Fish farm impacts on meiofauna and the microbenthic loop in Posidonia oceanica meadows

For about ten years, fish farming has been expanding all over the world. Even if this way of producing fishes is presented as a solution against overfishing, its impact on the surrounding environment can be important. For example, meadows of Posidonia oceanica, the endemic seagrass of the Mediterranean coastal zone, are fading close to those aquaculture, showing negative impacts on this hot spot of biodiversity.
This seagrass is used as an indicator of perturbations, although it does not react quickly, mainly because of its low turnover rate (1,5 y-1). So, it is proposed here to use meiofauna and the microbenthic loop (organic matter, bacteria, microphytobenthos and meiofauna) of this ecosystem to detect earlier perturbations due to fish farms. Moreover, the exergy index, measuring the distance between an ecosystem and its optimum state (climax), is also calculated on the microbenthic loop in order to show its interrest in ecological studies.
Study sites are both situated in the Gulf of Calvi (Corsica, France), in P. oceanica meadows, at a depth of 22 m. The studied fish farm is small (10 cages), situated offshore in front of Calvi and produces 40 tons of seabass (Dicentrarchus labrax) per year. The reference site is located in front of the research station STARESO (STation de REcherches Sous-marine et Oceanographiques). Sediment cores (diameter: 4,7 cm) were taken in November 2008 at both sites and sliced in four layers (0-1 cm, 1-2 cm, 2-5 cm and 5-10 cm), according to the expected abundance of meiofauna organisms. Biomass and abundance of every parts of the microbenthic loop were thus analysed.
Results concerning abundance, biomass and diversity of meiofauna organisms are presented here and compared with the rest of the microbenthic loop. Values of biomasses are also integrated in the calculation of the exergy index. Finally, this study evaluate the interest of meiofauna and the microbenthic loop to detect perturbations due to an aquaculture.