Nutrition azotée du phytoplancton dans les eaux brassées de la Manche occidentale

Uptake of nitrogenous nutrients (NO3-, NO2-, NH4+ and urea) by phytoplankton was measured over an annual cycle at a coastal station in the well-mixed waters of the western English Channel. Seasonal changes of nitrogen uptake showed a single, but spread-out, maximum in summer. The phytoplankton took up the various forms of nitrogen simultaneously. Ammonium uptake accounted for 48% of the total nitrogen uptake and was quantitatively important from the beginning of spring until early autumn. Nitrate uptake was relatively less important (32% of total nitrogen uptake) though nitrate was the major component of available inorganic 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.