The aim of this study was to distinguish between sources of the complex variety of Marennes-Oléron Bay suspended particulate organic matter (SPOM) contributing to the tropho-dynamics of the Marennes-Oléron oyster farming bay. Basic biomarkers (Chl a, C/N and POC/Chl a ratios), carbon and nitrogen stable isotopes from SPOM were analyzed and the microalgae community was characterized. The sampling strategy was bimonthly from March 2002 to December 2003; samples were taken from an intertidal mudflat. Four main sources contributed to the SPOM pool: terrigenous input from rivers, neritic phytoplankton, resuspended microphytobenthos and periodic inputs from intertidal Zostera noltii meadows. Seasonal fluctuations were observed in both years of the study period: (1) SPOM collected in the spring of 2002 (δ 13 C = −25‰ to −23‰) was mainly composed of fresh estuarine inputs; (2) SPOM from the summer and fall of 2002 and 2003 was predominantly neritic phytoplankton (δ 13 C = − 22‰ to −19‰); (3) SPOM from the winter of 2002, spring of 2003 and winter of 2003 (δ 13 C = −21 to −23‰) was composed of a mixture of decayed terrigenous river inputs and pelagic phytoplankton, which was predominantly resuspended microphytobenthos. In the summer of 2003-the warmest summer on record in southern France and Europe-SPOM was particularly enriched for 13 C, with δ 13 C values ranging from −14‰ to −12‰. Pulses in δ 13 C values, indicative of 13 C-enriched decaying materials, extended into the fall. These were attributed to benthic intertidal inputs, including both resuspended microphytobenthos and Z. noltii detritus. Changes in SPOM sources in Marennes-Oléron Bay may lead to differences in the quality of the trophic environment available for reared oysters.

Spatial and interannual variations of phytoplankton diversity were characterized along the French Atlantic Coast in relation to physical factors and large-scale climatic indices using phytoplankton surveys conducted from 1993 to 2010 in four geographical areas. This study relates phytoplankton diversity to oyster recruitment success by comparing a 'specific' versus a 'functional' diversity approach. Functional diversity was represented by functional groups of different phytoplanktonic functional traits (size, shape, toxicity), relevant for oyster feeding during growth, reproduction and larval development. Phytoplankton diversity patterns along the French Atlantic coast corresponded to a geographical distribution mainly driven by a latitudinal gradient and hydrodynamic features as confirmed by the functional characteristics of the indicator species recorded for each geographical area (C-S-R strategies of Reynolds, specific habitat). This geographical typology was less clear for functional diversity. Only few functional groups were explained by physical factors: the non-toxic small and solitary cells were in particular related to temperature, with high densities observed in the southern areas. Even if temporal variation was less important in explaining the phytoplankton diversity patterns, the main drivers explaining the interannual pattern were the large-scale climatic indices, mainly the Atlantic Multidecennal Oscillation. Functional groups were readily explained by climatic indices than species even if the relationships cannot be applied generally because of the non-linearity of the correlations (local and temporal variations). However, the potentially toxic, small and linear phytoplankton were anticorrelated to NAO. The functional approach thus brings constructive elements concerning the relationships between the prey assemblage of oysters and their physical drivers. Redundancy and co-inertia analyses appeared as complementary analyses in investigating phytoplankton pattern of variation, being particularly useful in analyzing geographical and temporal diversity fluctuation, respectively.

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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...

The structure and summertime production of planktonic communities and the role of nondiatom planktonic cells were studied in coastal ponds, which are areas traditionally used for fattening and greening table-sized oysters. The abundance and biomass of nano-microplanktonic protists were determined at weekly intervals between February 1998 and February 1999 in a coastal pond without oysters in the French Atlantic coast near La Rochelle. The production of these microbiotas was determined in the summer period. The structure of plankton communities revealed the following observations: (1) microphytoplanktonic cells were mostly diatoms and dinoflagellates, (2) microzooplanktonic cells were mainly ciliates, and (3) nanoplanktonic cells were represented by pigmented (80-90% of the nanoplankton biomass) and colorless nanoflagellates. Diatoms were dominated by Naviculiineae. Dinoflagellates were dominated by Peridiniales. Oligotrichida were predominant in the ciliate community. Protist biomass levels were nine times higher from April to August (summer period 1033 microg C L(-1)) than from September to March (winter period 114 microg C L(-1)). Whatever the season, nanoflagellates were dominant in the water column (66 and 53% of the entire protist biomass in the summer and winter periods, respectively). Nanoflagellates represented the highest production of nano-microplanktonic communities (76% of carbon protist production) in the coastal pond in summer and showed the shortest generation time (7.1 h). Dinoflagellates came after nanoflagellates in production (19.5% of carbon protist production). Diatoms represented only a supplementary carbon resource available for higher trophic levels, whereas, until now, they were considered as the principal food of oysters in coastal ponds. Ciliates were a small source of carbon, but their growth rate was high. We suggest, first, that nanoflagellates represented the primary resource available in the pond and could constitute an important food resource for higher trophic levels, such as oysters, farmed in this type of pond. Overall, the system appeared to be more autotrophic than heterotrophic. Because inorganic nutrients are quickly exhausted in a semiclosed pond, pigmented flagellates dominated the carbon biomass, production and biomass of bacteria were high (thus, the microbial food web appeared to be active in this pond), and mixotrophy seemed to be an important trophic mode there.