Etude de l’anthroposystème emblématique de l’étang de Berre : approches écosystémique et sociologique de l’impact du cténaire invasif Mnemiopsis leidyi

Abstract : The Berre Lagoon is an ecosystem historically disturbed by strong industrial discharges and significant freshwaters inputs from both natural and anthropogenic (EDF hydroelectric power station) origins. While rehabilitation policies initiated in 1994 are already showing some success, the introduction and proliferation of Mnemiopsis leidyi since 2005 could limit their effectiveness. The originality of this study is to associate oceanography and sociology in order to estimate the impact of M. leidyi on the functioning of this socio-ecosystem. We were able to show that the population of M. leidyi is maintained within a large range of temperatures (3 °C-28 °C) and salinities (10-30), with a quantity of carbon available ~ 3 mg C L-1 or more. In the laboratory, spawning by adults was observed at temperatures as low as 8 °C, while transition larvae can spawn at 10 °C. The temperature thus appears to be a determining factor in the dynamics of the population of this ctenophore. Larval stages see their development into adults blocked for temperatures below 14 °C. In summer the population is dominated by adults with high spawning rates (2 221 ± 2 496 eggs ind-1 d-1) supporting large blooms (up to 96 ind m-3). Ingestion of Mnemiopsis on the zooplankton community (mainly copepods and meroplankton larvae) was highly variable from 4 ± 2 prey ind-1 d-1 to 1 370 ± 97 prey ind-1 d-1 but was able to control up to 80% of the standing stock of zooplankton, reducing the top-down control over the phytoplankton communities. In addition, Mnemiopsis contributes poorly through its N-NH4 excretion (~3.8% at the most) to the pool of ammonium pool and to the regenerated production, also favouring phytoplankton growth. As a result, the population acts positively on maintaining the eutrophication of the lake by "top-down" and "bottom-up" effects. The absence of this ctenophore on during cold events and its recurrence several months later suggests either the existence of an external source or the presence of a refuge zone. The use of a Lagrangian modelling of particle transport coupled with measurements of the environmental variables has not only helped to understand the distribution of Mnemiopsis within the lagoon but also to identify a refuge zone (the Vaine lagoon) favorable to the maintenance of M. leidyi. The strong proliferation of M. leidyi in the Berre lagoon mainly affects professional fishermen. The clogging of nets, the mutilation of catches, the accelerated degradation of the material and the increase in the strenuousness induce an annual economic loss estimated at 50 %. Swimming is not affected except in case of large blooms during which beaches of the lagoon might be avoided; likewise, boating activities are impacted only in case of strong blooms when the organisms clog the cooling systems of the engine. Here in our interdisciplinary framework, the understanding of the biological and the anthropogenic interactions has shown that Mnemiopsis is tempering with the implementation of the rehabilitation efforts. In addition, the recent installation of the invasive highly potent hydromedusae Gonionemus vertens could also become a real threat as the widening of its range can be associated with the transplantation of eelgrass. Therefore, in the context of biological invasions, the prospects for the rehabilitation of the Berre Lagoon remain subject to the unpredictability of nature.
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Guillaume Marchessaux. Etude de l’anthroposystème emblématique de l’étang de Berre : approches écosystémique et sociologique de l’impact du cténaire invasif Mnemiopsis leidyi. Environnement et Société. Aix-Marseille Université, 2019. Français. ⟨tel-02143401⟩

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