Hydrodynamic influence on coral physiology: insights into the role of diffusion boundary layers in mitigating ocean acidification effects

Recent investigations suggest that a diffusion boundary layer (DBL) surrounding coral tissue may provide a potential buffer against the negative effects of ocean acidification. However, comprehensive studies on the DBL's role in supporting coral calcification are lacking, resulting in conflicting outcomes. This research analyses pH and oxygen dynamics within the DBL of reef-building corals (Acropora tenuis, Montipora digitata and Pocillopora acuta) developed at sites with different flow regimes. These corals underwent controlled indoor flume experiments with varying flow rates and pH levels to assess the impact on calcification, photosynthesis, and respiration. A cross-transplantation experiment further evaluated physiological responses to flow velocity changes in the naturally extreme semi-enclosed lagoon of Bouraké (New Caledonia). The outcomes of this investigation elucidate the impact of hydrodynamic conditions on the establishment of a DBL and, consequently, on coral physiology. Water flow resulted as a determinant factor with distinct effects on metabolic processes, determining species-specific responses that further differentiate among populations within the same species. A slow flow leads to substantial pH increases within the DBL in all three coral species and, specifically in A. tenuis, it ameliorates the adverse impact of ocean acidification, allowing corals to maintain higher calcification rates than in fast flow conditions. Additionally, the in-situ experiment reveals that the growth of species can also be influenced by the flow conditions of their originating site, highlighting a potential adaptation of certain populations. Specifically, P. acuta corals originating from areas with slow flow exhibit a decrease in their growth when transplanted to fast flow areas. These findings suggest that slow flow habitats, conducive to the formation of a DBL, may mitigate the adverse impact of ocean acidification and thus act as potential refuges for select coral species. Consequently, the DBL emerges as a species-specific potential buffering solution for corals under future climate change conditions.