Impacts of phytoplankton activity on dissolved nitrogen fluxes in the tidal reaches and estuary of the Tweed, UK

Monthly surveys were conducted from April to August 1997 to quantify the up take of dissolved nitrogen species (nitrate, ammonium and urea) and oxygen production in the tidal reaches and estuary of the Tweed. N-15 isotope labe lling was used in conjunction with in situ incubation methods to determine the uptake of dissolved nitrogen in surface water samples. Respiration, gro ss and net oxygen production were measured simultaneously. Phytoplankton ac tivity 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 con centration. Uptake of nitrate and urea was associated with phytoplankton bi omass and was highest during periods of net autotrophic production (May, Ju ne and August). Reductions in the total downstream fluxes of dissolved nitr ogen due to phytoplankton activity were appreciable despite the lon rates o f uptake of dissolved nitrogen and were found to be strongly dependent on t he flushing time of the estuary, (C) 1999 Elsevier Science Ltd. All rights reserved.