Dynamics of NH 4 + and NO 3 - Uptake in the Water Column of the Neuse River Estuary, North Carolina

Abstract

In an attempt to more fully understand the dissolved inorganic nitrogen dynamics of the Neuse River estuary, 15NH4+ and 15NO3− 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 NH4+ uptake occurred and varied both spatially and seasonally, accounting for as much as 95% of light uptake with the median being 33%. Apparent NH4+ uptake ranged from 0.001 μmol N 1−1 h−1 to 4.2 μmol N 1−1 h−1, with highest rates occurring during late summer-fall in the oligohaline estuary. Apparent NH4+ uptake was significantly related to NH4+ concentration (p<0.01); however, the regression explained <3% of the variation. Daily-integrated NH4+ uptake ranged from 0.1 mmol N m−2 d−1 to 133 mmol N m−2 d−1 and followed the trend of apparent uptake. Annual NH4+ uptake of the estuary was significantly lower in 1988 than for any other year. Dark uptake of NO3− was only 14% of maximum light uptake. Apparent NO3− uptake rates ranged from 0.001 μmol N 1−1 h−1 to 1.84 μmol N 1−1 h−1 with highest rates occurring in the oligohaline estuary. Apparent NO3− uptake was significantly related to NO3− concentration (p<0.01); however, the regression explained <5% of the variation. In general, NO3− uptake was only 20% of total dissolved inorganic nitrogen (DIN) uptake. Daily-integrated NO3− uptake ranged from 0.1 mmol N m−2 d−1 to 53 mmol N m−2 d−1 and followed similar patterns of apparent uptake. Annual NH4+ uptake was 11.39 mol N m−2 yr−1, 10.28 mol N m−2 Yr−1, 10.93 mol N m−2 yr−1, and 7.38 mol N m−2 yr−1, and 1.84 mol N m−2 yr−1, with the 4-yr mean being 10.0. Annual NO3− uptake was 3.12 mol N m−2 yr−1, 3.40 mol N m−2 yr−1, 1.96 mol N m−2 yr−1, and 1.84 mol N m−2 yr−1, with the 4-yr 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 primary productivity results in serious underestimates of estuarine nitrogen demand for the Neuse River estuary and may be true for other estuaries as well.