TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 66 (No. 5), pp. 621-631, 2010

Do River Discharge Rates Drive the Overall Functioning of the Pelagic Ecosystem over the Continental Shelf of the Bay of Biscay (NE Atlantic)? A Comparison of Two Contrasting Years with Special Reference to Anchovy (Engraulis encrasicolus L.) Nutritional State

Jean-Pierre Bergeron1*, Daniel Delmas2 and Noussithé Koueta3

1IFREMER, Centre Atlantique, Département Écologie et Modèles pour l'Halieutique (EMH), B.P. 21105, F-44311 NANTES CEDEX 03, France
2IFREMER, Centre Bretagne, Département Dynamique de l'Environnement Côtier (DYNECO), B.P. 70, F-29280, PLOUZANE, France
3Université de Caen, Laboratoire de Biologie et Biotechnologies Marines, Esplanade de la Paix, F-14032 CAEN CEDEX, France

(Received 10 January 2010; in revised form 16 June 2010; accepted 16 June 2010)

Abstract: In the 2000 and 2001 springtime periods, mesozooplankton samples were systematically taken during research cruises devoted to the study of the distribution and abundance of small pelagic fishes in the French part of the Bay of Biscay, NE Atlantic, continental shelf. Simultaneous activities of three enzymes were measured in these mesozooplankton samples, to characterize the main features of ecosystem function at a regional scale. The aim was to assess assimilation rates of carbohydrates, through pyruvate kinase activity, and protein, through trypsin activity, by the mesozooplankton community and the resulting overall productivity, through aspartate transcarbamylase activity. The highest influence on the pelagic environment was the strong difference in the freshwater outflow rates from the two main large rivers, the Loire and the Gironde, which was principally responsible for the inter-annual fluctuations of the nutrients in the Bay. This overall influence of river flow results in a characteristic pattern of sea-surface salinity, which was reflected by the phytoplankton biomass distribution. Mesozooplankton enzyme activities showed high variations between the two years. The contrast in the dynamics of the pelagic food web processes between the two consecutive years, with higher activity in 2001, affected the nutritional condition of the zooplanktivorous breeding anchovy Engraulis encrasicolus.

*Corresponding author E-mail: jean.pierre.bergeron@ifremer.fr

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