Evaluating Phytoplankton Productivity Outcomes to Managed Flow Alteration Actions in the San Francisco Estuary-Delta

Frances Wilkerson, San Francisco State University, San Francisco, CA, United States; San Francisco State University, Estuary and Ocean Science Center, San Francisco, CA, United States, Sarah Blaser, San Francisco State University, Tiburon, CA, United States, Richard C Dugdale, San Francisco State Univ, Tiburon, CA, United States, Jessica Wilson, San Francisco State University, Tiburon, United States and Ted Sommer, California Department of Water Resources, Division of Environmental Services, Sacramento, CA, United States
Abstract:
The San Francisco Estuary-Delta has been highly altered over the last two centuries due to human activities. This urban system is managed to ensure water supply for California. This has been accompanied by significant declines in native fish, including the endangered Delta Smelt. Proposed causes for these declines include water diversions and food limitation. Even though SF Estuary-Delta has plentiful nutrients, phytoplankton blooms are rare and it has been described as a high nutrient-low chlorophyll ecosystem with negative consequences on the pelagic food web. Managing agricultural return waters to the North Delta has been proposed to promote primary productivity and increase phytoplankton biomass downstream and support the fish community. Following a natural agricultural flow pulse in a wet year (2011) there was a diatom bloom and in 2016 a managed action was conducted to release excess agricultural irrigation water that resulted in increased chlorophyll downstream. A concern was whether this increased biomass was associated with elevated primary productivity, so making it useful as a food subsidy. To determine the effects of agricultural flow releases on primary productivity, we measured carbon and nitrogen uptake rates and chlorophyll before, during and after the 2018 North Delta Flow Action, when there was a managed augmented flow pulse using water from rice-fields. Unexpectedly there was no major increase in chlorophyll downstream and productivity rates were low even though the water was less turbid than in a control year (2017) with no altered flow. However nutrient concentrations in the flow pulse in 2018 were low, suggesting insufficient nutrients for a bloom to occur downstream. These results indicate that, in future managed actions, the quality of the source water being used to increase flow needs to be evaluated and we are testing use of short-term grow-outs/bioassays using the agricultural flow water. In conclusion, a holistic approach to adaptive management practices in other manipulated or altered estuaries and deltas during restoration or conservation efforts should be considered.
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