Peut-on utiliser l'indice d'exergie et la boucle microbenthique comme indicateurs de perturbations dans les herbiers à Posidonia oceanica (L.) Delile?

Nowadays, many anthropogenic perturbations threaten the Mediterranean Sea. Posidonia oceanica , its endemic phanerogam, is often used as a pollution descriptor. Unfortunately, it does not react very fast because of its low turnover rate (i.e. 1.5y-1). To solve that problem, this project proposes to use the microbenthic loop (meiofauna, bacteria, microphytobenthos and organic matter) of this important coastal ecosystem to detect anthropogenic pollutions earlier, because its components may react more rapidly than the posidonia plant. Every components of the loop are investigated and characterized, in order to determine their reactions to perturbations and use them in the calculation of the exergy index. This index comes from the thermodynamic theory of ecosystems and expresses the distance of an ecosystem, at a given state, from its equilibrium. The higher this index, the closer from its optimum state (climax) the ecosystem is. To calculate it, the biomass of each part of the studied ecosystem, weighted by a factor called β, is needed. This factor is estimated on a genetic basis and given in Jørgensen et al. (2005). The final aim of this project is to show if the exergy index can be used in the Mediterranean Sea as a tool in monitoring strategies, specifically in the sedimentary compartment of P. oceanica meadows. To realise these goals, a reference meadow was chosen near the research station STARESO (STAtion de REcherches Sous-marines et Oceanographiques) in Calvi Bay (Corsica, France). This meadow is studied since the 80’s and its good state of health is proven. Seasonal, spatial (small scale) and bathymetric variations in the microbenthic loop are studied there since 2007, by sediment cores (diameter=4.7cm, length=10cm). Those results are compared for 2008 with the patterns observed in a supposed perturbed site, situated close to a fish farm, in Calvi Bay too. In situ experiments were also led in summer 2009. During three months, a shading experiment and an enrichment of the sediment with sediment coming from under the aquaculture (once a week) were performed. Those experiments aimed to understand reactions of the microbenthic loop to those kinds of perturbations, that are often occurring close to a fish farm. Up to now, that study has led to a better understanding of seasonal and bathymetric patterns in the microbenthic loop of P. oceanica. The most important result obtained is the existence of an important spatial heterogeneity at small scale in that ecosystem. This phenomenon must absolutely be taken into account in every study based on the sediment in P. oceanica meadows.Utilisation de l'exergie pour mesurer et détecter les effets des perturbations affectant les herbiers à Posidonia oceanica (L.) Delile: caractérisation au sein du compartiment sédimentaire