Reciprocal effect between mangrove species and sediment biogeochemistry in French Guiana
Résumé
Spatial and seasonal variability of physicochemical parameters (salinity, pH, redox potential and sulphide concentration) were investigated in the sedimentary substrate of the mangrove-fringed coast of French Guiana in order to identify causes of species zonation and reciprocal effects between sediment and vegetation. Changes in salinity are mainly controlled by seasonal conditions and by location with respect to fresh water influx. No clear differences between values obtained under A. germinans and R. mangle. In contrast, a significant difference between the two species is shown by the depth distribution of redox potentials which values are remarkably high under Avicennia stands, so highlighting the ability of Avicennia to oxidize the sediment thanks to its radial cable roots. This phenomenon takes place uniquely the organic content is low. Conversely, in the older stages of Avicennia mangrove forests, the oxygen released by roots is probably insufficient to produce the decay of the large quantity of available organic matter. The low Eh values associated with the high sulphide concentrations denote sulphate-reducing processes similar to those observed under R. mangle stands. Nevertheless, the vertical shifting of water table produced by climatic influences modifies these processes. These results demonstrate that the sulphur content depends on the decay processes, which in turn depend on both the vegetal species and the sedimentary organic content. Consequently, this study suggests that, in contrast with many regions, neither salinity nor sulphide concentration are responsible for a zonation between A. germinans and R. mangle in the mangroves of French Guiana.