Chlorophyll is a measure of the biomass (abundance) of phytoplankton, the suspended microscopic algae that are the largest living component in San Francisco Bay. In South Bay, phytoplankton photosynthesis is the most important source of organic matter (food) for animals of the lower trophic levels (such as small crustaceans, worms, clams, and mussels). This 20-year series at USGS station 30 shows that the biomass of phytoplankton is highly variable.

Plot Description:
1. Each year there is a period of rapid growth of the phytoplankton population during spring, when chlorophyll concentration increases from about 1-2 milligrams per cubic meter to about 20 or 30 milligrams per cubic meter. This period of explosive population growth is called the spring bloom, which usually occurs in March or April.

2. The spring blooms are different each year. For example, the 1981 spring bloom was small, with peak chlorophyll concentrations of only 13 milligrams per cubic meter.

3. The spring blooms in 1983 and 1986 were very large, with chlorophyll concentrations reaching about 60 milligrams per cubic meter. These were both years of heavy precipitation and high river flow, so there is a connection between climate variability and phytoplankton population variability.