A study by UC Davis researchers found a 97% decline in phytoplankton, the microscopic foundation of the food chain. “Understanding the causes for the decline in the pelagic [water column] community is essential so that efficient solutions can be implemented,” says Bruce Hammock, a research scientist at the UC Davis School of Veterinary Medicine’s Aquatic Health Program. The invasive clams (Potamocorbula amurensis), originally from Asia, have been over-consuming phytoplankton and zooplankton for more than 30 years, and have long been understood to account for part of the fish population’s decline; the new study investigated the additional effects of exports. Beginning in the 1940s, fresh water from the Delta has been pumped by the federal Central Valley Project and the State Water Project to more than 20 million residents and millions of acres of farmland throughout California. The pumped water is unfiltered and includes the tiny plankton, dramatically reducing the amount of foundational food available to support life in the Estuary. Scientists associated with the study agree that finding ways to control the non-native clams will be essential to fixing the problem; the new findings also suggest that changes to the pumping schedule might be needed accommodate the needs of the estuary’s food chain—less pumping in the late summer and fall, for example, when water levels are lower and food demand is high. But Hammock thinks the idea should be tested before any real changes are made. “I’d like to see an experiment with moderate inflow [from the Sacramento and San Joaquin Rivers] and a substantial reduction in exports to test whether phytoplankton abundance increases,” he says

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Invasive clams and freshwater exports from the Delta have created dramatic and unsustainable changes in the San Francisco Estuary's foodweb over the past 50 years.

 A study by UC Davis researchers found a 97% decline in phytoplankton, the microscopic foundation of the food chain. “Understanding the causes for the decline in the pelagic [water column] community is essential so that efficient solutions can be implemented,” says Bruce Hammock, a research scientist at the UC Davis School of Veterinary Medicine's Aquatic Health Program. The invasive clams (Potamocorbula amurensis), originally from Asia, have been over-consuming phytoplankton and zooplankton for more than 30 years, and have long been understood to account for part of the fish population’s decline; the new study investigated the additional effects of exports. Beginning in the 1940s, fresh water from the Delta has been pumped by the federal Central Valley Project and the State Water Project to more than 20 million residents and millions of acres of farmland throughout California. The pumped water is unfiltered and includes the tiny plankton, dramatically reducing the amount of foundational food available to support life in the Estuary. Scientists associated with the study agree that finding ways to control the non-native clams will be essential to fixing the problem; the new findings also suggest that changes to the pumping schedule might be needed accommodate the needs of the estuary's food chain—less pumping in the late summer and fall, for example, when water levels are lower and food demand is high. But Hammock thinks the idea should be tested before any real changes are made. “I'd like to see an experiment with moderate inflow [from the Sacramento and San Joaquin Rivers] and a substantial reduction in exports to test whether phytoplankton abundance increases,” he says

About the author

Ashleigh Papp is a science writer based in San Francisco. She has a background in animal science and biology, she enjoys writing about emerging environmental issues, our oceans, and conservation-related science. For ESTUARY, she often covers wildlife. When not reading or writing, she's playing outside with friends or inside with her cat, Sandy.