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The vertical distribution of chlorophyll a, copepods, dissolved free amino acid concentration and the fixation of a4C by phytoplankton were monitored in the springs of 1983, 1987 and 1988 in the Ushant front region, shelf edge of the Celtic Sea and central Irish Sea, respectively. In each area, two stations characterized by mixed and stratified water conditions were compared. Vertical distributions of amino acids coincided with the distribution of copepods. A positive and significant correlation was found between the abudance of copepods and the concentration of amino acids dissolved in seawater. A negative and significant correlation was found between chlorophyll a and the concentration of amino acids. Enrichment of amino acids (_> 20 to 500 nM 1-t at specific depths) due to aspartic and glutamic acids, glutamine and ornithine, was assumed to reflect copepod feeding activity and faecal production. At these depths, the natural concentration and diversity of amino acids, including aspartic acid, glutamic acid, asparagine, serine, histidine, glutamine, arginine, threonine, glycine, alanine, tyrosine, valine, phenylalanine, ornithine and lysine, were high enough and in the correct proportions for triggering feeding and swimming and swarming behavior of copepods, as well as their remote detection of food at the micro-and meso-scales (1 to 10 m). This accumulation of amino acids also constitutes a potential additional source of organic nitrogen for bacteria and phytoplankton.

Physical, chemical, and biological parameters relevant to phytoplankton were measured in late summer 1982 across the southern part of the Ushant front. Daily primary production rates (g C m-' d-') increased and assimilation number decreased from the open sea to the frontal zone. However, productivity (mg C [mg Chl a]-' h-') in the frontal upper layers (> 10 O/ O light surface intensity) was significantly higher than in stratified and transitional waters. Offshore, no dinoflagellates but only nanoflagellates were observed, with diatoms (mainly Nitzschia seriata) in more coastal waters. On spring tides, diatoms were widespread over the transitional area, even on the stratified side. Physiological state of the phytoplankton was assessed on the basis of photosynthetic properties, adenylate energy charge, ATP: Chl a and POC: Chl a ratios, and specific growth rates. Physiological activity of nanoflagellates in well-stratified, nutrient-deplete waters was mainly influenced by light level. Physiological state of the diatom populations was not constant over the transitional area, but in the stratified frontal zone was greater in the upper layer. On the mixed side, by contrast, the physiological state of the phytoplankton was lower, particularly as regards light adaptation.

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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Small‐sized vegetative cells were found to co‐occur with normal‐sized cells in populations of the European bloom‐forming dinoflagellate Gymnodinium cf. nagasakiense Takayama et Adachi, currently known as Gyrodinium aureolum Hulburt, but not in populations of the closely related Japanese species Gymnodiniumn agasakiense. We examined how cell size differentiation may influence growth and cell cycle progression under a 12:12‐h light: dark cycle in the European taxon, as compared to the Japanese one. Cell number and volume and chlorophyll red fluorescence in both species varied widely during the photocycle. These variations generally appeared to be related lo the division period, which occurred at night, as indicated by the variations of the fraction of binucleated cells (mitotic index) as well as the distribution of cellular DNA content. “Small” cells of G. cf. nagasakiense divided mainly during the first part of the dark period, although a second minor peak of dividing cells could occur shortly before light onset. In contrast, “large” cells displayed a sharp division peak that occurred 9 h after the beginning of the dark period. The lower degree of synchrony of “small” cells could be a consequence of their faster growth. Alternatively, these data may suggest that cell division is lightly controlled by an endogenous clock in “large” cells and much more loosely controlled in “small” cells. Cells of the Japanese species, which were morphologically similar to “large” cells of the European taxon, displayed an intermediate growth pattern between the two cell types of G. cf. nagasakiense, with a division period that extended to most of the dark period.

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