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Résumé :
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Berre lagoon is a Mediterranean lagoon deeply impacted by industry and urban activities. Since 1966, a hydroelectric powerplant has discharged large quantities of freshwater and nutrients into the lagoon, inducing major ecosystemic changes. The lagoon ecosystem has declined to a eutrophic state with the loss of Zostera meadows and marine macrofauna. In 1994, an extensive monitoring network for Berre lagoon was set up. Different compartments of the ecosystem were measured: water quality, sediment quality, macrophytes (including Magnoliophyta), benthic macrofauna, fisheries, and ichthyofauna. Results show a pattern of change in the ecosystem linked to the different phases of eutrophication reduction. However, in 2018, a major ecological crisis occurred, inducing anoxia over more than 90 % of the lagoon surface area. Analysis of data from the monitoring network during and after this crisis, taking into account environmental and climatic factors, provides a basis for understanding the degradation of the different compartments of the ecosystem. The origin of this crisis is a `cocktail effect' of high spring nutrient inputs, high water temperature, strong water stratification, lack of wind, lack of Zostera meadows and high benthic biomass. This crisis highlights the extreme fragility of the Berre lagoon ecosystem and shows the importance of an ecosystemic approach for the monitoring network.
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