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JPR Advance Access published online on January 18, 2005
Journal of Plankton Research, doi:10.1093/plankt/fbh174
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Journal of Plankton Research, © Oxford University Press 2005; all rights reserved
Received August 26, 2004
Accepted December 22, 2004

Article

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

S. L’Helguen 1*, G. Slawyk 2, and P. Le Corre 1

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

* To whom correspondence should be addressed.
S. L’Helguen, E-mail: lhelguen{at}sb-roscoff.fr


  Abstract

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 between the rates of regeneration and uptake of urea was close to 1 on all time (seasonal, 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 y-1) was equivalent to about 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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