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

Abstract

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 µmol l-1, compared with 2.9 ± 1 µmol l-1 at a site further upstream, and 4.8 ± 1 µmol 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 (µmol3:µmol3) 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 µmol 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.