Impact of atypical ammonium concentrations on phytoplankton abundance and composition in fresh versus estuarine waters

The impact of atypically high ammonium (NH4+) concentrations delivered via treated wastewater effluent on phytoplankton community composition was investigated in a tidal slough connected with Suisun Bay in the northern part of San Francisco Bay. Input of effluent to a downstream location resulted in NH4+ concentrations of (mean +/- SD) 1021 +/- 380 mu mol l(-1), compared with 2.9 +/- 1 mu mol l(-1) at a site further upstream, and 4.8 +/- 1 mu mol l(-1) in Suisun Bay. Comparison of the diatom community at the downstream site in Pacheco Slough with that in Suisun Bay revealed a substantial overlap in species, including Cyclotella scaldensis, which dominated diatom species composition in both locations. The ratio of diatoms: other phytoplankton biomass (mu mol(3):mu mol(3)) suggested that diatoms contributed a greater proportion of total phytoplankton community biomass at the downstream location (48.6 +/- 87) versus in Suisun Bay (9.5 +/- 1) or upstream (9.5 +/- 8), and that diatoms can readily grow in the presence of NH4+ concentrations varying from 2 to 1350 mu mol l(-1). In the present investigation, species composition of the seeding population was found to be a more important predictor of final phytoplankton community composition than nutrient concentrations or ratios.