The Delta and Suisun Marsh region is rich in biodiversity, offering numerous ecosystem services and significant cultural value for many communities. However, it has been increasingly impacted by droughts, which are expected to become more frequent.
At the May meeting of the Delta Stewardship Council, Delta Lead Scientist Dr. Lisamarie Windham-Meyers spotlighted an article titled “Dry Me a River: Ecological Effects of Drought in the Upper San Francisco Estuary,” which was published by the Interagency Ecological Program (IEP) Drought Synthesis Team in the San Francisco Estuary and Watershed Science journal, which examines the impacts of droughts and explores the mechanisms driving these changes.
The Drought Synthesis Team used 40+ years of historical data and analyzed them with models and regressions to look at patterns during drought events.
There are several types of drought, two of which are meteorological drought and hydrological drought. Meteorological drought occurs when dry weather patterns prevail in an area, whereas hydrological drought manifests as low water supply in streams, reservoirs, and groundwater levels, typically following prolonged periods of meteorological drought. While meteorological drought can develop and dissipate quickly, hydrological drought tends to take much longer to emerge and recover.
The IEP Drought Synthesis Team developed a conceptual model to illustrate how drought impacts the upper San Francisco estuary. According to this model, freshwater flow is the primary factor determining the duration that water stays in a specific location (known as water residence time) and other water transport dynamics. During drought conditions, reduced flows result in less water exchange between locations, leading to longer water residence times. These diminished flows adversely affect habitat connectivity, which is essential for numerous organisms and the exchange of nutrients between aquatic and terrestrial systems.
The majority of food-web responses in the conceptual model could be attributed to the effects of drought on water movement patterns and by drought-associated increases in salinity, changes in water quality, or variations in the top-down effects of increased predation and increased grazing rates on phytoplankton and zooplankton.
Their models also highlighted the importance of region-specific responses to drought when assessing impacts, given the complexity and dynamics of the Delta. For example, in the South Delta, droughts were associated with increased water clarity, salinity, nutrient levels, chlorophyll concentrations, and zooplankton abundance. In contrast, Suisun Bay experienced decreased Delta outflow, reduced water exports, and lower abundances of zooplankton and certain fish species under drought conditions.
“For example, the graphic shows that the phytoplankton is increasing in the south-central region, but it’s actually decreasing in Suisun Marsh under these drought conditions,” said Dr. Windham-Meyers. “The reason for this is the longer residence time in the south-central area that is leading to greater phytoplankton blooms and greater zooplankton availability at the base of the food web.”
“But you can see by the little circle there, too, that it’s not the same phytoplankton and zooplankton. Things are changing because of salinity increases in the south Delta and also in Suisun Marsh. The point here is that the spatial responses are so profound that we need to start talking about these regions in a very clear way as having different responses to drought.”
ARTICLE: Dry Me a River: Ecological Effects of Drought in the Upper San Francisco Estuary
By: Rosemary Hartman, Elizabeth B. Stumpner, David H. Bosworth, Amanda Maguire, Christina E. Burdi, Interagency Ecological Program Drought Synthesis Team
Abstract: Droughts have major effects on estuaries because freshwater entry is one of the defining features of an estuary, and freshwater f low is an important variable that determines the interannual change in the environment. In the upper San Francisco Estuary (the Sacramento–San Joaquin Delta, Suisun Bay, and Suisun Marsh), the Mediterranean climate includes frequent multi-year droughts. To assess ecosystem responses to droughts in the upper estuary, the Interagency Ecological Program Drought Synthesis Team assembled a set of f low, water quality, chlorophyll, zooplankton, and fish data from 1975 to 2021 to test for differences between multi-year droughts and multi-year wet periods and tested for linear relationships between each variable and the Sacramento Valley Hydrologic Index (see definitions and relationships as outlined in Appendix A). Our models showed droughts decreased Delta outf low, project exports, zooplankton in Suisun Bay, and some fish species. We also found that droughts increasedwater clarity, salinity, nutrients, chlorophyll in the South Delta, zooplankton in the South Delta, and water residence time. Although our analyses only tested correlations, we hypothesized that most of the food-web responses could be traced to increased residence time, decreased transport rates, or both. However, some responses may have been caused by secondary effects, including shifts in salinity gradients, regional changes in water quality, or differences in top-down effects of increased predation and grazing rates. With increased frequency of droughts in the future, this increasingly low-outf low, warming, clearing estuary—which is invaded by non-native species and has low pelagic fish production—is rapidly becoming the new “normal.”
OTHER DELTA SCIENCE NOTES
The Draft Cyanobacteria Harmful Algal Bloom Monitoring Strategy was presented at the Delta Plan Interagency Implementation Committee, which endorsed the plan. The meeting included presentations from the State Water Board and the Department of Water Resources. Staff is currently working on integrating comments received during the review period into a final strategy to be released this summer.
The Delta Science Program has just completed a peer review for the Draft Fish and Aquatic Effects Analysis for the Long-Term Operations of the Central Valley Project and State Water Project. The review evaluated the analytical approach used to assess how the operations of the water projects in the Delta affect the aquatic environment and the exposure, response, and risk to selected species and whether quantitative and qualitative methods and risk assessment tools are used appropriately. The findings will inform the NEPA documents and the biological assessment. The final review documents can be found here.
