Ecological impacts of groundwater discharge to Mediterranean coastal lagoons

Abstract : Whilst the role of groundwater discharge and porewater fluxes (recirculation) as important pathways for nutrient delivery to coastal systems is increasingly being recognized, there remains limited evidence of its “downstream” ecological implications. This thesis aims at investigating the ecological role that groundwater flows play in some aspects of the functioning and vulnerability of coastal lagoonal ecosystems. Two contrasting lagoons on the French Mediterranean coastline were studied (La Palme and Salses-Leucate lagoons). Firstly, using concurrent water and radon mass balances, a comparison between the main nutrient sources for La Palme lagoon (karstic groundwater, recirculation, diffusion from sediments, inputs from a sewage treatment plant and atmospheric deposition) revealed that the recirculation of lagoon water through the lagoon sediments is the main source of both dissolved inorganic nitrogen (DIN) and phosphorous (DIP) to this lagoon. Secondly, the ecological impact of groundwater discharge and porewater recirculation fluxes was assessed by investigating their role in supporting primary production using nitrogen and carbon isotopes signatures. The nitrogen isotopic signatures in primary producers reflect predominantly the nitrogen isotopic signatures of the terrestrial (karstic) groundwater and porewater source in both La Palme and Salses-Leucate lagoons, demonstrating the important role of these sources in supporting primary production. The carbon isotope signatures indicate groundwater discharge as a significant source also of dissolved inorganic carbon to primary producers. Finally, in a different approach, the influence of terrestrial groundwater discharge on the growth of the Mediterranean mussel Mytilus galloprovincialis is documented. Variations in growth rate and condition index (tissue weight / shell weight) of mussels growing in and outside groundwater-influence in Salses-Leucate lagoon were examined. Mussels from the groundwater-influenced sites have higher growth rate and condition index compared to those from the control site, likely as consequence of both the higher winter temperatures and the groundwater-driven nutrient supply that increase the food availability to support mussel growth. Estimated growth rates from the groundwater-influenced sites are amongst the highest rates recorded in the Mediterranean region. The results of this thesis demonstrate the important role groundwater processes can play in coastal ecosystems.
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Aladin Andrisoa. Ecological impacts of groundwater discharge to Mediterranean coastal lagoons. Biodiversity and Ecology. Aix-Marseille Université, 2019. English. ⟨tel-02392125⟩

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