Mediterranean lagoons have been historically impacted by anthropogenic pressures. After more than two decades of environmental restoration efforts, most of these Mediterranean ecosystems showed an uneven recovery dynamic with respect to eutrophication, water chemical quality and are still impacted by seasonal or permanent deoxygenation. These observations are often attributed to the sediment that is acting as a source of legacy nutrients and pollutants, especially during oxygen depletion events, which can sustain eutrophication and water quality degradation. Depending on their eutrophication status, benthic habitats of Mediterranean lagoons can evolve from bare non-vegetated sediment to ecosystems dominated by opportunistic macroalgae, seagrass beds and perennial macroalgae. Their presence and resilience to deoxygenation events are expected to alter theoretical biogeochemical cycles, which are currently poorly studied. In a context of increasing deoxygenation favored by climate change, they are suspected to play an emerging and increasing role in the release of nutrients and pollutants from sediments that could delay the restoration of lagoon ecosystems. Four typical benthic habitats representing contrasted eutrophication status in Mediterranean lagoons (bare sediment, sediment with Chaetomorpha sp., sediment with Halopythis sp., and sediment with Zostera noltei) were exposed to long-term confinement using benthic chambers. Biogeochemical responses of these habitats were monitored for O₂ demand, nutrients (NH₄⁺, PO₄^3-), indicators of early diagenesis processes (Mn, Fe, sulfides) and trace elements, using water samples and sediment cores analysis. Benthic fluxes of oxygen and nutrients were the highest in habitats with Chaetomorpha sp. and Halopythis sp., macrophytes representing early stages in the recovery trajectory. When the oxygen demand exceeded its production, anoxia occurred within 1-2 days. This led to higher nutrient remobilization, their concentration in the water column then exceeded the threshold of "Bad" eutrophication status within a few days. Metal-oxide reduction in the surface sediment occurred under hypoxic conditions while sulfate reduction occurred under anoxic conditions. Significant releases of As, Sb, and Co were also observed under conditions of strong reduction and low pH in the presence of sulfides whereas precipitation and co-precipitation of sulfide with Fe, Co and Mo occurred under anoxic conditions. In bare sediments, punctual hypoxia was measured during nights with benthic fluxes of nutrients and trace elements from 2 to 100 times lower. In Zostera meadow, the O₂ consumption was not strong enough with respect to its production to induce deoxygenation events, indicating the high resilience of such habitats with respect to deoxygenation in these Mediterranean lagoons. This work documents how the eutrophication status in Mediterranean lagoons and their corresponding benthic habitats may modify the fate of oxygen, nutrients and trace elements in these coastal areas.

Primary Presenter: Julie Régis, Université de Nîmes (julie.regis@unimes.fr)

Authors:

Julie Regis, Univ. Nîmes, EA 7352 CHROME, rue du Dr Georges Salan, 30021 Nîmes, France  (julie.regis@unimes.fr)

Marion Richard, MARBEC, Univ. Montpellier, CNRS, IRD, IFREMER, Sète, France  (Marion.Richard@ifremer.fr)

Julie Leray, MARBEC, Univ. Montpellier, CNRS, IRD, IFREMER, Sète, France  (Julie.Le.Ray@ifremer.fr)

Beatrice Bec, MARBEC, Univ. Montpellier, CNRS, IRD, IFREMER, Sète, France  (beatrice.bec@umontpellier.fr)

Annie Fiandrino, MARBEC, Univ. Montpellier, CNRS, IRD, IFREMER, Sète, France  (annie.fiandrino@ifremer.fr)

Franck Lagarde, MARBEC, Univ. Montpellier, CNRS, IRD, IFREMER, Sète, France  (franck.lagarde@ifremer.fr)

Nicolas Cimiterra, MARBEC, Univ. Montpellier, CNRS, IRD, IFREMER, Sète, France  (Nicolas.Cimiterra@ifremer.fr)

Vincent Ouisse, MARBEC, Univ. Montpellier, CNRS, IRD, IFREMER, Sète, France  (vincent.ouisse@ifremer.fr)

Dominique Munaron, MARBEC, Univ. Montpellier, CNRS, IRD, IFREMER, Sète, France  (Dominique.Munaron@ifremer.fr)

Christian Grenz, Aix Marseille Univ, Université de Toulon, CNRS, IRD, MIO, Mediterranean Inst Oceano, Marseille, France  (christian.grenz@mio.osupytheas.fr)

Bruno Deflandre, University of Bordeaux, CNRS, EPOC, EPHE, UMR 5805, F-33615, Pessac, France  (bruno.deflandre@u-bordeaux.fr)

Chrystelle Montigny, HSM, Université de Montpellier, IRD, CNRS, Montpellier, France  (chrystelle.montigny@umontpellier.fr)

Rémi Freydier, HSM, Université de Montpellier, IRD, CNRS, Montpellier, France  (remi.freydier@umontpellier.fr)

Sophie Delpoux, HSM, Université de Montpellier, IRD, CNRS, Montpellier, France  (sophie.delpoux@umontpellier.fr)

Patrick Verdoux, Univ. Nîmes, EA 7352 CHROME, rue du Dr Georges Salan, 30021 Nîmes, France  (patrick.verdoux@unimes.fr)

Isabelle Techer, Univ. Nîmes, EA 7352 CHROME, rue du Dr Georges Salan, 30021 Nîmes, France  (isabelle.techer@unimes.fr)

Sylvain Rigaud, Univ. Nîmes, EA 7352 CHROME, rue du Dr Georges Salan, 30021 Nîmes, France  (sylvain.rigaud@unimes.fr)