SEASONAL-VARIATIONS IN NITROGEN UPTAKE IN SHALLOW WELL-MIXED WATERS (WESTERN ENGLISH-CHANNEL)

Seasonal changes in inorganic (NH4+; NO3-; NO2-) and organic (urea) ni trogen 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. Nitra te uptake accounted for a third of the total during spring phytoplankt on growth, but decrease in summer. This decrease was related to the lo w 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 up take), increasing to 59 % in summer. In contrast with nitrate, ammoniu m concentrations during summer were sufficiently high to saturate upta ke and prevent nitrogen limitation. Ammonium supplied about half of th e nitrogen required by the phytoplankton in an annual cycle (48 %), fo llowed 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 sugge st that light controls phytoplankton growth throughout the seasonal cy cle.