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MEPS 255:27-41 (2003)  -  doi:10.3354/meps255027

Dynamics of particulate organic matter δ15N and δ13C during spring phytoplankton blooms in a macrotidal ecosystem (Bay of Seine, France)

N. Savoye1,5,*, A. Aminot2, P. Tréguer1, M. Fontugne3, N. Naulet4, R. Kérouel2

1Laboratoire des Sciences de l¹Environnement Marin, CNRS UMR 6539, Institut Universitaire Européen de la Mer, place Nicolas Copernic, Technopole Brest-Iroise, 29280 Plouzané, France
2Département Ecologie Côtière, Institut Français de Recherche pour l¹Exploitation de la Mer, Technopole Brest-Iroise, 29280 Plouzané, France
3Laboratoire des Sciences du Climat et de l¹Environnement, UMR 1572 CEA/CNRS, Domaine du CNRS, Avenue de la terrasse, 91198 Gif-sur-Yvette, France
4Laboratoire d¹Analyse Isotopique et Electrochimique de Métabolismes, CNRS UMR 6006, Université de Nantes, 2, rue de la Houssinière, BP 92208, 44322 Nantes Cedex 03, France
5Present address: Analytical and Environmental Chemistry, Vrije Universiteit Brussels, Pleinlaan, 2, 1050 Brussels, Belgium

ABSTRACT: Two cruises (April and June 1997) were carried out in the Bay of Seine, a nitrate- and ammonium-enriched ecosystem of Western Europe, to identify the major mechanisms that control δ15N and δ13C in spring particulate organic matter (POM). Particulate organic nitrogen (PON) δ15N ranged between 0.8 and 5.2‰ in April and between 2.2 and 6.2‰ in June, while particulate organic carbon (POC) δ13C ranged between -24.3 and -19.7‰, and between -20.0 and -16.2‰ during the same periods. During spring 1997, POM was highly dominated by autochthonous phytoplankton. It is shown that the variation of PON δ15N is due to both nitrate mixing between river and marine waters and fractionation of N stable isotopes during nitrate utilization by phytoplankton. Therefore, similarly to what was previously shown for open ocean, δ15N can be used as a proxy of spring fractional nitrate utilization in coastal ecosystems. It is also shown that POC δ13C in spring is controlled by POC concentration and C:N ratio (in addition to Œtemperature effects¹), which are considered here as indicators of primary production and phytoplankton degradation, respectively. The co-variation of δ13C and δ15N describes the spring phytoplankton dynamics: at the start of phytoplankton development, nitrate concentration is high (low δ15N) and phytoplankton production is low (low δ13C); then primary production increases (δ13C becomes higher) and the nitrate pool diminishes (δ15N becomes higher); at a later stage, the nitrate pool is depleted (high δ15N), part of the phytoplankton becomes degraded and production is still high (high δ13C).

KEY WORDS: C and N stable isotopes · Phytoplankton · Coastal ecosystem · Nitrate utilization · Bay of Seine

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