Nitrogen recycling in a North Carolina coastal river1

The amount of nitrogen recycling in the lower Chowan River (northeastern North Carolina) was estimated from differences between inflow and assimilation. Inflow was much higher in winter (e.g. 1,127 t N in January) than in summer (185 t in August), because of the wintertime high flow rates and high concentrations of particulate (PN) and dissolved inorganic nitrogen (DIN). About 69% of the total annual nitrogen inflow was dissolved organic N (DON), 13% was PN, and the remaining 18% was mostly ammonium. As much as 33% of the incoming DON may be broken down in this section of the river. Assimilation by the river plankton, computed from short term “C and ‘“N uptnkc measurements, averaged 100 g C * m-2*yr-’ and 32 g N. The seasonal patterns of inflow and uptake of DIN in the river were completely out of phase; during January inflow was 12 times higher than uptake, whereas during summer DIN uptake exceeded inflow by as much as 75 times. For the whole year, as much as two-thirds of the DIN assimilated may have come from regeneration within the river. Apparently, most of the DIN that was regenerated was ammonium. Thus, nutrient cycling seems to be important __ near the mouths of such coastal rivers. Virtually nothing is known about nitrogen cycling in coastal rivers. Often it has been assumed that in these systems both nutrient input and output are high and therefore internal cycling is unimportant and the river is similar to a pipe or conduit (Likens 1975) for the movement of water and materials. On the other hand, calculations from production studies (Windom et al. 1975; Hobbie et al. 1975) have led others to assume that nitrogen entering these aquatic systems must recycle a number of times per year. To test these assumptions, we made an experimcntal study of nitrogen cycling near the mouth of a North Carolina coastal river. An important process in aquatic nitrogen cycling seems to be regeneration of ammonium. In systems isolated from the sediments (epilimnion of a lake, open ’ This research was supported by the Office of Water Resources and Technology, U.S. Department of the Interior, through the University of North Carolina Water Resources Research Institute (project B-077-NC). ‘L Present address: Institute for Coastal and Marine Resources, East Carolina University, Grccnvillc, N.C. 27834. :j Present address: The Ecosystems Center, Marine Biological Laboratory, Woods IIolc, Mass. 02543. ocean) regeneration of ammonium by zooplankton may control the rate of photosynthesis (Eppley et al. 1973; Smith and Whitledge 1977; Harrison 1978). In shallow systems much ammonium is regenerated from sediments; this may contribute a large fraction of that present (IIartwig 1974; Hale 1975). One technique which gives the true regeneration rate of ammonium is to incubate samples with lsNH,+ and measure the dilution of the label with unlabeled ammonium (Alexander 1970; Harrison 1978). Regeneration can also be estimated from measurements of the rate of change of ammonium concentration in samples incubated with water and sediments (Hartwig 1974; Hale 1975) or with water alone (Harrison 1974); these are measurements of net flux as the organisms in the sample will be continuously taking up some rcgencrated nitrogen. Another indirect technique, which we have used, involves comparison of the measured rates of ammonium assimilation by plankton with the rates of ammonium input (by runoff) into the system, Assimilation rates of ammonia and nitrate by the river plankton were measured every 2 weeks. We also calculated inflows and outflows of nitrate, ammonium, and organic nitrogen for 1 year. The