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International audience; Diatoms are found in almost all aquatic environments as long as light is available for their photosynthesis. In coastal ecosystems, pelagic diatoms are extensively studied, but their benthic counterparts, more discreet and more difficult to access, are often ignored. Yet living at much lower light intensities, they are a key compartment of coastal ecosystems functioning, being able to produce as much biomass as pelagic diatoms that live in the well-lit surface waters. Benthic diatoms differ considerably from pelagic forms on several aspects: First, pelagic and benthic diatoms do not follow the same seasonal dynamics: In the bay of Brest, we showed that benthic diatoms are the first to bloom in the season, providing food and energy to the ecosystem right from the beginning of spring (with 60% of the total biomass until April). The system then moves from a system dominated by benthic biomass in early spring to a system where the pelagic biomass dominates. Regarding biodiversity, most benthic diatoms species are different from those found in the water column, they are mostly pennate forms and are far more diverse. On the physiological side, benthic diatoms are incredibly well adapted to low light intensities. However, the mechanisms which result in this adaptation remain unclear. We search for the role of the nanostructure of the frustule in the capture of light, but no difference between pelagic and benthic diatoms could be highlighted so far. Of particular interest is that benthic diatoms able to migrate into the sediment, are adapted to the high Si(OH)4 concentrations found in their surrounding and evidence a multiphasic Si uptake with very high half saturation constant (KS). Their capacity to reverse benthic Si(OH)4 fluxes at the interface indicate the important role they also play in the coastal silica cycle and particularly in controlling the pelagic diatom production. It remains to examine the coupling between carbon and silica at the water/ sediment interface and to determine which factors control the benthic production. Further studies are also needed to understand which evolution has allowed pennate diatoms to colonize so successfully the benthic habitat.

TOPHYPAC is a field study that was conducted in the Charente river estuary (Marennes-Oléron bay). The main goal was to assess the possible impact of chemical contamination with copper and pesticides on phytoplankton communities. Physical, chemical (nutrients, metals and pesticides) and biological (phytoplankton community) parameters were monitored in the field during three years, from 2011 to 2014. In 2012 and 2013, field sampling was coupled with experimental exposure of phytoplankton communities to copper and to an herbicide cocktail, after transportation to the lab: the effects were assessed using photosynthesis endpoint and cellular densities. TOPHYPAC allowed to set the framework of Charente estuary contamination with pesticides: herbicides glyphosate (and its metabolite AMPA) and metolachlor were almost permanently found at the most upstream station (Fort Lupin) from the studied area. The most elevated concentrations were noticed during the first part of each year. Samples from spring and early summer also exhibited other herbicide substances: dimethenamid, acetochlor, bentazon, mesotrion, metamitron. Phytoplankton evolution in the natural environment was studied as a whole but also on the following community subsets: micro-phytoplankton, nano-phytoplankton, pico-phytoplankton and cyanobacteria Synechococcus sp.. The most influent driver on the whole phytoplankton community abundance was shown to be temperature, related to seasons; however other variables such as salinity, tubidity and solar radiation were also significant drivers for the community subsets. The analysis model used highlighted an abnormal low abundance period for Synechococcus sp. during spring 2012. At the same time, a massive freshwater discharge that brought pesticides was also noticed: this event was not identified as responsible for this drop, but it could weaken the communities. The experimental part of the study showed a marked sensitivity of phytoplankton communities that were exposed to copper during the year 2012: especially, Synechococcus sp. were shown to be impacted by copper, at concentrations at the µg.L-1 level. The herbicide cocktail used during the year 2012 (glyphosate, metolachlor and mesotrion) induced no significant effect on phytoplankton communities, whatever the concentration tested and the season. During the year 2013, the modified cocktail (glyphosate, S-metolachor, dimethenamid and metamitron) led to significant but slight effects on photosynthesis with no variation related to stations or seasons. Tests run from spring to autumn during two years did not demonstrate phytoplankton sensitivity towards the tested herbicides at environmental relevant concentrations., TOPHYPAC est une étude sur l’estuaire de la Charente (bassin de Marennes-Oléron). L’objectif général du projet était d’évaluer l’impact éventuel de la contamination chimique par le cuivre et les pesticides sur les communautés phytoplanctoniques. Un suivi environnemental comportant des paramètres physiques, chimiques tels que les nutriments, métaux et pesticides, et biologiques (phytoplancton) a été réalisé de 2011 à 2014 ; ce suivi a été couplé, en 2012 et 2013, à une approche expérimentale sur les communautés de phytoplancton transplantées en laboratoire et exposées au cuivre et à deux mélanges d’herbicides. Les effets des contaminants testés ont été recherchés sur la photosynthèse et les abondances du phytoplancton. Le projet TOPHYPAC a permis de dresser un état des lieux de la contamination de l’estuaire par les pesticides : les analyses ont révélé une présence quasiment permanente des herbicides glyphosate (et son métabolite l’AMPA) et métolachlore au niveau de la station la plus en amont (Fort Lupin) du site d’étude. Les concentrations les plus importantes sont retrouvées dans la première partie de l’année. La période de printemps et début d’été est également celle où d’autres herbicides sont présents dans l’environnement tels que : diméthénamide, acétochlore, bentazone, mésotrione, métamitron. L’évolution du phytoplancton dans son milieu naturel a été analysée de manière globale mais aussi sur des sous-groupes : micro-phytoplancton, nano-phytoplancton, pico-phytoplancton et les cyanobactéries Synechococcus sp. L’influence prépondérante de la température, liée à la saisonnalité, a été démontrée sur l’abondance globale de la communauté ; cependant des paramètres tels que la salinité ou la turbidité, ainsi que le rayonnement, influencent aussi notablement les différents sous-groupes. Le modèle utilisé a mis en évidence une période pendant laquelle les Synechococcus sp. étaient anormalement peu abondants, au printemps 2012. La survenue concomitante d’une importante dessalure accompagnée de pesticides ne semble pas être en cause, mais pourrait constituer un facteur fragilisant les communautés. Au niveau expérimental, les expositions des communautés au cuivre ont montré une sensibilité plus importante de celles-ci en 2012 : en particulier, une sensibilité très marquée des Synechococcus sp. au cuivre a été démontrée, avec des valeurs seuils d’effets parfois de l’ordre du µg.L-1. Le mélange d’herbicides utilisé en 2012 (glyphosate, métolachlore et mésotrione) n’a conduit à aucun effet significatif aux concentrations testées sur les communautés, quelle que soit la période. En 2013, le mélange, quelque peu différent (glyphosate, S-métolachore, diméthénamide et métamitron), a entraîné des effets significatifs modérés, uniquement sur la photosynthèse, et relativement stables au niveau spatial (stations) et temporel (saisons). Il semble que les essais réalisés du printemps à l’automne au cours de deux années consécutives ne permettent pas de conclure à une sensibilité particulière du phytoplancton aux concentrations environnementales des herbicides testés.