IZOFLUX

Seagrass meadows are considered as an essential component of the coastal lagoons. The plants colonize a wide range of sediment type and contribute to the stabilization of substrate by trapping the sediment particles because of the reduction of erosion constraints. Accordingly, they play a key role in the biogeochemical cycles of lagoons. The main objective of the IZOFLUX project (3-years) is to clearly characterize the relationship between the benthic processes and fluxes in the sediments colonized by the seagrass meadows by considering the recent knowledge and the new technologies in the field of the benthic biogeochemistry of biogenic elements, the microbiology associated with these cycles, and the sedimentary hydrodynamics. The greatest originality of the project is its integrative approach, which would enable for the first time to convert a cover of eelgrass in term of flux at the lagoon scale, while characterizing the associated processes. The ambitious nature of such an approach is only possible by creating a synergy between the various communities of biogeochimists, benthologists, microbiologists, sedimentologists, modelors, which are all scientific actors of site. The major point of the IZOFLUX project is without any doubt the complementary skills of these actors. Work will focus mainly on the Arcachon Basin, which inhabits the largest eelgrass meadows of Europe. However, this eelgrass cover has significantly decreased since 2005 and this regression process is going on. So, It is urgent of studying the eelgrass ecosystem to identify the causes and the consequences that could have this regression on the risk of eutrophication of the lagoon. This risk is directly related to the biogeochemical cycles and to the dynamics of sediment particles. As a multisite approach, we plan to visit contrasted lagoons as well: one in a poor ecological state where a attempt of re-establishment of eelgrass has recently started (Etang de Berre), the other in good condition ecological (a languedocian lagoon). The IZOFLUX Project is articulated around the following objectives: 1) To understand the effects of eelgrass on the biogeochemistry of sediments by using in situ methodologies to work on various spatial (from the roots to the ecosystem scales) and temporal scales (tidal, diurnal and seasonal cycles). 2) To quantify the benthic fluxes of oxygen, dissolved inorganic nitrogen and phosphorus, as well as sedimentary particles by recent and innovative technologies including the eddy-correlation system, in situ chamber and ultrasonic sound altimetry. 3) To define quantitatively the reaction pathways associated to the N2 production in the new paradigms on the benthic cycle of nitrogen (denitrification, anammox; role of foraminifera) 4) To build a diagenetic model, which consider all the benthic mechanisms. The complementary skills of actors, the strategy adapted including both field and experimental approaches and the transfer towards the modelling will enable us for the first time to provide an answer on the relations between the density of eelgrass, their regression or their re-establishment, and benthic fluxes for in fine defining in a complete way the role of the eelgrass meadows on the biogeochemistry and the sedimentation of a coastal system.