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

Abstract

The impact of atypically high ammonium (NH 4 + ) 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 NH 4 + concentrations of (mean SD) 1021 380 mol l -1 , compared with 2.9 1 mol l -1 at a site further upstream, and 4.8 1 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 (mol 3 :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 NH 4 + concentrations varying from 2 to 1350 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.