Ecological Factors Acting on the Microfauna in Mangrove Swamps. The Case of Foraminiferal Assemblages in French Guiana

Thirty-seven surface sediment samples were collected in January 1999 and July 2000 in the mangrove swamps and mud flats of French Guiana where strong seasonal contrasts occur. Sampling stations were selected to provide information on the foraminiferal assemblages associated with different environmental conditions, on the mud banks and in the mangrove forest. A total of 44 species have been identified. The main parameter acting on the distribution of foraminiferal assemblages is the hydrodynamics of the estuary, characterized by the double influence of coastal water and low-Ca fresh water, with drastic seasonal changes. The coastal end member is dominated by the calcareous species Ammonia tepida, A. parkinsoniana and Cribroelphidium spp., the continental (freshwater) end member is characterized by Miliammina fusca and Trochamminita irregularis. The calcareous species penetrate into the estuary, even into the mangrove forest during the dry season, but totally disappear from the estuary during the rainy season. The second parameter acting on the distribution of foraminiferal assemblages is the vertical elevation that acts indirectly by influencing the time of aerial exposure and the colonization by mangrove trees. Both canopy and litter of the mangrove forest protect the sediment from heating, drying and increases in salinity due to sun and wind. Foraminifera are very rare or absent on the open mud banks affected by mud cracks, but begin to grow as soon as young Avicennia are present. In the mangrove swamps, local conditions influence the composition of the assemblages and the taphonomic processes.