Phytoplankton in the Bay of Seine (France). Influence of the river plume on primary productivity.

Phytoplankton distribution and primary productivity were investigated in th e Bay of Seine (eastern English Channel, France) in spring and early summer of 1992 and 1994. In 1992, the horizontal distribution of phytoplankton sp ecies was determined over the whole Bay of Seine. In 1994, species distribu tion and primary productivity were studied along the salinity gradient of t he Seine plume and in the neighbouring marine waters. Phytoplankton distrib ution was characterised by the permanent diatom dominance from early spring to early summer. The spring bloom did not occur uniformly over the bay, bu t was initiated in the marine waters, in the middle of the bay and progress ed towards the coast where it developed, in June, in the diluted waters of the Seine plume. Phytoplankton distribution was also characterised by the d ominance of large diatoms (Rhizosolenia) in marine waters and by the domina nce of small species (Skeletonema costatum, Asterionella glacialis) in the plume waters. The greatest abundance of large diatoms was generally encount ered below the euphotic zone. This accumulation seemed to be related to cel l sedimentation and advection of marine waters below the diluted plume wate rs. The daily carbon productivity range was 1.4 to 3 g m(-2) d(-1). The hig hest values were measured in the plume waters (S > 28), suggesting the prim ary productivity in the plume was more controlled by Light than by nutrient s. The photic zone productivity/chlorophyll a ratios (P/B; producted carbon per unit of Chla) 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 corr esponded 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. (C) Elsevier, Paris.