Mélissa NARAYANINSAMY

Effect of eutrophication on the functioning of the mangrove ecosystem of Lamentin (Martinique): role of available particulate organic matter, blue carbon and greenhouse gas dynamics

Mangroves also play an important role in blue carbon sequestration. Indeed, they are considered a carbon sink to mitigate greenhouse gas-induced climate change. Blue carbon is a term used to describe carbon captured by oceans and coastal ecosystems. The input of labile organic matter, such as algal organic matter, tends to alter and more generally increase the rate of mineralization of refractory organic matter. This phenomenon, named "priming effect", contributes to carbon dioxide emissions by coastal ecosystems. Eutrophication leads to accelerated primary production including algal blooms, accumulation of organic matter and excessive oxygen consumption (Paerl, 2006). The priming effect could be intensified by the increased supply of labile organic matter associated with the eutrophication phenomenon.

The estuary mangrove, an ecosystem with strong physical constraints (tides, storms, etc.) is home to a remarkable plant and animal diversity.In addition, and despite the relatively modest surface of mangroves in the world (about 0.5% of coastal areas), these wetlands are characterized by a high primary production (more than 218 Tg C a -1) and participate significantly in the maintenance of an important secondary aquatic production. The large biomass of mangroves and their intrinsic physiological properties make this ecosystem, despite the small surface area colonized, an important carbon reservoir, estimated at 15% of global storage. In fact, mangroves but also mangrove sediments are now recognized as atmospheric CO2 capture sinks but also sources of organic and inorganic carbon for intertropical coastal areas. Concerning the other remarkable greenhouse gas, CH4, there are still many uncertainties on its balance within this ecosystem.

Mangroves are also subject to increasing anthropogenic pressures that affect the hydrological regimes and the functionality of estuaries in the intertropical zone. Moreover, the deforestation of mangroves for aquaculture or agriculture and the concentration of populations on the coast without wastewater treatment, alter the quality of the environment, the availability of nutrients, the decrease of dissolved oxygen in the water column and the balance between production and respiration. All these disturbances have an impact on the exchange of greenhouse gases (CO2 and CH4) in the waters and sediments of the mangrove and on its functioning as observed through the qualitative change of the Particulate Organic Matter (POM) of the waters and sediments and on the structure of the food web.

The main objective of this thesis is to highlight the effect of eutrophication on the origin and quality of the available SOM, its storage (blue carbon) and its use by consumers (mainly crabs) on the ecosystem functioning, and more broadly on the dynamics of greenhouse gases, within the mangrove of Lamentin in Martinique.

Mots-clefs : Blue carbon, Mangrove, Crabs