Spatial Distribution of Seawater Carbonate Chemistry and Hydrodynamic Controls in a Low-Inflow Estuary

Abstract:

Estuaries are vulnerable to ocean acidification due to their often-reduced buffering capacity and eutrophication. Morro Bay is a short and seasonally hypersaline, low-inflow estuary located on the central California Coast that recently experienced a rapid collapse of eelgrass, the major biogenic habitat. Compared to classic estuaries, the impact of acidification and the mechanisms driving the distribution of carbonate parameters in low-inflow estuaries are not well understood. Here, we combine physical and chemical seawater analysis to better understand in situ modifications to waters in a low-inflow estuarine environment and consider the impact on the local ecosystem. Sampling transects were completed in the main channel of Morro Bay in June, August, and September, bracketing both a high and low tide on each date, to characterize the spatial distribution of carbonate chemistry and hydrodynamic parameters during the dry, low-inflow season. Temperature, salinity, alkalinity, and dissolved inorganic carbon (DIC) all increased from the mouth to the back of the estuary, with larger values observed during the low tide. pH values decreased towards the back of the bay, but there was little variation in pH and pCO₂ between high and low tide, despite the increase in DIC, indicating that the back bay waters were well buffered. High alkalinity and DIC in the back bay were not fully explained by evaporation-driven increases in salinity. However, a strong correlation between temperature and alkalinity provides evidence that increased flushing time (estimated using a salt-budget model approach) could largely account for the modification. The physical decoupling of the mouth and back bay, as indicated by significantly longer flushing times in the back bay, likely drove the observed modification of alkalinity and DIC. This, in turn, buffered the estuary from large changes in pH and pCO₂ that might otherwise be expected due to photosynthesis and respiration. It is expected that a recovery of eelgrass in the back bay would lead to decreases in DIC and increases in estuarine pH, altering observed spatial distributions. The unique hydrodynamic exchange in seasonally low-inflow estuaries and its potentially large role in influencing local carbonate chemistry and ocean acidification warrants further study.