Pj. Shaw et al., Impacts of phytoplankton activity on dissolved nitrogen fluxes in the tidal reaches and estuary of the Tweed, UK, MAR POLL B, 37(3-7), 1998, pp. 280-294
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.