Phytoplancton en baie de Seine. Influence du panache fluvial sur la production primaire

Abstract

Ahstract -Phytoplankton in the Bay of Seine (France). Influence of the river plume on primary productivity.Phytoplanklon distribution and primary productivity were investigated in the Bay of Seine (eastern English Channel, France) in spring and early summer of 1992 and 1994. In 1992, the horizontal distribution of phytoplankton species was determined over the whole Bay of Seine. In 1994, species distribution and primary productivity were studied along the salinîty gradient of the Seine plume and in the neighbouring marine waters. Phytoplankton distribution was characterised by the permanent diatom dominance from early spring to carly summer. The spring bloom did not occur uniformly over the bay, but was initiated in the marine waters, in the middle of the bay and progressed towards the coast where it developed, in June, in the diluted waters of the Seine plume. Phytoplankton distrÎbution was also characterised by the dominance of large diatoms (Rhizosolenia) in marine waters and by the dominance of small species (Skeletonema costatum, Asterionella glacialis) in the plume waters. The greatest abundance of large diatoms was generally encountered below the euphotic zone, This accumulation seemed to be related to cell sedimentation and advection of marine waters below the diluted plume waters. The daily carbon productivity range was 1.4 to 3 g m~2 d'I. The highest values were measured in the plume waters (S > 28), suggesting the primary productivity in the plume was more controlled by Iight than by nutrients. The photic zone productivityl chlorophyll a ratios (PIB ; producted carbon per unit of Chia) were close to 40 mg mg-I d~l in the plume waters. P/B exceptionally reached 90 mg mg-I d-1 in early summer. This value corresponded to a fast growing population of S. costatum. In marine waters, PIB varÎed from 22 mg mg-I d~J , in spring, to 43 mg mg~1 d-I , in early summer. © Elsevier, Paris phytoplankton 1 primary productivity 1 Bay of Seine Résumé -La distribution des populations phytoplanctoniques et la production primaire ont été étudiées, au printemps et au début de l'été, en 1992 sur l'ensemble de la baie de Seine et en 1994, dans le gradient de salinité du panache de la Seine et dans les eaux marines adjacentes. La distribution du phytoplancton est marquée par la dominance persistante des diatomées du début du printemps au début de l'été. Leur développement n'est pas uniforme. La floraison printanière des diatomées débute dans les eaux marines, au centre de la baie et progresse vers la côte où elle se développe, en juin, dans les eaux dessalées. La distribution du phytoplancton est aussi caractérisée par la prédominance des diatomées de grande taille (Rhizosolenia) dans les eaux marines et par celle des espèces de petite taille (Skeletonema costatum, Asterionella glacialis) dans les eaux du panache. Les densités maximales des diatomées de grande taille sont généralement observées en dessous de la couche éclairée; ces accumulations semblent liées, d'une part, à la sédimentation des cellules et, d'autre part, à l'a...

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