TOPHYPAC. Tolérance des communautés phytoplanctoniques aux phytosanitaires dans le panache de la Charente. Programme Évaluation et réduction des risques liés à l’utilisation des Pesticides

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.