1. Foodweb support for the threatened Delta smelt: Microzooplankton dynamics in the low salinity zone of San Francisco Bay J.K. York 1, B. Costas 1, G. McManus 1, A. M. Slaughter 2, T. Ignoffo 2, W. Kimmerer 2 1 Department of Marine Sciences, University of Connecticut, Groton, CT 2 Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA In summer microzooplankton grazing presented a strong “top-down” control on phytoplankton biomass. Copepod grazing pressure decreased the growth rate of microzooplankton. The decrease in microzooplankton did not result in the expected increase in phytoplankton or bacterial biomass. This may be because of a more complex food web, with multiple food items making up the diet of P. forbesi. Future work will focus on a broader understanding of the contribution of microzooplankton to copepod diets, in an effort to determine whether copepods (the preferred food for Delta Smelt), may be food limited. Results Plot shows results from 2 (of 10 total) representative sampling dates. Phytoplankton growth rates (y-intercept) ranged from 0.19 to 1.97 doublings per day, increasing from spring to summer. Microzooplankton grazing rates (slope) ranged from 0.04 to 1.66 per day, with a general trend towards higher rates in summer. In Spring 9-42% of phytoplankton primary production was grazed by microzooplankton; in Summer up to 84% was grazed. Our work is part of a collaborative research program to characterize the foodweb of the low salinity zone (LSZ) of the northern San Francisco Estuary (SFE) to assess the potential for food limitation of Delta Smelt. Our focus is the role of microzooplankton (20-200µm) which are the main consumers of phytoplankton in most estuarine systems. In addition, they are part of the “microbial loop” by which dissolved organic matter is included in the food web. Dilution Experiments -- Determine the effect of dilution of filtered Bay water on phytoplankton net growth rates in bottle incubations. Increased growth in more dilute treatments indicates release of phytoplankton from microzooplankton grazing pressure. Trophic Cascade Experiments -- Determine the cascade of effects of copepod grazing on food web by experimentally manipulating copepod concentrations (low, medium, high). Lower microzooplankton growth rates (due to copepod grazing) are expected to result in increases in phytoplankton biomass due to release from grazing pressure. Additional work - determine microzooplankton abundance by microscopic enumeration - assess microzooplankton bactivory by experiments with fluorescently labeled bacteria The authors wish to thank Captain David Morgan and David Bell for their assistance aboard R/V Questuary, as well as the Captain and crew of R/V Polaris. Funding for this project was provided by CALFED Science Program Grant # SCI-05-C107. Conclusions and implications Materials and methods Introduction Acknowledgments For more information, email: joanna.york@uconn.edu Further information San Francisco Bay and Delta Suisun Bay San Pablo Bay Sacramen to River San Joaquin River San Francisco 020 Kilometers Carquinez Strait CA sampling area 0.5-5 psu Mar-Aug 2006 Microzooplankton Herbivory Trophic Cascade Experiments Research Questions: 1)How does microzooplankton grazing impact phytoplankton (herbivory)? 2) How does microzooplankton grazing impact bacteria (bactivory)? 3) How does copepod grazing impact microzooplankton, and how does this affect lower trophic levels? net growth rate (day -1 ) bacteria microzooplankton phytoplankton apparent growth rate (day -1 ) Copepod grazing decreased the growth rate of microzooplankton. The decrease in microzooplankton abundance had no effect on lower trophic levels (phytoplankton, bacteria). This preliminary evidence suggests that P. forbesi may also graze on phytoplankton. July * April** Pseudodiaptomus forbesi (L -1 ) dilution