Foodweb support for the threatened Delta Smelt: Summary of program objectives and preliminary results Anne Slaughter 1 and The CALFED Foodweb Research Team 1,2,3,4 1 Romberg Tiburon Center, San Francisco State University 2 U.S. Geological Survey, Menlo Park, CA 3 Department of Biology, Georgia Southern University 4 Department of Marine Sciences, University of Connecticut Preliminary Results This collaborative research program is underway to characterize the foodweb of the low salinity zone (LSZ) of the northern San Francisco Estuary (SFE). Recent evidence indicates that several species of estuarine fish, including Delta Smelt (and their copepod prey), may be food limited, suggesting a link between their declines and changes Water and plankton net samples were collected weekly at three salinities (0.5, 2 and 5 psu) from March 14 to August 23, The following data and samples were collected at each salinity: -water column salinity, temperature and clarity profiles (CTD with attached PAR sensor, Secchi disk) -surface water collection via bucket -plankton (ring) net tows Funding for this project was provided by CALFED Science Program Grant #SCI-05-C107 Sampling Program Introduction Acknowledgment Posters 1 R.A. Cohen, A.M. Slaughter, E.J. Carpenter. The effects of salinity on phytoplankton and dissolved organic carbon availability. 2 V.E. Hogue, A.E. Parker, R. Dugdale, A. Marchi, F. Wilkerson. The role of excess ammonium in reducing phytoplankton in the San Francisco Estuary. 3 T.R. Ignoffo, A.Gould, W.J. Kimmerer. Growth and development of Limnoithona tetraspina, the most abundant copepod in the estuary. 4 U.E. Lidstrom, A.M. Slaughter, R.A. Cohen, E.J. Carpenter. Phytoplankton production within the Low Salinity Zone. 5 A.E. Parker, V.E. Hogue, F. Wilkerson, R. Dugdale. Evaluating the potential contribution of bacterial carbon for higher trophic levels. 6 J.K. Thompson, F. Parchaso. Grazing potential of Corbula amurensis on lower trophic levels. 7 J.K. York, B. Costas, G. McManus, A.M. Slaughter, T. Ignoffo, W. Kimmerer. Microzooplankton dynamics in the Low Salinity Zone of the San Francisco Estuary. Oral Presentation W.J. Kimmerer. Foodweb support for the threatened delta smelt: Subtle interactions may be a cause of the pelagic organism decline. (Tuesday 1:10 PM, POD session) The CALFED Foodweb Research Team at lower trophic levels. Exogenous organic carbon likely provides important support to the LSZ foodweb. Our research examines the base of the foodweb, from phytoplankton and bacteria to copepods, in the LSZ. The data presented here resulted from the first year of a 2- year field sampling and experimental program that included weekly sampling cruises during two periods (spring and summer) when larval, juvenile and adult delta smelt are present. During this time, we focused on the potential influence of ammonium concentration on nitrate uptake (and therefore growth of phytoplankton), phytoplankton production and lysis (a potential source of DOC), bacterial production, microzooplankton and zooplankton grazing and production and abundance of and grazing impact by Corbula amurensis. San Francisco Estuary Suisun Bay San Pablo Bay Sacramento River San Joaquin River San Francisco 020 Kilometers Carquinez Strait CA sampling area psu Mar-Aug 2006 Delta Smelt (and other fish) Benthic grazing effects 6 Phytoplankton biomass rarely exceeded 10 µg L -1 unless Corbula amurensis grazing was <0.3 m 3 m -2 d -1 (Data based on analyses of DWR D7 bivalve samples and chl a values, shallow water of Grizzly Bay) Surface salinity (top; horizontal lines represent nominal salinities) and position of each station (bottom), including X2 (calculated distance, from the Golden Gate, of the 2 psu bottom isohaline). X2 and surface salinity diverged in spring, primarily due to collection of surface measurements when salinity was strongly stratified; X2 is also not well-predicted when less than ~55 km. Later in the season, the 2 psu distance and X2 were in closer agreement – an anomalously wet year Red lines indicate field sampling period for this project. Green numbers indicate rank of each month (in terms of monthly outflow); e.g., April 2006 was the wettest April since 1980, May 2006 was the third wettest May since 1980, etc. net Delta outflow (1000 m 3 s -1 ) Phytoplankton production 4 Mar Apr May Jun Jul Aug Several peaks in total chlorophyll biomass were observed in spring and summer 9% to 84% of primary production passed to microzooplankton grazers 43% Aug 22 71% Aug 15 70% Jul 18 84% Jul 11 17% May 09 20% May 02 9% Apr 18 38% Apr 11 42% Mar 21 21% Mar 14 grazing/ growth Microzooplankton grazing rate (day -1 ) Phytoplankton growth rate (day -1 ) Microzooplankton grazing 7 growth rate d -1 Copepod production 3 Limnoithona tetraspina is the most abundant copepod in the LSZ and had a growth rate of ~10% d -1 (mean of all juvenile stages) Effects of salinity on phytoplankton 1 More saline phytoplankton lysed when exposed to lower salinities, potentially releasing DOC for bacterial production Hours chlorophyll (µg L -1 ) Skeletonema spp. Saline (5 psu) cells  0.5, 2, 5 psu 5.0 psu 2.0 psu 0.5 psu Treatments Scenedesmus spp. Fresh (0.5 psu) cells  0.5, 2, 5 psu Nitrate concentrations decreased following phytoplankton peaks; nitrate concentrations were similar at all salinities Nutrients psu 2.0 psu 0.5 psu chlorophyll (µg L -1 ) nitrate (µM) Bacterial production 5 Free-living bacteria contributed 67-93% to bacterial production