Sacramento River Phytoplankton Studies 2016


Some previous research on the Sacramento River found declining phytoplankton concentrations at downstream locations. The cause for the observed phytoplankton decline is not easily discernible because the losses do not appear to be driven by effluent-derived nutrient concentrations. While phytoplankton-consuming organisms, such as clams and zooplankton, are present in the river, they are not abundant enough to fully explain the declines. In 2016, Regional San staff and consultants conducted scientific and hydraulic modeling studies to evaluate the factors that might potentially limit phytoplankton growth within the Sacramento River, both upstream and downstream of the EchoWater Resource Recovery Facility. These studies consisted of the following:

  • Surveyed the river between Knights Landing and Isleton to estimate the abundance and diversity of phytoplankton, zooplankton, and clams along the river; changes in phytoplankton growth rates; differences in water chemistry; and other environmental factors
  • Modeled river flow to identify, for each survey location, the proportion of flow arising from different upstream sources
  • Conducted a controlled mesocosm experiment to determine the relative importance of clam grazing, zooplankton grazing, river-water sources, and nutrient concentrations on phytoplankton growth


Our study found that phytoplankton biomass was regulated by multiple factors in the lower Sacramento River, including dilution, light limitation, respiration, and clam and zooplankton grazing, with the importance of individual factors differing by location. Our findings emphasize the need for practitioners of restoration and management programs to evaluate multiple potential factors when attempting to enhance phytoplankton production in the Sacramento River and Delta.

This research is now published in the scientific journal San Francisco Estuary and Watershed Science and is available as “open source”:

Investigating Factors Contributing to Phytoplankton Biomass Declines in the Lower Sacramento River. 2023. Mussen, T.D., Driscoll, S., Cook, M.E., Nordin, J.D., Guerin, M., Rachiele, R., Smith, D.J., Berg, G.M., Thompson, L.C. San Francisco Estuary and Watershed Science, 21(1). Retrieved from×1287p 

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