Mangrove forests are coastal tropical wetlands that fulfill critical functions for the neighboring ecosystems. They are often under the influence of effluents from urban centers or shrimp farms but are usually considered as resistant to these anthropogenic inputs. The aim of this study was to determine the actual resistance of such ecosystem to a long-term disturbance. For this purpose we used an in situ experimental system set up in Mayotte Island (Indian Ocean) to evaluate the capacities of mangroves for the bioepuration of domestic wastewaters.

Since 2007, pretreated domestic wastewaters (fresh water enriched in nitrogen and phosphorus) are discharged into plots dominated by two varieties of mangrove trees. The vegetation, fauna (crabs and meiofauna), sediments and pore water are followed, either continuously (litter production and crab population) or punctually (vegetation structure, leaf areas, leaf pigment concentration, meiofauna communities, physico-chemistry of water and sediments).

Anthropogenic inputs over a long period led to an increase in vegetation growth associated with an increase in leaf pigment content and surface and tree productivity. A decrease in crabs density and diversity and a modification of meiofaunal community structure were also observed. These results were season-dependent.

Our results indicate that long-term anthropogenic inputs can induce a strong modification of the productivity and structure of some compartments of mangrove ecosystem, without major dysfunction. A short interruption in wastewater discharges also suggested a high resilience of crab populations. These results will be completed with an assessment of the microbial compartment, particularly the communities involved in nitrogen cycle.