European salt marshes diversity and functioning: the case study of the Mont Saint-Michel bay, France

The macrotidal Mont Saint-Michel bay has beenstudied intensively since 1990. The objectives ofthis study, supported by the European Union, wereto understand various processes underlying thefunctioning of this hydrosystem with a special focuson organic matter and nutrient fluxes betweensaltmarshes and marine waters. This paper presents asynopsis of these studies. The tidalflats are unvegetated and primary production isexclusively due to microphytobenthos communitiesdominated by diatoms. Halophile plant communitiescolonize the top parts of the tidal flats. Theircomposition and production vary according to amaturity gradient and sheep grazing. In ungrazedsaltmashes, production ranged from 1080 gDW m-2·yr-1in the lower marsh to 1990 gDW m-2·yr-1in the upper marsh whereas it was only 200 to500 gDW m-2·yr-1in Salicorniaspp.dominated pioneer zones and sheep grazed areas. Mostof this organic matter (OM) was trapped in situ,processed by fungi and bacteria, and then releasedseaward via tidal fluxes, groundwater and runoff as particulate OMand nutrients: –497 kg N, –1200/–1000 kg P-PO4and –9900/–4200 kg inorganic carbon).