Phytoplankton uptake of ammonium, nitrate and urea in the Neuse River

Uptake rates of ammonium, nitrate and urea were measured during the spring, summer and autumn (2001) in the eutrophic, nitrogen (N) limited Neuse River Estuary (NRE), North Carolina, USA. Ammonium was the dominant form of N taken up during the study, contributing approximately half of the total measured N uptake throughout the estuary. Nitrate uptake declined significantly with distance downstream com-prising 33 % of the total uptake in the upper estuary but only 11 and 16 % in the middle and lower estuary, respectively. Urea uptake contributed least to the total pool in the upper estuary (16%), but increased in importance in the middle and lower estuary, comprising 45 and 37 % of the total N taken up, respectively. The importance of regenerated N for fuelling phytoplankton productivity in the mesohaline sections of the NRE is demonstrated. The contribution of urea to the regenerated N pool suggests that internal regen-eration of dissolved organic N may support a large proportion of the phytoplankton primary production and biomass accumulation in the middle and lower NRE. These results suggest that N-budgets based on dissolved inorganic N uptake rates alone will seriously under estimate phytoplankton N uptake