Mesozooplankton of the lower San Francisco Estuary: spatio-temporal patterns, ENSO effects and the prevalence of non-indigenous species

Abstract

The San Francisco Estuary (SFE) is a large, highly dynamic and ecologically important estuary on the west coast of the USA. We collected zooplankton and hydrographic data over a 3-year period (1997–1999) at six stations spanning the lower SFE to investigate long-term changes through comparison with a 1980– 1981 survey of the area, El Nino-Southern Oscillation (ENSO) effects and the prevalence of non-indigenous species. Spatio-temporal patterns in the zooplankton were primarily regulated by seasonal and inter-annual fluctuations in marine and riverine forcings. Non-metric multidimensional scaling accounted for 82.7% of the variation in zooplankton composition. The strongest correlates of the three axes were salinity (r 1⁄4 0.692), distance to marine influence and temperature (r 1⁄4 0.492 and 0.309, respectively) and a lagged multivariate ENSO index (r 1⁄4 0.299). Five zooplankton communities were identified, which showed predictable spatiotemporal progressions through the lower estuary. Inter-annual differences between El Nino and La Nina periods were substantial and appeared to be related to variable strengths of riverine and oceanic forcing. Between 1980–1981 and 1997– 1999, a dramatic change occurred in the zooplankton of the lower SFE, such that dominance has shifted from native calanoid copepods to non-indigenous (Asian) cyclopoids. These changes in the zooplankton are expected to have significant but as yet unknown impacts on higher tropic levels and food web dynamics in the lower SFE.