In the spring of 2020, Regional San staff studied how environmental conditions can affect phytoplankton living in Delta river channels. Many factors can influence the amount of phytoplankton occurring in Sacramento–San Joaquin River Delta, including nutrient concentrations, water clarity, water flow rates, and grazing by clams and zooplankton. One method to investigate the relative importance of these factors is by measuring how phytoplankton respond to a dramatic change in environmental conditions, such as nutrient concentrations, water clarity, or flow rate. The United States Bureau of Reclamation owns and operates the Delta Cross Channel Gates, which allows water from the Sacramento River to flow through a large constructed channel into the Mokelumne River and South Delta. Regional San conducted an experiment measuring phytoplankton abundance and water quality parameters before, during, and after the Delta Cross Channel Gates were closed for four consecutive days. This experiment tested how reduced river flow rates (or increased water residence time) can affect phytoplankton living in Delta river channels. We used a kayak to collect samples downstream of the Delta Cross Channel gates and at other selected locations in the northeast Delta for comparison, including the Sacramento River, Snodgrass Slough, Georgiana Slough, North Fork Mokelumne River, and South Fork Mokelumne River.
Our hypothesis was that phytoplankton concentrations downstream of the Delta Cross Channel gates would gradually increase after the gates were closed, relative to phytoplankton concentrations in the lower Sacramento River or Georgiana Slough, where river flow rates would remain similar throughout the experiment. Our samples are currently being analyzed to determine if phytoplankton biomass, phytoplankton taxonomic composition, or nutrient concentrations changed during the experimental period. Findings from this study will inform water operations, shallow water habitat restorations, and nutrient management programs, by helping to identify the combination of environmental factors that regulate phytoplankton abundance in the Delta.