Variations saisonnières de l'absorption de l'azote dans un système brassé peu profond (Manche occidentale)

Seasonal changes in inorganic (NH4+; NO3-; NO2-) and organic (urea) nitrogen uptake were studied with N-15 as a tracer at a shallow station (depth: 22 m) in permanently well-mixed waters of the Western English Channel. Uptake of all the four nutrients was maximum in spring. Nitrate uptake accounted for a third of the total during spring phytoplankton growth, but decrease in summer. This decrease was related to the low ambient nitrate concentrations in early summer and to an inhibition of uptake by high concentrations in late summer. Ammonium was taken up in significant amounts from the beginning of spring (46 % of total uptake), increasing to 59 % in summer. In contrast with nitrate, ammonium concentrations during summer were sufficiently high to saturate uptake and prevent nitrogen limitation. Ammonium supplied about half of the nitrogen required by the phytoplankton in an annual cycle (48 %), followed by nitrate (33 %). Urea and nitrite were taken up at relatively low rates during a major part of the year and satisfied only 15 and 5 % of the total phytoplankton nitrogen requirements. Our results suggest that light controls phytoplankton growth throughout the seasonal cycle.