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

Abstract

Phytoplankton NH 4 + and NO 3 − 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 NO 3 − in the ambient dissolved inorganic nitrogen pool. The predominance of NH 4 + uptake occurred because of the general suppression of NO 3 − assimilation at NH 4 + concentrations in excess of 2 μ m. This suppression, however, was not as universal as has been reported for other systems, and it is suggested that the extremely high NO 3 − 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 NH 4 + concentrations were low towards the end of the spring bloom.