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

Aaron L Strong,Christopher A Francis,Raphael M Kudela,Ivy B Huang,Sara E Driscoll,Gert L Van Dijken,Stephen G Monismith,Matthew M Mills,Kevin R Arrigo,G Mine Berg

doi:10.1525/elementa.2021.040

Abstract

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.

Highlights

The Sacramento River is the largest river in California and extendss 400 miles from Mount Shasta to San Francisco Bay (SFB)
After incubation for 48 h, Chl a concentration increased under all nutrient and light conditions (Figure 3), though increases and nutrient enrichment effects were greater in the high light (HL) treatment (Figure 3A–C) than the low light (LL) treatment (Figure 3D–F)
Light as the limiting factor for Lower Sacramento River phytoplankton We present clear evidence of phytoplankton growth under the HL treatments when cells were supplied with nutrients, as either individual substrates (NH4þ or NO3–) or in effluent, as well as under the high ambient nutrient concentrations (Table 1) of the Lower Sacramento River downstream from the wastewater effluent outfall from the Sacramento Regional Wastewater Treatment Plant (SRWTP)

Summary

Introduction

The Sacramento River is the largest river in California and extendss 400 miles from Mount Shasta to San Francisco Bay (SFB). The decrease in productivity has cascaded up the food web (Kimmerer, 2006), leading to an overall decline in fish production throughout the northern SFB and the Delta (Sommer et al, 2007). This phenomenon is known as pelagic organism decline (POD) and has been a major focus of federal and state agency

Methods

Results

Discussion

Conclusion