Freshwater Flow Affects Subsidies of a Copepod (Pseudodiaptomus forbesi) to Low-Salinity Food Webs in the Upper San Francisco Estuary

Abstract

Productivity of the food web supporting small pelagic fishes in the upper San Francisco Estuary is chronically low, and some of the native fish species are in a long-term decline. The low-salinity (oligohaline) zone (LSZ) is particularly depauperate in phytoplankton and zooplankton. Based on prior empirical studies, it is hypothesized that freshwater flow increases the subsidy of a key copepod prey species (Pseudodiaptomus forbesi) from its freshwater population center into the LSZ. We combined hydrodynamic and particle-tracking modeling with Bayesian analysis in a box-model approach to estimate the magnitude of this subsidy and its dependence on freshwater flow rates. Net gains of P. forbesi into the LSZ came mostly from freshwater, landward regions of higher copepod abundance. The subsidy increased with freshwater flow, a finding that supports previous empirical analyses. However, in the context of persistent drought and ongoing climate change, the levels required to achieve a detectable net gain may be difficult and costly to achieve.