Enhanced Regional Variability of Seawater pCO2 Under Human and Natural Interference in a Nearshore Coral Nature Reserve (Dongshan Island, China)

Abstract

AbstractCO2 signatures differ among locations, even within the same coastal habitat. However, the driving mechanism behind this variation is not well understood due to the presence of both human and natural stresses. This study examines seawater pCO2 and its deviation from the regional average (defined as pCO2 anomaly) during spring, summer, and winter at two contrasting zones (trial and core zones) of Dongshan coral habitat. The impact of solar, tidal, and biological cycles on pCO2 anomaly is relatively small under the designed sampling scheme. Results show that there is a large positive pCO2 anomaly in the turbid water of the trial zone during all studied seasons, while the large negative pCO2 anomaly in the core zone only occurs during the summer upwelling season. This indicates that CO2 signatures among locations may have seasonality. The physical and biological contributors to pCO2 anomaly are then quantitatively revealed based on a first‐order Taylor decomposition and biological carbon metabolisms. The results suggest that organic carbon metabolism of marine phytoplankton dominated pCO2 anomaly in all three seasons, while inorganic carbon metabolism also enlarged pCO2 anomaly, especially during summer. Physical contributions were minor. Moreover, significant linear correlations between turbidity (related to local human activities), chlorophyll (related to coastal upwelling) and changes in dissolved inorganic carbon (induced by organic carbon metabolism) indicate human and natural influences on bioprocesses, which may enhance pCO2 anomaly. Finally, the Dongshan coral habitat was estimated as a source of atmospheric CO2, with an annual efflux of approximately 50 t C.