Ecosystem Recovery in Progress? Initial Nutrient and Phytoplankton Response to Nitrogen Reduction from Sewage Treatment Upgrade in the San Francisco Bay Delta

Abstract

The San Francisco Bay Delta has been an estuary of low productivity, with causes hypothesized to relate to light limitation, grazing by invasive clams, and polluting levels of NH4+ discharge from a wastewater treatment plant. Suppression of phytoplankton NO3− uptake by NH4+ has been well documented, and thus this estuary may have experienced the counterintuitive effect of depressed productivity due to wastewater NH4+ enrichment. In 2021, a new wastewater treatment plant came online, with a ~75% reduction in nitrogen load, and within-plant nitrification, converting the discharge to NO3−. The expectation was that this change in nitrogen loading would support healthier phytoplankton production, particularly of diatoms. Here, responses of the post-upgrade Bay Delta phytoplankton were compared to five years of data collected pre-upgrade during the fall season. Indeed, increased chlorophyll a accumulation in the estuary was documented after the implementation of the upgraded wastewater treatment and photophysiological responses indicated comparatively less stress. Major differences in river flow were also observed due to drought conditions during the decade covered by this study. While short-term favorable effects were observed, understanding longer-term ecological feedback interactions that may follow from this major nutrient change under variable flow conditions will require more years of observations.