Tamara E. C. Kraus Megan B. Young Carol Kendall Calla M. Schmidt 2018 <p><span>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 (</span>¹⁴<span>N,<span>&nbsp;</span></span>¹⁵<span>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&nbsp;(POM) in a portion of the Sacramento River, California, during river-scale nutrient manipulation experiments that involved halting wastewater discharges high in ammonium (NH</span>₄⁺<span>). 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</span>₄⁺<span>. Comparison of<span>&nbsp;</span></span><i>δ</i>¹⁵<span>N-POM and<span>&nbsp;</span></span><i>δ</i>¹⁵<span>N-phytoplankton (</span><i>δ</i>¹⁵<span>N-PHY) revealed that their<span>&nbsp;</span></span><i>δ</i>¹⁵<span>N values followed broadly similar trends. However, after 3 days of downstream travel in the presence of wastewater treatment plant (WWTP) effluent,<span>&nbsp;</span></span><i>δ</i>¹⁵<span>N-POM and<span>&nbsp;</span></span><i>δ</i>¹⁵<span>N-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 NH</span>₄⁺<span><span>&nbsp;</span>after 3 days of downstream transport. An apparent gradual increase over time in the proportion of NH</span>₄⁺<span><span>&nbsp;</span>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 NH</span>₄⁺<span><span>&nbsp;</span>concentrations.</span></p> application/pdf 10.5194/bg-15-353-2018 en EGU Use of flow cytometry and stable isotope analysis to determine phytoplankton uptake of wastewater derived ammonium in a nutrient-rich river article