NITROGEN NUTRITION OF THE PHYTOPLANKTON I N THE WELL-MIXED WATERS OF THE WESTERN ENGLISH-CHANNEL

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.