Use of flow cytometry and stable isotope analysis to determine phytoplankton uptake of wastewater derived ammonium in a nutrient-rich river

Abstract

Abstract. Anthropogenic alteration of the form and concentration of nitrogen (N) in aquatic ecosystems is widespread. Understanding availability and uptake of different N sources at the base of aquatic food webs is critical to establishment of effective nutrient management programs. Stable isotopes of N ( 14 N, 15 N) are often used to trace the sources of N fueling aquatic primary production, but effective use of this approach requires obtaining a reliable isotopic ratio for phytoplankton.In this study, we tested the use of flow cytometry to isolate phytoplankton from bulk particulate organic matter (POM) in a portion of the Sacramento River, California, during riverscale nutrient manipulation experiments that involved halting wastewater discharges high in ammonium (NH + 4 ). Field samples were collected using a Lagrangian approach, allowing us to measure changes in phytoplankton N source in the presence and absence of wastewater-derived NH

Elevated Nitrate Preference Over Ammonium in Aquatic Plants by Nitrogen Loadings in a City River

“…Plant δ 15 N TN was mainly controlled by source δ 15 N and N isotope effects of N uptake processes (Evans, 2001; Kohzu et al., 2008; Schmidt et al., 2018). In our study, the source δ 15 N NH4+ and δ 15 N NO3‐ in the water of different river sections did not substantially differ between seasons and years (Figure S3 in Supporting Information ).…”

“…Second, the observed N isotope effects, that is, δ 15 N differences between plant tissues and N sources (Figure S4 in Supporting Information S1), may differ among plant species with different physiologies (Evans, 2001;Granger et al, 2004Granger et al, , 2010Schmidt et al, 2018). Any physiological differences that can cause differences in N-uptake kinetics may influence the N isotope effects of specific plants.…”

“…So far, isotope effects were assumed negligible during the physical processes of N transportation into plant tissues (Evans et al., 1996). However, both NO 3 − reduction and NH 4 + assimilation in plants would cause substantial 15 N enrichment in residual NH 4 + and NO 3 − , respectively (Evans, 2001; Evans et al., 1996; Liu et al., 2014; Schmidt et al., 2018). The effluxes of 15 N‐enriched NH 4 + and NO 3 − from plant tissues lead to lower δ 15 N in aquatic plants than external N sources (Mariotti et al., 1982; Pastor et al., 2013; Robinson, 2001).…”

“…In river ecosystems, the assimilation of ammonium (NH 4 + ) and nitrate (NO 3 − ) by aquatic plants plays a vital role in retaining or removing anthropogenic N pollutants and regulating river primary productivity and ecological functions (Howarth et al, 2011;Miyajima et al, 2009;Peipoch et al, 2014). Within the framework of river N biogeochemistry, relatively more studies have been conducted on river N chemistry and microbial processes with less on plant N-use processes.Aquatic plants are diverse in species and rely on ammonium (NH 4 + ) and nitrate (NO 3 − ) as dominant N sources (Adamec, 2010;Schmidt et al, 2018). Physiological indices (e.g., nutrient concentrations, enzyme activities, and isotopic compositions) of aquatic plants have been measured to indicate river N pollution (Dailer et al, 2010;Schmidt et al, 2018).…”

“…Within the framework of river N biogeochemistry, relatively more studies have been conducted on river N chemistry and microbial processes with less on plant N-use processes.Aquatic plants are diverse in species and rely on ammonium (NH 4 + ) and nitrate (NO 3 − ) as dominant N sources (Adamec, 2010;Schmidt et al, 2018). Physiological indices (e.g., nutrient concentrations, enzyme activities, and isotopic compositions) of aquatic plants have been measured to indicate river N pollution (Dailer et al, 2010;Schmidt et al, 2018). Total N (TN) contents in aquatic plants have been reported to indicate the elevated N availability or N loadings in river water (Baker et al, 2010;Benson et al, 2008).…”