S. Lhelguen et al., NITROGEN NUTRITION OF THE PHYTOPLANKTON I N THE WELL-MIXED WATERS OF THE WESTERN ENGLISH-CHANNEL, Oceanologica acta, 16(5-6), 1993, pp. 653-660
Uptake of nitrogenous nutrients (NO3-, NO2-, NH4+ and urea) by phytopl
ankton was measured over an annual cycle at a coastal station in the w
ell-mixed waters of the western English Channel. Seasonal changes of n
itrogen uptake showed a single, but spread-out, maximum in summer. The
phytoplankton took up the various forms of nitrogen simultaneously. A
mmonium uptake accounted for 48% of the total nitrogen uptake and was
quantitatively important from the beginning of spring until early autu
mn. Nitrate uptake was relatively less important (32% of total nitroge
n uptake) though nitrate was the major component of available inorgani
c nitrogen. Nitrate uptake rates were comparable to those of ammonium
in winter and spring, but were much lower than the latter in summer as
a result of inhibition by high ambient ammonium concentrations. Urea
and nitrite contributed much less (respectively 13 and 7%) to nitrogen
nutrition of phytoplankton. Regenerated-production accounts for about
70% of the annual production; this proportion is much higher than in
several other coastal ecosystems.