Abstract :
[en] Tropical near-shore coastal ecosystems receive 60% of the world freshwater and an equivalent fraction of organic matter. Thus, these regions are expected to have a major role in the overall budgets of CO2 and CH4, two major greenhouse gases, in the coastal and global oceans. As a contribution to the understanding of the role of the coastal ocean and continental aquatic environments in the global cycle of these gases, we report the seasonal variability of partial pressure of CO2 (pCO2), CH4 concentration, related air-water fluxes of CO2 and CH4 and ancillary data in several contrasted tropical coastal ecosystems in terms of geomorphology, lithology of the drainage basin, freshwater and seawater inputs, and riparian population and related anthropogenic pressure (land use change, aquaculture, waste waters release, eutrophication, and invasive species proliferation). We investigated waters surrounding two forested mangrove sites (Tam Giang and Kiên Vàng) located in Ca Mau Province (South-West Vietnam), in five lagoons (Grand-Lahou, Ebrié, Potou, Aby and Tendo) and three rivers (Comoé, Bia and Tanoé) flowing into these lagoons in Ivory Coast. Data from the two forested mangrove sites in South-West Vietnam were obtained during the dry and rainy seasons, providing for the first time information on the seasonality of dissolved inorganic carbon (DIC) and air-water CO2 fluxes in the water surrounding mangrove ecosystems. Our data suggest an increase of heterotrophic activity in sediments and/or the water column during the rainy season that could be due to an increase of carbon inputs from soil flushing, probably from the land surrounding the mangrove forests. The airwater CO2 fluxes we computed are consistent with the few data available so far in waters surrounding mangrove forests, and confirming that this emission of CO2 is significant for the carbon budget of mangrove forests, and also for the regional CO2 budget at tropical and subtropical latitudes. Data in lagoons and rivers of Ivory Coast were obtained during four cruises covering the main climatic seasons (high dry season, high rainy season, low dry season and low rainy season). The three rivers were oversaturated in CO2 and CH4 with respect to atmospheric equilibrium, and the seasonal variability of pCO2 and CH4 concentrations was due to dilution during the flooding period. The invasive floating hyacinth Eichhornia crassipes that cover these rivers can significantly contribute to the CO2 emission to the atmosphere from the aquatic compartment due to root respiration and organic matter degradation derived from these plants. However, floating macrophytes are atmospheric CO2 sinks. The surface waters of the Potou, Ebrié and Grand-Lahou lagoons were oversaturated in CO2 and CH4 with respect to the atmosphere during all seasons. In contrast, the Aby and Tendo lagoons exhibit enhanced over-saturation in CH4 but under-saturation in CO2 because of their permanent haline stratification (unlike the other lagoons) that seemed to lead to higher phytoplankton production and export of organic carbon below the pycnocline. However, the permanent stratification also leads to anoxic bottom waters favorable to a large CH4 production. Thus, the largest CH4 over-saturations and diffusive air-water CH4 fluxes were observed in the Tendo and Aby lagoons while they can act as a sink for atmospheric CO2. We highlight the importance of physical settings (permanent versus seasonal stratification) in controlling the organic C flows, modulating the atmospheric CO2 source-sink status, and the intensity of CH4 emissions to the atmosphere in these near-shore coastal ecosystems.
