AMMONIUM ASSIMILATION AND REGENERATION BY SIZE-FRACTIONATED PLANKTON IN PERMANENTLY WELL-MIXED TEMPERATE WATERS

Ammonium assimilation and regeneration by size-fractionated plankton w ere measured for 1 year at a coastal station in the permanently well-m ixed waters of the western English Channel. The lowest assimilation an d regeneration rates (<1 ng-at N l(-1) h(-1)) were in winter and the h ighest (>25 ng-at N l(-1) h(-1)) in summer. Vertical profiles showed a light-dependent pattern in assimilation and regeneration, with the ma ximum rates at intermediate depths and the lowest at the base of the e uphotic zone. Nanoplankton (1-15 mu m) assimilated and regenerated mor e ammonium throughout the year than net-(15-200 mu m) and picoplankton (<1 mu m). Assimilation in net- and nanoplankton was regulated by cha nges in biomass rather than by photosynthetic efficiency. Assimilation in picoplankton was mainly bacterial, but the autotrophic contributio n to it became substantial in spring-summer. Ciliates and bacteria wer e more important for ammonium regeneration than flagellates. Assimilat ion to regeneration ratios varied as a function of size class: from 1 in picoplankton through 1.8 in nanoplankton to 2.4 in netplankton. Amm onium regenerated in the whole water column exceeded assimilation requ irements in the euphotic zone and this may explain the accumulation of ammonium in spring-summer observed in these waters.