Implications of Shifting Stratification Dynamics on Phytoplankton Blooms in San Francisco Bay Under Future Climate Scenarios

Although south San Francisco Bay (SFB) is highly enriched in nutrients, it experiences low phytoplankton productivity relative to many other nutrient-enriched estuaries. SFB’s phytoplankton production is modulated by multiple factors, including strong tidal mixing, which limits vertical stratification and light availability in the system’s highly-turbid waters to short windows (days). Historically, SFB has not exhibited classic eutrophication conditions despite high ambient nutrient concentrations; however, a shift in drivers may change the stratification dynamics that governs the ecosystem balance. Extreme wet and dry seasons and extreme subseasonal storms are projected to increase in California, with a potential to shift hydrodynamic conditions, altering stratification and thus phytoplankton dynamics. These changing dynamics could lead to more phytoplankton blooms, harmful algal blooms, and low dissolved oxygen with larger spatial extent and higher temporal frequency. To evaluate the risk of potential future nutrient-related impacts in SFB, we compiled a 30-year daily dataset of forcing data. Using statistical and probabilistic methods, we examine the effect of these forcings on the Bay under various conditions through this dataset and hydrodynamic modeling. Both observations and model output are interpreted in the context of forecasted shifts in precipitation patterns, in which extremely wet water years are taken as a proxy for projected precipitation conditions. The effect of shifting precipitation patterns on stratification dynamics is also interpreted with respect to phytoplankton bloom frequency.