Vickie CALFED2006

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Foodweb support for the threatened Delta smelt: The Role of Excess Ammonium in Reducing Phytoplankton in San Francisco Estuary V. E. Hogue, A. E. Parker, R. C. Dugdale, F. P. Wilkerson, A. Marchi 1 Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 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 at lower trophic levels. These changes include reduced spring phytoplankton blooms that may be caused in part by the form of available nitrogen for growth. High levels of ammonium (of anthropogenic sources) may restrict phytoplankton growth on the ambient nitrate. The authors wish to thank Ulrika Lidstrom of the Carpenter lab for the chlorophyll-a collection. We would also like to thank Captain David Morgan and David Bell for their assistance aboard R/V Questuary. Funding for this project was provided by CALFED Science Program Grant # SCI-05-C107. Introduction Acknowledgments Literature cited Vickie Hogue:, Alex Parker:, Dick Dugdale:, Frances Wilkerson:, and Al Marchi: A pdf version of this poster and the above citations are available on the Dugdale-Wilkerson website: Further information Nitrate (µM) Ammonium (µM) ρ15NO3 or ρ15NH4 (µmoles L-1 h-1) Dugdale R. C., Wilkerson F. P., Hogue V. E., Marchi A. (in review). The role of ammonium and nitrate in spring bloom development in San Francisco Bay. Parker A. E., Hogue V. E., Dugdale R. C., Wilkerson F. P. (in prep). Inorganic nitrogen speciation and the development of phytoplankton blooms in estuaries; evidence from San Francisco Bay. Wilkerson F. P., Dugdale R. C., Hogue V. E., Marchi, A., 2006.  Phytoplankton blooms and nitrogen productivity in San Francisco Bay.  Estuaries and Coasts, Vol. 29, no. 3, 401-416 Foodweb support for the threatened Delta smelt includes LSZ phytoplankton Phytoplankton productivity in LSZ has been thought to be limited due to low light from high sediment load or grazing by the invasive clam Corbula amurensis The LSZ has high concentrations of inorganic nutrients but the interaction of the two principal forms of dissolved nitrogen, nitrate (NO3) and ammonium (NH4) may also reduce phytoplankton productivity To establish whether this might be occurring in the low salinity habitat of the juvenile delta smelt, concentrations of DIN (NO3, NH4), urea, and phosphate were measured, along with chlorophyll biomass and 15NO3 and 15NH4 uptake NH4 is used for growth but high concentrations suppress the uptake of NO3 by phytoplankton If NO3 uptake can occur, maximal rates exceed those of NH4uptake, so that maximum phytoplankton growth rate is higher on NO3 than on NH4, provided light is adequate Development of spring blooms (initiated with improved irradiance) are likely modulated (and limited in biomass) by high concentrations of NH4 (>4 µM) which enter the system through sewage treatment, agricultural practices, or other anthropogenic sources (Dugdale et al. in review) Chlorophyll-a (µg L-1) Nitrate and ammonium concentrations were low in the spring and summer of 2006 compared to recent years (Wilkerson et al. 2006) The extreme rainfall during this sampling season appeared to have diluted the nutrient concentrations in the SFE Diluted ammonium concentrations maintained levels at which nitrate uptake was not inhibited. However, nitrate uptake was limited by the lower than normal nitrate concentrations Because ammonium concentration levels were not inhibitory and nitrate could be accessed, nitrate uptake then contributed equally with ammonium uptake to phytoplankton biomass Heavier relative rainfall in spring and summer 2006 (see poster Slaughter et al.) This lead to a striking decrease in salinity range between subembayments Nitrate and ammonium concentrations were lower than expected suggesting a dilution by heavy rainfall However, silicate concentrations appeared to increase, most likely due to increased input from rivers and streams Phytoplankton (>97%) removed from water Mixed algal culture added to water Phytoplankton recover and grow in Central and San Pablo Bay water No Phytoplankton growth in “Bad Suisun” water Enclosure experiment in which resident phytoplankton were removed from water and then algae (originally isolated from northern SFE) added. There was no chlorophyll-a increase in Suisun Bay water. 5.0 psu Chlorophyll-a (µg L-1) Chlorophyll-a (µg L-1) Salinity (psu) Salinity (psu) 2003 2006 Central Bay Enclosure experiments with water from 3 embayments during March 2005 show NO3 drawdown to start once NH4 is < 4 µM, and then nitrate uptake increases and chlorophyll-a accumulates. There is a lag effect in Suisun enclosures (Parker et al. in prep). Nitrate (µM) ρ15NO3 (µmoles L-1 h-1) Ammonium (µM) Chlorophyll-a (µg L-1) Nutrient (µM) 2000-2005: “Normal” Years 2006: An Anomalous Year 2006 Time Series Data

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