Effects of Ammonium-rich Wastewater Effluent on Phytoplankton Productivity in Experimental Mesocosms from the northern San Francisco Estuary

Ammonium (NH₄)-rich effluent is released by a large municipal wastewater treatment facility (WWTF) in the northern San Francisco estuary. Long-term monitoring of elevated NH₄ in the estuary has been correlated with low Chl-a concentrations, and some studies suggest that increased NH₄ supply has shifted the community from diatoms to flagellates. Our aim was to understand how nitrogen form, specifically NH₄-rich effluent, influences phytoplankton community in the lower Sacramento River.

Mesocosm (10 L enclosure) experiments were conducted in conjunction with the USGS California Water Science Center study (Nitrogen Dynamics along the Sacramento River and Links to Phytoplankton Dynamics), a large scale field manipulation in which WWTF discharge was halted for extended periods (~15 hours) in October 2013 and May of 2014. Whole water mesocosm experiments included comparisons of various nitrogen sources (effluent-NH₄, NH₄Cl and NO₃), and were incubated in ambient light conditions and water temperatures. Development of phytoplankton communities was monitored by measuring Chl-a biomass, nutrient draw-down, function group identification and isotope N-uptake rates.

The reduction of NH₄-rich effluent in situ did not alter total phytoplankton biomass, however uptake of NO₃ was able to occur when NH₄-effluent was reduced. Enclosures from upstream of effluent discharge (low DIN) were nitrogen limited and accumulated very little Chl-a over the study period, while amending with nitrogen (NH₄Cl, NO₃, NH₄-effluent) resulted in high Chl-a, and similar N-uptake and nutrient drawdown. Enclosures did not show differences between growth on effluent-based NH₄ and NH₄Cl solution at ~40µM NH₄, however river data revealed a depression in N-uptake rates at high NH₄-effluent concentrations. Elevated NH₄ itself has been shown to impact phytoplankton communities, but we must also consider the potential toxicity of other constituents when studying systems with wastewater-based nitrogen pollution.