Response of Lower Sacramento River phytoplankton to high-ammonium wastewater effluent

Since the 1980s, the San Francisco Bay Delta ecosystem has experienced large declines in primary production. Hypothesized reasons for this decline include (1) suppression of nitrate (NO3−) uptake, and thus phytoplankton growth, due to high concentrations of ammonium (NH4+), and (2) wastewater NH4+-induced changes in phytoplankton community composition away from large-celled diatoms. These twin hypotheses implicate NH4+ loading from the Sacramento Regional Wastewater Treatment Plant effluent outfall in explaining declines in primary production in the region. They have been controversial within the water resources management community and have stimulated a lengthy public scientific and regulatory debate. Here, in an effort to resolve this debate, we present results from a 48-h incubation experiment with surface water from both upstream and downstream of the Sacramento Regional Wastewater Treatment Plant effluent outfall, a major source of NH4+ loading to the ecosystem. We amended this water with either NH4+, NO3−, or full wastewater effluent. All assays were incubated under high light (52% of incident irradiance) or low light (6% of incident irradiance). NO3− uptake rates were suppressed to near zero in all treatments with either added NH4+, added wastewater effluent, or high in situ NH4+ concentrations. Yet, phytoplankton uniformly grew well on all dissolved inorganic nitrogen sources, including effluent and NH4+. Diatom species were the most abundant taxa at all stations, and diatom cell abundances increased at greater rates than all other taxa over the course of the experiment. Among all treatments, the light treatment had the greatest effects on chlorophyll a accumulation and phytoplankton growth rates. Our results suggest that high NH4+ loading is not a driver of the lower productivity in the San Francisco Bay Delta. Although phytoplankton preferred NH4+ to NO3− when both were available in our experiment, the form of dissolved inorganic nitrogen had no effect on growth rates or species composition.