Dynamics of NH,+ and NO,- Water Column of the Neuse North Carolina

In an attempt to more fully understand the dissolved inorganic nitrogen dynamics of the Neuse River estuary, r5NHq+ and 15N0,uptake rates were measured and daily depth-integrated rates calculated for seven stations distributed along the salinity gradient. Measurements were made at 2-3-wk intervals from March 1985 to February 1989. Significant dark NH,+ uptake occurred and varied both spatially and seasonally, accounting for as much as 95% of light uptake with the median being 33%. Apparent NH,+ uptake ranged from 0.001 pmol N 1-I h-’ to 4.2 pmol N 1-l h-l, with highest rates occurring during late summer-fall in the oligohaline estuary. Apparent NH,+ uptake was significantly related to NH,+ concentration (p < 0.01); however, the regression explained <3% of the variation. Daily-integrated NH,+ uptake ranged from 0.1 mm01 N mm2 d-’ to 133 mmol N me2 d-l and followed the trend of apparent uptake. Annual NH,+ uptake of the estuary was significantly lower in 1988 than for any other year. Dark uptake of N03was only 14% of maximum light uptake. Apparent NO,uptake rates ranged from 0.001 pmol N ll’ h-l to 1.84 umol N ll’ h-’ with highest rates occurring in the oligohaline estuary. Apparent NO,uptake was significantly related to NO,concentration (p < 0.01); however, the regression explained <5% of the variation. In general, NO,uptake was only 20% of total dissolved inorganic nitrogen (DIN) uptake. Daily-integrated NO,uptake ranged from 0.1 mmol N mm2 d-r to 53 mmol N mm2 d-l and followed similar patterns of apparent uptake. Annual NH,’ uptake was 11.39 mol N mm2 yt-‘, 10.28 mol N m-* yr’, 10.93 mol N m-* yr’, and 7.38 mol N m-* yt-‘, with the 4yr mean being 10.0. Ammal NO,uptake was 3.12 mol N m-* yt-‘, 3.40 mol N m-r p-l, 1.96 mol N m-r yt-‘, and 1.84 mol N m-r II-‘, with the 4yr mean being 2.6. The total annual DIN uptake was more than twice published estimates of phytoplankton DIN demand, indicating that there is an important heterotrophic component of DIN uptake occurring in the water column. The extrapolation of nitrogen demand from prhnary productivity results in serious underestimates of estuarine nitrogen demand for the Neuse River estuary and may be true for other estuaries as well Introduction solved inorganic nitrogen (DIN) can result in inEstuaries are continuously enriched with a wide creased phytoplankton production and biomass, array of chemicals from the watershed, including which may in turn be followed by increased popthe plant growth-stimulating nutrients nitrogen ulations of consumer animals. This trophic transfer (N) and phosphorus (P). Many estuaries successcontributes to the value of estuaries for human fully incorporate and attenuate large and/or highpurposes (Neilson and Cronin 1981). ly variable pulses of nutrient inputs by a combiExcessive nutrient enrichment can be detrimennation of processes, including assimilation into tal to estuaries as has been demonstrated in a numstanding stock of primary producers, recycling via ber of instances around the nation. High nutrient grazing, and bacterial decomposition, sedimentalevels may lead to blue-green algal blooms in the tion, and export. Elevated concentrations of disfresher portions of estuaries and to dinoflagellate blooms in the more saline stretches. We chose to concentrate our research emphasis on N rather i Present address: Institute of Ecosystem Studies, Box AB, than P because many studies (e.g., Goldman et al. Millbrook, New York 12545-0129. 1973; Nixon 1981; Boynton et al. 1982; Nixon and