Diel Behaviors of Zooplankton Interact with Tidal Patterns to Drive Spatial Subsidies in the Northern San Francisco Estuary

Spatial subsidies and habitat connectivity are critical factors in estuarine trophic webs. Advection and tidal dispersion of organic matter including plankton from productive regions such as wetlands can subsidize consumers in less productive areas. These dispersive fluxes have generally been assumed to result from tidal mixing along concentration gradients, but other mechanisms of dispersion may be important. We estimated fluxes of the calanoid copepod Pseudodiaptomus forbesi between a restored marsh and a connected channel in the northern San Francisco Estuary in summer 2018 using continuous flow data and hourly abundance data over four tidal cycles. Late copepodites and adults were demersal, abundant in the water column only at night, and abundance was uncorrelated with tidal flows. Over the tidal day, dispersive fluxes of copepods were variable. However, over the entire summer, tidal flows were flood dominant at night when the copepods were in the water column, driving an estimated dispersive flux into the marsh. Dispersion at the marsh will change seasonally as tidal patterns and copepod abundance change. Our results show that the transport of zooplankton in shallow tidal systems is regulated by the interactions of diel zooplankton behavior with long-term tidal patterns. Similar interactions in other systems will result in transport based on site-specific hydrodynamics and zooplankton behavior, and could move zooplankton up the concentration gradient rather than down. Patterns of zooplankton behavior and currents occur on a wide variety of time scales; thus, researchers must take a long-term perspective to understand these interactions.