California Sea Grant College Program

Based on an analysis of a 37-year time series, zooplankton biomass and species composition appear to have changed profoundly in Suisun Bay and Sacramento-San Joaquin Delta, in response to invasive species introductions and hydrological conditions. Results are consistent with other studies linking a majority of species interactions in the upper San Francisco Estuary to non-native invasions. The Asian clam and several exotic zooplankton species appear to have capitalized, directly and indirectly, on the long drought from 1987 to 1994 and exacerbating water management practices. During this period of sustained high-salinity conditions, larger native copepods – the preferred prey of larval fishes – were replaced by smaller Asian zooplankton, notably Limnoithona tetrapsina, likely introduced via ballast water. In addition to the new species, total zooplankton biomass dropped significantly during the last four decades; however, the pace and timing of the decline does not explain the sudden collapse of pelagic fishes around 2002. While the shear abundance of smaller non-native zooplankton appears to have offset the loss of biomass from larger native zooplankton, it is plausible, even likely, that the new plankton community has degraded food quality for larval fishes. The Delta Science Fellow is currently exploring this idea and its link to larval fish survivorship.