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JPR Advance Access originally published online on January 18, 2005
Journal of Plankton Research 2005 27(3):263-270; doi:10.1093/plankt/fbh174
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Journal of Plankton Research Vol. 27 No. 3 © Oxford University Press 2005; all rights reserved

Seasonal patterns of urea regeneration by size-fractionated microheterotrophs in well-mixed temperate coastal waters

S. L’Helguen1,*, G. Slawyk2 and P. Le Corre1

1 Laboratoire de Chimie Marine, Observatoire Océanologique de Roscoff, CNRS/INSU UMR 7127 Et Université de Bretagne Occidentale, BP 74, 29682 Roscoff, Cedex, France and 2 Laboratoire D’OcéAnographie Et de Biogéochimie, UMR CNRS 6535, Campus de Luminy, Case 901, 13288 Marseille, Cedex 09, France

* Corresponding author: lhelguen{at}sb-roscoff.fr

Received August 26, 2004; accepted in principle November 2, 2004; accepted for publication December 22, 2004; published online January 18, 2005

Urea regeneration by size-fractionated plankton was measured over an annual cycle at a coastal station in the permanently well-mixed waters of the western English Channel. Rates of urea regeneration in the <200 µm fraction varied from 0.6 to 20.6 nmol N L–1 h–1. Regeneration rates were lowest in winter and highest in summer. The ratio of the rates of regeneration to uptake of urea was close to 1 on all time (seasonal and nycthemeral), and space (vertical) scales indicating that regeneration by microheterotrophs supplied the totality of urea used by phytoplankton. On an annual basis, urea regenerated by the microheterotrophs (0.98 mol N m–2 year–1) was equivalent to ~33% of the total regenerated N (urea + ammonium). The major part of urea regeneration was due to the nanoplankton (51%) and microplankton fractions (36%). Regeneration of urea in the picoplankton was detectable only from April to October and represented, on an average, 25% of the total urea regenerated during this period. Urea regeneration in micro- and nanoplankton fractions was mainly associated with ciliates and in the picoplancton fraction with bacteria.


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