Ammonium assimilation and regeneration by size-fractionated plankton in permanently well-mixed temperate waters
L.C.rre, P.; Wafar, M.; L'Helguen, S.; Maguer, J.F.
Journal of Plankton Research 18.3: 355-370
1996
ISSN/ISBN: 0142-7873
DOI: 10.1093/plankt/18.3.355
Accession: 037322304
Ammonium assimilation and regeneration by size-fractionated plankton were measured for 1 year at a coastal station in the permanently well-mixed waters of the western English Channel. The lowest assimilation and regeneration rates ( 25 ng-at N/l/h) in summer. Vertical profiles showed a light-dependent pattern in assimilation and regeneration, with the maximum rates at intermediate depths and the lowest at the base of the euphotic zone. Nanoplankton (1-15 mu m) assimilated and regenerated more ammonium throughout the year than net- (15-200 mu m) and picoplankton (<1 mu m). Assimilation in net- and nanoplankton was regulated by changes in biomass rather than by photosynthetic efficiency. Assimilation in picoplankton was mainly bacterial, but the autotrophic contribution to it became substantial in spring-summer. Ciliates and bacteria were more important for ammonium regeneration than flagellates. Assimilation to regeneration ratios varied as a function of size class: from 1 in picoplankton through 1.8 in nanoplankton to 2.4 in netplankton. Ammonium regenerated in the whole water column exceeded assimilation requirements in the euphotic zone and this may explain the accumulation of ammonium in spring-summer observed in these waters.