Biogeochemical evidence of flow re-entrainment on the main fringing reef of La Reunion Island

Understanding factors influencing seawater chemistry variability in coral reef environments is a major challenge to improve predictions of their evolution in the context of ocean acidification. In this study, autonomous sensors for current speed and direction, photosynthetically active radiation (PAR), temperature, salinity, dissolved oxygen (DO) and pH_T were deployed three times between 2021 and 2022 offshore and on three reef flat sites of the main fringing reef of La Reunion Island. Discrete sampling of seawater for DO, pH_T and total alkalinity (TA) at different times of the day complemented the monitoring. Diurnal variation of those variables on the reef flat was mainly driven by benthic community metabolism but hydrodynamics and reef geomorphology played also a key role. DO and pH_T variations were decoupled in time, especially at night when we observed DO rebounds while pH_T values were stationary. The reef flat was also largely TA depleted compared to offshore waters. We hypothesize that the strong offshore-alongshore current redirected TA depleted water exiting from reef channels into the reef flat. This seawater re-entrainment could also explain specific variations of DO and pH_T values. This result highlights the important role of reef geomorphology in modulating changes in seawater chemistry. Neglecting this phenomenon could lead to substantial errors in the estimation of carbonate budgets when using the Eulerian approach.