Nitrogen metabolism of the eutrophic Delaware River ecosystem1

A comprehensive investigation of the nitrogen cycle in the Delaware River was carried out using 15N tracers to measure rates for important transformations of nitrogen. Daily, depth-averaged 15N rates for the principal inorganic nitrogen species were consistent with rates derived from longitudinal profiles of concentration in the river. The data indicated that nitrification was a rapid, irreversible sink for NH4+, with export of the product N03from the study area. Utilization of NO,by primary producers was negligible, owing to low irradiance levels and to high NH,+ concentrations. The oxygen sag near Philadelphia was found to result from oxygen demand in the water column, with only minor benthic influence. Reaeration provided the major oxygen input. Nitrification accounted for about 1% of the net oxygen demand near Philadelphia but as much as 25% farther downstream. Many rivers and coastal waters act as treatment facilities for large inputs of primary-treated sewage. Microbial processes convert the organic material and inorganic nitrogen to more oxidized forms. Water quality declines owing to low oxygen concentrations, phytoplankton blooms, and high turbidity due to sewage-derived suspended particulate material. Investigation of the nitrogen cycle in such systems must account for the complexity and dynamic nature of the various nitrogen transformations and for the influence of light (Lipschultz et al. 198 5; Stanley and Hobbie 198 1; Dugdale and Goering 1967; Garside 198 l), dissolved oxygen content (Goreau et al. 1980; Lipschultz et al. 198 1; Lipschultz 1984), and flushing rates (Wofsy et al. 198 1). Most investigations of nitrogen cycling have been focused on a restricted subset of metabolic processes, principally phytoplankton uptake of NH,+ and N03-. Recent studies, however, have emphasized the need to expand the number of processes under investigation in order to understand the nitrogen cycle in a system (e.g. see Glibert et al. 1982; Lipschultz et al. 1985; Olson 198 1; McCarthy et al. 1984). We describe here 15N tracer experiments on the Delaware River intended to permit ’ This work was funded by NSF grant BSR 8316359 and by EPA grant R8 10219-01-O to Harvard University. identification of the important processes from an expanded set of nitrogen transformations and to relate the measured rates to observed changes in inorganic nitrogen concentration. Rates were integrated over depth and time to account for their sensitivity to light (Lipschultz et al. 198 5). This permitted quantitative comparisons and identification of the principal processes in the ecosystem. Net production (or loss) rates were then calculated for each inorganic nitrogen species at seven stations along the river near Philadelphia, and net production rates derived from 15N incubations were compared to net rates derived from river concentration data using a mass conservation model. We thank L. Kerkhof for assistance in the laboratory and fieldwork and M. Zahniser for help in design and use of the emission spectrometer.