly lower at pH 7.7 vs. pH 8.1 (p<0.05), as well as with the combined effect of pH and seagrass presence (p=0.05). These results are counterintuitive to the expectations of the theoretical models, and indicate that seagrass presence in close proximity to Porites spp. may negatively impact calcification, potentially serving as a competitor in an environment with limited resources.