Quantifying Effluent Dissolved Organic Nitrogen (EDON) Uptake by Microbial Communities Along a Salinity Gradient in the York River

Effluent discharged from water reclamation facilities (WRFs) contains dissolved organic nitrogen, termed effluent dissolved organic nitrogen (EDON), that subsequently enters coastal waterways. It is still unclear at what rate EDON can be taken up by microbial communities relative to other nitrogen (N) substrates. Bench-scale sequencing batch reactors (SBRs), used to mimic WRFs, were supplied with 15N-labeled ammonium (15NH4+) to produce 15N-labeled EDON (EDO15N) that was subsequently used to measure uptake rates along a salinity gradient of the York River, Virginia, USA, in the spring and summer. Although NH4+ dominated influent N pools, only a small fraction (4.1%) of EDON was produced from NH4+ microbial assimilation in biological treatment processes. When added as a short-term (4-h) tracer, the EDO15N was taken up by estuarine microbes at rates 0.01–0.434 μmol N L−1 h−1, which are similar to rates of NH4+ and nitrate uptake. When added to 48-h bioassays, EDON stimulated phytoplankton growth more at the lower salinity (0–8‰) sites (8.5–13.8 μg Chl a L−1) than at the higher salinity (20‰) site (up to 0.4 μmol Chl a L−1). The microbes in the 0.7–5 μm size fraction had significantly higher EDO15N uptake rates than the larger size fraction (e.g., > 5 μm, p < 0.05). Taken together with urea and amino acids, DON plays a more important role in N nutrition for microbes during the summer months. This study provides the first EDO15N uptake rates using EDO15N produced from 15NH4+ in SBRs, and the results provide conclusive evidence that organic N in effluent is biologically available to estuarine microbes.