Along the French Mediterranean coast, a complex of eight lagoons underwent intensive eutrophication over four decades, mainly related to nutrient over-enrichment from continuous sewage discharges. The lagoon complex displayed a wide trophic gradient from mesotrophy to hypertrophy and primary production was dominated by phytoplankton communities. In 2005, the implementation of an 11 km offshore outfall system diverted the treated sewage effluents leading to a drastic reduction of anthropogenic inputs of nitrogen and phosphorus into the lagoons. Time series data have been examined from 2000 to 2013 for physical, chemical and biological (phytoplankton) variables of the water column during the summer period. Since 2006, total nitrogen and phosphorus concentrations as well as chlorophyll biomass strongly decreased revealing an improvement in lagoon water quality. In summertime, the decline in phytoplankton biomass was accompanied by shifts in community structure and composition that could be explained by adopting a functional approach by considering the common functional traits of the main algal groups. These phytoplankton communities were dominated by functional groups of small-sized and fast-growing algae (diatoms, cryptophytes and green algae). The trajectories of summer phytoplankton communities displayed a complex response to changing nutrient loads over time. While diatoms were the major group in 2006 in all the lagoons, the summer phytoplankton composition in hypertrophic lagoons has shifted towards green algae, which are particularly well adapted to summertime conditions. All lagoons showed increasing proportion and occurrence of peridinin-rich dinophytes over time, probably related to their capacity for mixotrophy. The diversity patterns were marked by a strong variability in eutrophic and hypertrophic lagoons whereas phytoplankton community structure reached the highest diversity and stability in mesotrophic lagoons. We observe that during the re-oligotrophication process in coastal lagoons, phytoplankton shows complex trajectories with similarities with those observed in freshwater lake systems.

Carbon stable isotopes and fatty acids were measured in the suspended particulate organic matter (POM) of the Thau lagoon to study its qualitative temporal changes in relation to environmental factors and to identify the food sources of bivalves over a one-yr-cycle in relation to their growth. Reciprocally, the impact of shellfish farming on POM was also studied. Oysters and mussels were sampled and measured for biometry, stable isotopes and fatty acid composition. Water samples were collected at two sites, both inside and outside of the shellfish farming area, to determine concentrations in POM, chlorophyll a (Chl a) and stable isotopes. Carbon isotopes and fatty acids in bivalves reflected seasonal changes in food sources, which varied consistently with the environment. Seasonal changes in δ 13 Highlights ► Carbon isotopes and fatty acids in bivalves reflected seasonal dietary changes. ► Terrestrial organic matter and bacteria can contribute to the diet of bivalves. ► Mussels and oysters showed significant differences in their fatty acids. ► Periods of high growth rate in bivalves were mainly fuelled by diatoms. ► Stable isotopes allow investigating the effects of bivalve aquaculture.

The decrease in water nutrients confirms oligotrophication in the period 1998-2016. ► The decrease in phytoplankton biomass was associated with a decrease in diatom abundance. ► The dominant taxa shifted from Skeletonema-Chaetoceros to Chaetoceros-Pseudo-nitzschia. ► The median proportion of dinoflagellates relative to diatoms increased significantly. ► The total dinoflagellate abundance did not change significantly.

The consumption and assimilation rates of the woodlouse Armadillidium vulgare were measured on leaf litters from five herb species grown and naturally senesced at 350 and 700 µl l CO. Each type of litter was tested separately after 12, 30 and 45 days of decomposition at 18°C. The effects of elevated CO differed depending on the plant species. In Medicago minima (Fabaceae), the CO treatment had no significant effect on consumption and assimilation. In Tyrimnus leucographus (Asteraceae), the CO treatment had no significant effect on consumption, but the elevated CO litter was assimilated at a lower rate than the ambient CO litter after 30 days of decomposition. In the three other species, Galactites tomentosa (Asteraceae), Trifolium angustifolium (Fabaceae) and Lolium rigidum (Poaceae), the elevated CO litter was consumed and/or assimilated at a higher rate than the ambient CO litter. Examination of the nitrogen contents in these three species of litter did not support the hypothesis of compensatory feeding, i.e. an increase in woodlouse consumption to compensate for low nitrogen content of the food. Rather, the results suggest that in herbs that were unpalatable at the start of the experiment (Galactites, Trifolium and Lolium), more of the the litter produced at 700 µl l CO was consumed than of that produced at 350 µl l because inhibitory factors were eliminated faster during decomposition.