The response of San Francisco Bay Delta phytoplankton communities to experimental ammonium, nitrate, and wastewater effluent additions and changes in irradiance

Aaron L Strong1, Gert van Dijken2, Matthew M Mills2, Ivy Bifu Huang3, Christopher Francis2, Stephen G Monismith4 and Kevin R Arrigo2, (1)Stanford University, Stanford, CA, United States, (2)Stanford University, Earth System Science, Stanford, CA, United States, (3)Stanford University, Dept. of Civil and Environmental Engineering, Stanford, CA, United States, (4)Stanford University, Environmental Fluid Mechanics Laboratory, Stanford, CA, United States
Abstract:
The Bay Delta ecosystem exhibits unusually low levels of productivity, given the high levels of inorganic nutrient loading to the system. It has been suggested that anthropogenic ammonium (NH4+) loading inhibits diatom nitrate (NO3-) uptake and ultimately prevents blooms from occurring. From May 5-May 9, 2015, we conducted experimental manipulations and water column profiling research in the Bay Delta, in order to assess the ecological effects of anthropogenic nitrogen loading in the lower Sacramento River on phytoplankton growth, community structure, and dissolved inorganic nitrogen (N) uptake rates. We collected surface water at three locations, two located above and one below the Sacramento Regional wastewater treatment plant’s diffuser pipe and incubated water for 48 hours. Experimental treatments included control, +NH4+ to 60 µM, +NO3- to 7.5 µM, and added whole wastewater effluent containing 60 µM NH4+. The water was incubated at ambient water temperature in 10-L cubitainers at two light levels: 50% and 5% of surface irradiance. Over two days, chlorophyll a (Chl a) concentrations increased seven-fold in response to both the +NH4+ and effluent additions at the upstream stations in the 50% light treatment, suggesting strong phytoplankton growth in response to added NH4+. Organic carbon accumulation showed similar patterns, and inorganic nutrients, including dissolved silica were all significantly drawn down. At all stations, Chl a accumulation was strongly affected by light limitation in the 5% light treatment. We also report changes in phytoplankton community structure, based on microscopy enumeration and HPLC pigment analysis. among treatments over the 48 hours. Additionally, we assessed variations in NO3- and NH4+ uptake and CO2 fixation across treatments over the course of the incubation. Taken as a whole, our results suggest that NH4+ from wastewater effluent does not inhibit phytoplankton growth in the Bay Delta, contrary to what has been previously suggested, and that light limitation plays a major role in controlling the productivity of the lower Sacramento River.
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