Role of the Diffusion Boundary Layer in coral reefs exposed to extreme environmental conditions. A potential tool to cope with climate change

Recent research has suggested that the build-up of a Diffusion Boundary Layer (DBL) around the coral’s tissue can potentially create a buffer to ocean acidification and oxygen depletion. However, to date few studies have investigated the DBL’s potential role in supporting corals’ calcification and highlighted contrasting results. In this study I analysed pH and oxygen dynamics within the DBL of two populations of the reef-building corals A. tenuis, M. digitata and P. damicornis, originating from two sites with markedly different flow regimes, under a combination of slow/fast flow and ambient/low pH in indoor flumes. Furthermore, I attempted to determine the effect of water flow under different pH conditions upon corals' calcification, photosynthesis and respiration. Finally, I assessed in situ physiological responses of corals to changes in flow speeds through a crossed transplant experiment carried out in the lagoon of Bouraké, a naturally extreme and variable environment in New Caledonia. The results of the present study show that hydrodynamic conditions, characterising the environment in which coral grow, have a deep impact on the build-up of a DBL and, therefore, on coral’s physiology. Water flow seems to have different effects on metabolic processes, driving species-specific responses that can further differ among individuals within the same species. Slow flows leaded to significant increases in pH and oxygen concentration within the DBL of all three species, and in A. tenuis and M. digitata ameliorated the negative impact of ocean acidification, allowing corals to sustain higher calcification rates in slow flows than in fast flows. These findings suggest that slow flow habitats, where a DBL can form, may act as a potential refugia for some species of corals and thus the DBL must be considered as potential tool in the conservation of coral reefs under future climate change conditions.