Uptake and regeneration of nitrogen in the well-mixed waters of the English Channel: new and regenerated productions.

Nitrate and ammonium uptake rates were measured in spring and summer in dee p and shallow well-mixed waters of the English Channel during different cru ises between 1986 and 1994. In the deep waters, nitrate uptake was relative ly low during phytoplankton development and ammonium uptake represented mor e than 70 % of the total uptake irrespective of the season. In the shallow waters, nitrate uptake during spring phytoplankton growth was high and repr esented about 75 % of the total uptake. Ammonium uptake became substantial towards the end of spring and summer. The high contribution of ammonium to the nitrogenous nutrition of phytoplankton over the whole of the well-mixed waters is related to a high rate of recycling of nitrogen in the water col umn. Ammonium regeneration by microheterotrophs can satisfy between 62 % an d the totality of the phytoplankton nitrogen requirements. The high primary production in deep well-mixed waters is not supported by allochthonous nit rogen supply but by an intense in situ regeneration of nitrogen. The situat ion is similar in shallow well-mixed waters, except during the spring bloom . (C) Elsevier, Paris.