Elevated ammonium concentrations from wastewater discharge depress primary productivity in the Sacramento River and the Northern San Francisco Estuary

Primary production in the Northern San Francisco Estuary (SFE) has been declining despite heavy loading of anthropogenic nutrients. The inorganic nitrogen (N) loading comes primarily from municipal wastewater treatment plant (WTP) discharge as ammonium (NH4). This study investigated the consequences for river and estuarine phytoplankton of the daily discharge of 15 metric tons NH4-N into the Sacramento River that feeds the SFE. Consistent patterns of nutrients and phytoplankton responses were observed during two 150-km transects made in spring 2009. Phytoplankton N productivity shifted from NO3 use upstream of the WTP to productivity based entirely upon NH4 downstream. Phytoplankton NH4 uptake declined downstream of the WTP as NH4 concentrations increased, suggesting NH4 inhibition. The reduced total N uptake downstream of the WTP was accompanied by a 60% decline in primary production. These findings indicate that increased anthropogenic NH4 may decrease estuarine primary production and increase export of NH4 to the coastal ocean.

► Wastewater discharge changed the chemistry and productivity of the Sacramento River. ► Secondary wastewater treatment results in substantial NH4 downstream of the sewage discharge. ► Elevated NH4 (1) prevents phytoplankton NO3 use (2) suppresses NH4 uptake (3) depresses primary production. ► The Sacramento River NH4 load to the Northern SFE suppresses phytoplankton blooms. ► These results provide a mechanism to help explain bottom-up impacts on the SFE pelagic food web.