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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.

The evolution of nutrient distribution has been followed in spring on the Armorican shelf in association with hydrological features and phytoplankton development. Data were collected on a sampling grid of 20 stations on the continental shelf during four cruises (SATIR 8 to 11) between March and June 1982. Haline fronts (Ushant haline front, coastal haline front) were found in late winter and early spring; their outlines follow closely the outlines of the summer thermal fronts. The origin of these haline fronts comes mainly from the outflow towards the English Channel of the waters of the Loire river on the South Brittany shelf. At this time of the year, a conservative mixing is observed between offshore waters with relatively low nutrient concentrations and coastal waters nutrient-enriched by the river inputs. A particularly early phytoplankton development has been observed in March in the haline frontal zones. Nutrient assimilation and phytoplankton development take place sequentially in spring on the Armorican shelf: a time lag of nearly three months exists between the initiation of the bloom in the South Brittany low-salinity waters and in the central part of the continental shelf. The initiation of phytoplankton development results mainly from the establishment of a haline or thermal stratification. A simple model of critical depth (Sverdrup, 1953; Riley, 1957) taking into account the evolution of the global solar irradiance and the depth of the mixed layer permits the prediction of periods of initiation of phytoplankton development on the Armorican shelf and explains the evolution of the nutrient distributions.