Pore-water chemistry in mangrove sediments: relationship with species composition and developmental stages. (French Guiana).
Résumé
Spatial and seasonal variability of sedimentary salinity, pH, redox potential and solid phase sulphide concentration were investigated in a range of mangrove communities along the coast of French Guiana. Seasonal depth distributions of these parameters and organic content were compared within Avicennia, Rhizophora and mixed mangrove stands at different stages of plant development. Mangrove communities and variable surface water inputs strongly impact sediment and ground water properties. In the upper sediment, changes in salinity are mainly controlled by seasonal conditions, transpiration and proximity of fresh water influx, whereas we suggest that constant basal salinity results from an accumulation of salt that has migrated as a result of density driven convection processes. There are no clear differences between the depth distributions of salinity obtained beneath Avicennia germinans and Rhizophora mangle stands, implying that plant zonations are not primarily controlled by soil salinity in this environment. Nevertheless, R. mangle grows in places subjected to the greatest variability in freshwater influxes, suggesting that Rhizophora might require or withstand occasional inundation by fresh water. Beneath Rhizophora stands, sediment properties reflect anaerobic and sulphidic conditions close to the sediment surface. In contrast, beneath Avicennia stands, sediment geochemistry mostly depends on the stages in forest development, on contents in sedimentary organic matter and on seasonal changes. In the early stage of Avicennia settlement, the sediment at the level of radial, pneumatophore-bearing cable roots, displays permanent suboxic conditions with Eh values reaching 400 mV. These high Ehs are interpreted as an effect of the oxidation produced by the cable root system. The development of mature Avicennia stands results in accumulation of sedimentary organic matter and promotes low Ehs and the reduction of pore-water sulphate. Near cable root level, the oxidation process observed in pioneer mangroves results in a reoxidation of solid sulphides produced previously. During dry conditions, the desiccation of the upper sediment adds its oxidation effects to those of root activity. As a result, suboxic processes dominate in the upper, 20-cm-thick layer; organic matter decomposition and sulphur oxidation strongly acidify the sediment. Below 20 cm, the sediment is anaerobic and sulphidic. Hence, sulphide concentrations depend on the edaphic conditions controlling decay processes and appear to be a consequence rather than a cause of the observed zonation of vegetal species. The small size of A. germinans propagules might have a significant influence on the extensive development of this plant community along the highly dynamic coastline of the Guianas. This study demonstrates that the different properties of pore-water were intimately linked and that the explanation of the evolution of this forest reflects a combination of multiple parameters. Moreover, it appeared that the organic content played a key role along with the species composition and the seasonal variations (waterlogging, desiccation).