Temporal and spatial variability in phytoplankton ammonium and nitrate uptake in the Delaware Estuary



Phytoplankton NH4+ and NO3 uptake was examined along the longitudinal salinity gradient of the Delaware Estuary over several seasonal cycles using 15N-tracer techniques. Saturated nitrogen uptake rates increased directly with water temperature and reached a maximum of 380 nmol Nl−1h−1 during summer. This temperature dependence was related primarily to changes in the rate of maximum chlorophyll specific uptake, which varied exponentially between 2 and 70 nmol N [μg Chl h]−1 over a temperature range of 2–28°C. Despite these high uptake rates, balanced growth (C:N⋍7:1) could be maintained over the diel light cycle only by highly efficient nitrogen uptake at low light intensities and dark uptake below the photic zone and at night (dark uptake=25% maximum uptake). Ammonium fulfilled 82% of the annual phytoplankton nitrogen demand in the estuary despite dominance of NO3 in the ambient dissolved inorganic nitrogen pool. The predominance of NH4+ uptake occurred because of the general suppression of NO3 assimilation at NH4+ concentrations in excess of 2 μm. This suppression, however, was not as universal as has been reported for other systems, and it is suggested that the extremely high NO3 concentrations found in the estuary contribute to this pattern. Nitrate was a significant source of nitrogen only during periods of high phytoplankton production in summer, and when NH4+ concentrations were low towards the end of the spring bloom.


School of Marine Science and Ocean Engineering

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Estuarine, Coastal and Shelf Science



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