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

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Phytoplankton 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 salinity gradient of the Seine plume and in the neighbouring marine waters. Phytoplankton distribution was characterised by the permanent diatom dominance from early spring to early 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 distribution 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 −1. The highest values were measured in the plume waters (S > 28), suggesting the primary productivity in the plume was more controlled by light than by nutrients. The photic zone productivity/ chlorophyll a ratios (P/B; producted carbon per unit of Chl a) were close to 40 mg mg −1 d −1 in the plume waters. P/B exceptionally reached 90 mg mg −1 d −1 in early summer. This value corresponded to a fast growing population of S. costatum. In marine waters, P/B varied from 22 mg mg −1 d −1, in spring, to 43 mg mg −1 d −1, in early summer.