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Anthropogenic alteration of the form and concentration of nitrogen (N) in\udaquatic ecosystems is widespread. Understanding availability and uptake of\uddifferent N sources at the base of aquatic food webs is critical to\udestablishment of effective nutrient management programs. Stable isotopes of N\ud(14N, 15N) are often used to trace the sources of N fueling aquatic\udprimary production, but effective use of this approach requires obtaining a\udreliable isotopic ratio for phytoplankton. In this study, we tested the use\udof flow cytometry to isolate phytoplankton from bulk particulate organic\udmatter (POM) in a portion of the Sacramento River, California, during\udriver-scale nutrient manipulation experiments that involved halting\udwastewater discharges high in ammonium (NH4+). Field samples were\udcollected using a Lagrangian approach, allowing us to measure changes in\udphytoplankton N source in the presence and absence of wastewater-derived\udNH4+. Comparison of δ15N-POM and\udδ15N-phytoplankton (δ15N-PHY) revealed that their\udδ15N values followed broadly similar trends. However, after 3 days\udof downstream travel in the presence of wastewater treatment plant (WWTP)\udeffluent, δ15N-POM and δ15N-PHY in the Sacramento River\uddiffered by as much as 7 ‰. Using a stable isotope mixing model\udapproach, we estimated that in the presence of effluent between 40 and\ud90 % of phytoplankton N was derived from NH4+ after 3 days of\uddownstream transport. An apparent gradual increase over time in the\udproportion of NH4+ in the phytoplankton N pool suggests that either\udvery low phytoplankton growth rates resulted in an N turnover time that\udexceeded the travel time sampled during this study, or a portion of the\udphytoplankton community continued to access nitrate even in the presence of\udelevated NH4+ concentrations

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