Biological regulation of pH during intensive growth of phytoplankton in two eutrophic estuarine waters

Estuarine inorganic eutrophication results in high biomass of phytoplankton and elevates pH in the surface layer. However, its effects on the carbonate system are not well understood. We conducted incubation experiments using low-, moderate- and high-salinity waters enriched with nutrients in Delaware Bay and the Pearl River Estuary to examine effects on the carbonate system. The results showed that as phytoplankton grew, pH increased and dissolved inorganic carbon (DIC) decreased. However, the decrease in DIC was not fully accounted for by the production of organic carbon. This indicates that some C was missing during high phytoplankton growth. The missing C appeared to depend on the water buffering capacity, as it was largest when pH was highest in the low-salinity estuarine water. Total alkalinity (TA) decreased more in the low-salinity water with nutrient additions in both estuaries. We proposed that carbonate precipitation was formed at high pH driven by high phytoplankton growth, which is confirmed by a model simulation of TA. The model simulation based on the stoichiometric ratio of -Delta 106DIC: Delta 17TA demonstrated the difference between simulated and observed TA. The difference disappeared when carbonate precipitation formation was considered in the model, indicating carbonate precipitation formation could account for the missing C, the decrease in TA and the increase in pH. We concluded that the mechanism of CO2 being released from carbonate precipitation due to bloom-induced high pH benefits further photosynthesis, thus enhancing the biological pump efficiency in estuarine and coastal ecosystems.