Foodweb support for the threatened Delta smelt:Foodweb support for the threatened Delta smelt:Microzooplankton dynamics in the low salinity zone of San Francisco BayMicrozooplankton dynamics in the low salinity zone of San Francisco Bay

J.K. YorkJ.K. York11, B. Costas, B. Costas11, G. McManus, G. McManus11, A. M. Slaughter, A. M. Slaughter22, T. Ignoffo, T. Ignoffo22, W. Kimmerer, W. Kimmerer22

1 1 Department of Marine Sciences, University of Connecticut, Groton, CTDepartment of Marine Sciences, University of Connecticut, Groton, CT2 2 Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CARomberg 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 moredilute treatments indicates release of phytoplankton from microzooplankton grazingpressure.

• Trophic Cascade Experiments -- Determine the cascade of effects of copepodgrazing on food web by experimentally manipulating copepod concentrations (low, medium, high). Lower microzooplankton growth rates (due to copepod grazing) areexpected to result in increases in phytoplankton biomass due to release fromgrazing 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 PabloBay

Sacramento River

San Joaquin

River

San

Francisco

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CarquinezStrait

CA

sampling area0.5-5 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?

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net g

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• 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