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Wetlands, especially marshes, support many services such as carbon catchment control or water purification led by primary producers such as phytoplankton and microphytobenthos (PB). The impact of the sedimentary compartment, as source and sink of essential nutrients for the water column, is often neglected in the study of their dynamics and water purification capacity of the systems. This work compared monthly (between February 2020 and April 2021) the benthic and pelagic primary producers' dynamics in two anthropised freshwater marshes (Marans and Genouillé), with the simultaneous follow-up of physico-chemical parameters of the water column and nutrient fluxes at the sediment-water (SWI) interface. It was suggested a strong contribution of phytoplankton (pumping) and the benthic compartment (denitrification) to the water purification of these two nitrates (NO

International audience; In freshwater systems, sediment can be an important source for the internal loading of phosphate (PO4). The limiting character of this element in such system leads to consider this phenomenon in terms of eutrophication risks and water quality stakes. A four-months follow-up (January, March, April and May 2019) was carried out in a strong PO4 limited secondary channel from an artificial irrigation system of Charente Maritime (France) to link the mobilization of remineralization products in the upper 6 cm layer of sediment (conventional core slicing/centrifugation and DET probes) and the phytoplankton biomass dynamics in the water column. Results showed congruent patterns between the temporal succession of the organic matter mineralization processes in the sediment and the primary biomass dynamics in the water column. In January and March (considered in winter), PO4 proved to be retained by adsorption onto iron oxides in anoxic sediment since pore water nitrate inhibited for about a month the respiration of metal oxides in the first cm of sediment, thus limiting PO4 availability and the phytoplankton growth. In April and May (early spring), after exhaustion of pore water nitrate, the dissolutive reduction of iron oxides released PO4 into pore water generated a significant diffusive outgoing flux from the sediment to the water column with a maximum in April (-1.10E-04±2.81E-05 nmol cm-2 s-1). This release coincided with the nanophytoplankton bloom (5.50 µg Chla L-1) and a potential increase of PO4 concentration in the water column. This work provides some insight on the importance of benthic-pelagic coupling in anthropogenic systems. This conceptual model has to be deployed on other sites of interest where internal loading of P takes precedence over external inputs and nitrate mitigation drives its benthic recycling and ultimately its bioavailability. This is to be essential to characterize the aquatic environment quality in order to limit eutrophication risks.

International audience; The increase in extreme events such as storms is one of the major threats that coastal ecosystems will have to face in the near future. In such a context, both maturation and ecological successions processes remain at the core of ecology to better anticipate the changes to ecosystem biodiversity and functions facing environmental stressors. However, these concepts are mainly approached through closed experimental studies that oversimplify the mechanisms. A survey was carried out on a ‘natural’ and open ecosystem subjected to an acute disturbance, i.e. a marine submersion of freshwater drained marshes, occurring after a storm. Plankton biomass, production and taxonomic/functional phytoplankton diversity were followed weekly at four stations over 2 months. Most of the stations were disrupted by this acute disturbance and displayed gradual growth and development, as described in the classical maturation process. The main differences between stations were attributed to the heterogeneity of the communities before the storm, the intensity of the disturbance and the different human actions performed to recover the freshwater environment. The concept of ‘ecological resilience’ was thus better suited than ‘engineering resilience’ for such open systems facing constant fluctuations in environmental drivers. With regard to ecological succession, the more impacted stations were marked by a significant change in taxonomic beta-diversity, with numerous stochastic processes, due to taxa dispersion. They first exhibited a convergence in functional traits due to the increase in nutrient availability drained from the catchment basin and then an increase in divergence when nutrients became limited.

The functional diversity of two planktonic functional compartments, the nano-microphytoplankton and the mesozooplankton was used in order to better understand i) the drained marshes functioning and their related ecological functions, ii) the impacts of human control (replenishment) and human activities on the catchment basin (urbanization and catchment basin size). It was based on a monthly seasonal survey on 7 freshwater drained marshes. Both nano-microphyto- and mesozooplankton displayed high seasonal variations linked to the environmental fluctuations and human control on sea lock gates. Winter presented the lower biomasses of both compartments. Winter that is characterized by low water temperature, low light availability and high flood is actually related to the dominance of tychopelagic phytoplankton and K-strategists zooplankton. Spring and summer were characterized by i) the succession of pelagic large cells, small cells and then taxa with alternatives food strategies due to nitrogen limitation and phosphorous desorption from the sediment leading to eutrophication processes and ii) the dominance of r-strategists for mesozooplankton. The artificial summer replenishment acts positively on water quality by decreasing the eutrophication processes since the nitrogen inputs limit the proliferation of phytoplankton mixotrophs and diazotrophs and increase the ecological efficiency during the warm period. Both small and large catchment basins may lead to summer eutrophication processes in drained marshes since the largest ones imply higher hydrodynamic features at the root of large inputs of nitrogen nutrient favoring the phytoplankton development while the smallest ones exhibit hypoxia problems due to high proliferation of macrophytes. Urbanized marshes are less subjected to eutrophication during summer than non urbanized marshes due to more recurrent nutrient inputs from urban waste. However they exhibited a lower ecological efficiency. The results suggest that a better management of the hydrodynamics of such anthropogenic systems can avoid eutrophication risks on coastal areas.

International audience; The degradation of water quality and the multiple conflicts of interest between users make marsh restoration very important. A Water Quality Evaluation System (WQES) was developed for river systems by the European Water Framework Directive (WFD). Some form of biologically-based, habitat-specific reference standard seems absolutely essential for wise management and stewardship of marsh ecosystems. The goal of this study was to develop a statistical method to define and to characterize a water body typology for drained marshes of the Charente-Maritime wetlands on the French Atlantic coast, placing particular emphasis on environmental factors as hydraulic functioning, human activities and pedological substratum. The Charente-Maritime marshes represent a good field study because of his high diversity of types of marshes and of anthropogenic activities in a restrictive area thus erasing spatial climatic effect (latitude effect). The statistical method developed here had permitted to define and characterize 12 different water bodies, 7 in freshwater (F1 to F7) and 5 in salt water marshes for the Charente-Maritime area. This typology demonstrated an important link between the size catchment area, nitrate concentrations, and leaching of precipitation from cultured soils. Even though the Charente-Maritime marshes are strongly impacted by humans, they may still retain the ability to remove nitrate. The increasing gradient of water renewal in the freshwater marshes from F1 to F7 explained the decreasing gradient of eutrophication. A better management of the hydrodynamic of the marshes can avoid eutrophication risk on the coastal sea area. Reliance on the WFD parameter set necessarily placed limits on the kinds of interpretations that could be made and on the study's potential contribution to the basic science of marshes. Ecologically-based insights regarding both external flows (links between ecosystems, meta-ecosystem theory) and internal flows (structure of the planktonic food web) seem an essential prerequisite for further advances in the study of marsh ecosystems.

We conducted a weekly survey at two stations of a French coastal marsh during the transition from winter to spring, when the sea lock gates were closed. Field measurements and laboratory experiments were combined in order to describe the structure and dynamics of planktonic food webs. Physico-chemical parameters were measured, and food web typology was described for the first time in the marsh using plankton biomass and internal flux assessment. Both stations changed from a “biological winter” to food webs identified as “multivorous” and passed through to a herbivorous food web. However, food web structure differed significantly between the two stations at the end of the study. Station A remained as a multivorous food web, while station B changed to a strong multivorous food web. After the sea lock gates closing (end of March), an increase in phosphorus concentrations in the water column at station B may have controlled bacterial and phytoplankton development and could explain, at least in part, the differences between the two stations. Our study suggests moreover that differences in water renewal between the two stations could have been responsible for the differences observed. Sea lock gates closing seems to be responsible for the rapid changes observed in food web structure, suggesting that the effects of human hydraulic management on ecosystem functioning in marshes is not yet well understood.