Phytoplankton ecology of the San Francisco Bay system -the status of our current understanding

Although past studies of phytoplankton dynamics in the San Francisco Bay system are limited in number and scope, they have provided sufficient information to define gross spatial and temporal patterns. Annual changes in the density and composition of phytoplankton populations differ among major geographic areas within the system, and recent studies suggest that phytoplankton dynamics in each major portion of San Francisco Bay are governed by a unique set of environmental factors. The annual maximum abundance of phytoplankton in central San Francisco Bay during spring may be a direct consequence of diatom blooms that occur in coastal waters during the upwelling season. The spring maximum of phytoplankton abundance in South Bay may also result from the dispersion of neritic diatoms from offshore during some years, although the 1978 spring maximum resulted from rapid in situ growth of microflagellate populations. Apparently, stratification of the South Bay water column (initiated by movement of Delta-derived low-density water from the northern reach) creates a shallow surface layer where flagellates are given sufficient solar irradiation to maintain rapid growth rates. Phytoplankton populations in the northern reach of San Francisco Bay apparently are most strongly regulated by the physical accumulation of suspended particulates by gravitational circulation, the rapid growth of planktonic algae over shoals, and phytoplankton dynamics in coastal waters and/or tributaries. Because few research efforts have been implemented to define environmental factors that regulate phytoplankton dynamics, basic unanswered (or unasked) questions remain. There is need (1) to define those functional groups of plank tonic algae responsible for fixing inorganic carbon and energy, and then to follow pathways of energy and material transfer from the phytoplankton to other trophic levels, (2) to define the relationships between the physics of water movement and phytoplankton dynamics, and (3) to identify those physical-chemicalbiological factors most responsible for regulating phytoplankton population size and composition, and then to quantify the response of algal population growth to changes in these important environmental factors. The quality of estuarine waters is reflected in and a consequence of the phytoplankton community because the density (i.e. abundance) and composition of plankton populations both respond to environmental stress and can, in turn, cause environmental stress. The species composition and population density of phytoplankton are sensitive to environmental changes, and continual documentation of phytoplankton population dynamics can provide an invaluable record of water quality, can signal if radical changes occur within an estuarine system, and can offer clues to the causes of changes when they do occur. Planktonic algae affect concentrations of dissolved gases (oxygen and carbon dioxide), concentrations of dissolved inorganic and organic substances, and affect pH. Finally, the photosynthetic fixation of inorganic carbon by phytoplankton offers a source of organic carbon and energy for higher trophic levels and ultimately determines the success