On the following subject: "Study of the emblematic anthroposystem of the Etang de Berre: ecosystemic and sociological approaches to the impact of the invasive cténaire Mnemiopsis leidyi".
Graduate school: Environmental Sciences (251)
Director: Delphine Thibaut, Senior Lecturer
Co-director: Cécilia Clayes, Senior Lecturer
Abstract
Study of the emblematic anthroposystem of the Etang de Berre: ecosystemic and sociological approaches to the impact of the invasive cténaire Mnemiopsis leidyi
The Etang de Berre is a lagoon ecosystem that has historically been disrupted by heavy industrial discharges and significant natural and man-made freshwater inputs (EDF hydroelectric power station). While the policies to rehabilitate the Etang de Berre, initiated in 1994, are beginning to bear fruit, the introduction and proliferation of Mnemiopsis leidyi in 2005 could limit their effectiveness. The originality of this study is to combine 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 M. leidyi population is able to maintain itself over a wide range of temperatures (3°C-28°C) and salinities (10-30), for a quantity of available carbon > 3 µgC L-1. We have demonstrated in the laboratory that adults can oviposit as early as 8°C, and that transient larvae can oviposit as early as 10°C. Temperature therefore appears to be a determining factor in the population dynamics of this cetarian. Larvae are unable to develop into adults at temperatures below 14°C. In summer, the population is dominated by adults with high reproduction rates (2,221 ± 2,496 eggs ind-1 d-1) supporting large blooms (up to 96 ind m-3). The ingestion of Mnemiopsis on the zooplankton community (mainly copepods and meroplankton larvae) is highly variable from 4 ± 2 prey ind-1 d-1 to 1,370 ± 97 prey ind-1 d-1, but shows a capacity to control up to 80 % of the zooplankton stock reducing the top-down control on the phytoplankton communities. In addition, Mnemiopsis makes a small contribution via its N-NH4 excretion (up to 3.8 %) to the ammonium pool and regenerated production, which also promotes phytoplankton growth. As a result, the population has a favourable effect on maintaining the eutrophication of the Etang de Berre by means of a "top-down" and "bottom-up" effect. The absence of this cetenary during cold events and its reappearance several months later suggests either the existence of a source external to the lagoon or the presence of a refuge zone. The use of Lagrangian modelling of particle transport coupled with measurements of environmental variables not only enabled us to understand the distribution of Mnemiopsis in the pond but also to identify a refuge area (Vaine pond) favourable to the maintenance of M. leidyi. The heavy proliferation of M. leidyi in the Etang de Berre mainly affects professional fishing. The heavy clogging of nets, the mutilation of catches, the accelerated deterioration of equipment and the increase in drudgery result in an annual economic loss estimated at 50 % by fishermen. There is little impact on bathing, except in the event of heavy proliferation, during which time the lagoon's beaches may be deserted. Similarly, boating is only inconvenienced in the event of heavy proliferation, when ctenaria clog engine cooling circuits. In an interdisciplinary context, an understanding of biological and human interactions has shown that Mnemiopsis is an obstacle to the implementation of rehabilitation efforts. In addition, the recent establishment of the invasive stinging hydromedusa Gonionemus vertens could also be a threat due to the expansion of its range linked to the transplantation of eelgrass beds. Consequently, in a context of biological invasions, the prospects for the rehabilitation of the Etang de Berre remain subject to the unpredictable nature of nature.
Key words: Mnemiopsis leidyi, Invasive species, Ecophysiology, Interdisciplinarity, Etang de Berre
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