Use of Toxicity Identification Evaluation Procedures to Clarify the Relationship Between Ammonium Concentrations and Phytoplankton Blooms in the San Francisco Bay Estuary, California, USA.

This article investigates the potential inhibitory effects of ammonium and other contaminants on phytoplankton growth in the northern San Francisco Bay Estuary, a region historically supporting larval fish production through phytoplankton blooms. Using toxicity identification evaluation (TIE) procedures on 16 ambient water samples collected during spring 2012, the study found no evidence that ammonium concentrations (2.5–9.2 µM) inhibited phytoplankton growth; rather, ammonium appeared to support growth even at levels up to 12 µM in controlled spiking experiments. Treatments targeting metals (via EDTA chelation) and nonpolar organic contaminants (via solid-phase extraction) similarly showed no significant toxicity, with copper toxicity thresholds exceeding ambient levels and detected herbicides such as diuron present at concentrations well below known effect levels. The findings emphasize that environmental factors like turbidity, flow, and clam grazing likely play more substantial roles in limiting bloom development than ammonium or contaminant toxicity, underscoring the importance of empirical validation in complex, multistressor estuarine systems.