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 exercise of this approach requires obtaining a dependable isotopic ratio for phytoplankton. In this study, or we tested the exercise 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 tall in ammonium (NH4+). Field samples were collected using a Lagrangian approach, or allowing us to degree changes in phytoplankton N source in the presence and absence of wastewater-derived NH4+. Comparison of δ15N-POM andδ15N-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, or 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.
Source: usgs.gov