High levels of ammonium (NH4+) may limit the growth of larger phytoplankton (diatoms) by inhibiting nitrate (NO3-) uptake. The primary objectives of this project are to use stable isotope analyses to track the degree to which phytoplankton in the San Francisco Estuary absorb ammonium vs. nitrate and the sources of these nutrients (i.e., effluent from a nearby treatment plant, dissolved organic matter or river inputs). Of particular interest is whether the phytoplankton community switches from using NO3- to NH4+, as NH4+ concentrations rise, and whether diatoms are more sensitive to changes in NH4+ concentration than smaller phytoplankton species. Findings will have implications for determining the food resources available for pelagic fishes and for regulating the sources of nutrient pollution.