Impacts of phytoplankton activity on dissolved nitrogen fluxes in the tidal reaches and estuary of the Tweed (North East UK)

Monthly surveys were conducted from April to August 1997 to quantify the uptake of dissolved nitrogen species (nitrate, ammonium and urea) and oxygen production in the tidal reaches and estuary of the Tweed. 15N isotope labelling was used in conjunction with in situ incubation methods to determine the uptake of dissolved nitrogen in surface water samples. Respiration, gross and net oxygen production were measured simultaneously. Phytoplankton activity in the Tweed resulted in low rates of dissolved nitrogen uptake (<0.4 nmol N l?1 h?1) and caused small changes in the dissolved oxygen concentration. Uptake of nitrate and urea was associated with phytoplankton biomass and was highest during periods of net autotrophic production (May, June and August). Reductions in the total downstream fluxes of dissolved nitrogen due to phytoplankton activity were appreciable despite the low rates of uptake of dissolved nitrogen and were found to be strongly dependent on the flushing time of the estuar