The Role of Protistan Microzooplankton in the Upper San Francisco Estuary Planktonic Food Web: Source or Sink?

Abstract

Decline of native pelagic species in estuarine systems is an increasing problem, especially for native fishes in the San Francisco Estuary and Delta (SFE-D). Addressing these losses depends on understanding trophodynamics in the food web that supports threatened species. We quantified the role of microzooplankton (heterotrophic-mixotrophic protists <200 μm) in the food web of the upper SFE-D. We sampled protist plankton abundance and composition at two sites (Suisun Bay and Grizzly Bay) approximately monthly from February 2004 to August 2005 and conducted dilution experiments during spring and summer of both years in Suisun Bay. Heterotrophs dominated the protist community in Suisun Bay and Grizzly Bay, particularly in the <20 μm size range, and peaks in protistan microzooplankton biomass were associated with high phytoplankton biomass. In both years, microzooplankton grazing rates were high (0. 5-0. 7 day-1) during the spring and lower (̃0.2 day-1) during summer. Phytoplankton growth rates peaked in April 2004 (̃0.7 day-1) but were much lower (<0. 1 day-1) in spring 2005, despite relatively high abundance. Thus, microzooplankton grazing consumed as much as 73% of phytoplankton standing stock during spring and̃15% of standing stock during summer of both years. Combined with earlier results, we conclude that microzooplankton can be important mediators of carbon and energy flow in the upper SFE-D and may be a "source" to the metazoan food web. © 2011 Coastal and Estuarine Research Federation.