Phytoplankton are suspended, single-celled plants (algae). The phytoplankton community is the largest component of living biomass in San Francisco Bay, and we measure the distribution of chlorophyll as a proxy of this biomass. Unlike salinity and TSS, chlorophyll is a nonconservative quantity -- it changes in response to processes of production and consumption within the Bay, as well as inputs and transports. Although nutrient concentrations are very high in San Francisco Bay, the Bay does not have the noxious or toxic blooms of algae that are observed in many other estuaries that receive large inputs of nutrients from waste and land runoff.

Description of Numbered Regions:

1. Bay waters usually have small populations of phytoplankton, with chlorophyll concentrations typically less than 5 mg/m³.
2. In the South Bay, the phytoplankton population has a period of explosive growth (a bloom) in March or April. During these blooms, the phytoplankton abundance (measured as chlorophyll concentration) increases more than ten times.
3. Prior to 1987, the North Bay had abundant phytoplankton in summer, with chlorophyll concentrations reaching 30 to 60 mg/m³. But these episodes of elevated summer biomass disappeared in 1987 after the Asian clam Potamocorbula invaded the estuary and began to consume phytoplankton cells as fast as they could reproduce.
4. The patterns of the spring phytoplankton blooms change from year to year, with large and prolonged blooms during years of exceptional river flow such as in 1995.