Articles | Volume 15, issue 1
Biogeosciences, 15, 353–367, 2018
Biogeosciences, 15, 353–367, 2018

Research article 17 Jan 2018

Research article | 17 Jan 2018

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

Calla M. Schmidt1, Tamara E. C. Kraus2, Megan B. Young3, and Carol Kendall3 Calla M. Schmidt et al.
  • 1University of San Francisco, 2130 Fulton St, San Francisco, CA 94117, USA
  • 2USGS California Water Science Center, 6000 J Street, Placer Hall, Sacramento, CA 95819, USA
  • 3USGS National Research Program, 345 Middlefield Rd, Menlo Park, CA 94025, USA

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 (14N, 15N) 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 river-scale nutrient manipulation experiments that involved halting wastewater discharges high in ammonium (NH4+). 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 NH4+. Comparison of δ15N-POM and δ15N-phytoplankton (δ15N-PHY) revealed that their δ15N values followed broadly similar trends. However, after 3 days of downstream travel in the presence of wastewater treatment plant (WWTP) effluent, δ15N-POM and δ15N-PHY in the Sacramento River differed by as much as 7 ‰. Using a stable isotope mixing model approach, we estimated that in the presence of effluent between 40 and 90 % of phytoplankton N was derived from NH4+ after 3 days of downstream transport. An apparent gradual increase over time in the proportion of NH4+ in the phytoplankton N pool suggests that either very low phytoplankton growth rates resulted in an N turnover time that exceeded the travel time sampled during this study, or a portion of the phytoplankton community continued to access nitrate even in the presence of elevated NH4+ concentrations.

Short summary
In this study we measured phytoplankton use of nitrogen from wastewater treatment plant effluent in a river. The project was designed to test a new technique for isolating phytoplankton from river water prior to isotopic analysis in order to use trace nitrogen source use under environmental conditions. This study helps us understand the fate of anthropogenic nitrogen in aquatic ecosystems, which is essential to establishing effective nutrient management plans.
Final-revised paper