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Impact of shrimp farming effluent on benthic organic matter dynamics and their implications for biogeochemical processes in a mangrove (New Caledonia)

Team 6: Source and transfer of organic matter in aquatic ecosystems

MOLNAR Nathalie

Year of defence: 

In New Caledonia, the mangrove is used as a buffer zone between the shrimp farms and the lagoon, in order that the mangrove acts as a biofilter that would allow the treatment of the organic load of effluents. While the impacts of effluents on surface water in the mangroves are relatively well understood, their effects on the benthic compartment, at both geochemical and biological, remained largely unknown. The objective of this thesis was to understand the ability of the mangrove to process the effluent in its substrate. A dual problematic was defined, understanding the processes of mineralization of organic matter below different mangrove stands, and monitoring the effluent in a spatiotemporal way, using molecular and isotopic tools. Sampling was conducted during a year in order to cover the cycle of the farm, as well as seasonal variations, in two mangroves, one receiving effluents from a shrimp farm, and the other serving as control area. The MO contained in the effluent is dispersed homogeneously in the mangroves by the tides, some is deposited and is subjected to decay processes, but some is exported from the ecosystem, the mangroves cannot buffer the entire organic load. The zonation of mangrove clearly plays a key role in the natural treatment of effluents. Due to the organic nature of its substrate, and the fact that it is daily swept by tides, the Rhizophora stand appears as a zone of preferential dispersion of effluents. Its functioning does not appear altered by the effluent discharge. As for Avicennia stand, despite the amount of MO which is deposited, the substrate does not switch to anoxia due to intense bioturbation resulting from the density of crabs and the physiological activity of the root system, which are both boosted by the contribution of effluents. However, it may not be used as a discharge area because of the modification of the sedimentation and consequently of the immersion time, 

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