This case study describes several assessment endpoints examined in San Francisco Bay, including dissolved oxygen; turbidity and light; chlorophyll, primary production, algal blooms, and species composition; and benthic primary producers such as seagrass. All these assessment endpoints are affected by the loading and concentration of causal variables nitrogen and phosphorus and the quality and quality of algal biomass that result from nutrient levels. Algal biomass, in turn, affects many estuarine conditions including hypoxia in bottom waters and water clarity of the water column.
San Francisco Bay is divided into two estuaries: (1) The North Bay is the tidal estuary of the Sacramento and San Joaquin rivers, and is primarily surrounded by agricultural land in its watershed; and (2) The South Bay is a marine lagoon in the densely populated urban watershed between Oakland, San Francisco, and Silicon Valley. The North Bay of San Francisco Bay receives high nutrient loads from agricultural lands while the South Bay receives its nutrient loads from urban wastewater discharge. Both bays receive high loads of suspended solids in the wet season. The water column is typically mixed by wind and tidal currents and does not experience persistent stratification. Residence time varies from days in the North Bay during large floods to months in the South Bay during the dry season. High turbidity continues in the dry season as the result of tidal and wind-driven currents.
San Francisco Bay receives high inputs of nutrients and suspended sediments from urban and agricultural sources in the watershed. As a result of those inputs in combination with shallow depths and continual resuspension, San Francisco Bay has high concentrations of suspended sediments and high turbidity. Those conditions limit phytoplankton production and eliminate submerged aquatic vegetation. Phytoplankton biomass in San Francisco Bay is dominated by diatoms, and spring blooms routinely occur. There are, however, no documented harmful algal blooms.
In the 1970s, the San Francisco Bay routinely experienced large algal blooms and seasonal or episodic hypoxia. Pursuant to the implementation of advanced wastewater treatment in the watershed, however, chlorophyll a concentrations have trended downward and occurrences of hypoxia are much less frequent and in some areas, have been eliminated. One of the consequences of lower primary productivity is that there has been a decline in zooplankton and planktivorous fish stocks.
Water quality stations established by various research groups are located throughout San Francisco Bay and are used to collect nutrient, Secchi disk, temperature, DO, and other data. Data are used to examine spatial and temporal trends and relationships between parameters such as the relationship between nutrients and salinity (National Estuarine Experts Workgroup 2010).
Reference:
National Estuarine Experts Workgroup. 2010. Nutrients in Estuaries: A Summary Report of the National Estuarine Experts Workgroup 2005–2007. U.S. Environmental Protection Agency. Accessed October 2016. https://www.epa.gov/sites/production/files/documents/nutrients-in-estuaries-november-2010.pdf.