THE USE OF LIPID MARKERS TO DEFINE SOURCES OF ORGANIC-MATTER IN SEDIMENT AND FOOD-WEB OF THE INTERTIDAL SALT-MARSH-FLAT ECOSYSTEM OF MONT-SAINT-MICHEL BAY, FRANCE

Salt marsh plants and seven surface sediment samples along a transect in the intertidal Bat area of Mont-Saint-Michel Bay were analysed for fatty acids and sterols. The presence of lipid markers of halophytes ( long-chain fatty acids, 18 :3 omega 3, and phytosterols) in the surfac e layers of the sediment confirms the export of organic matter from th e salt marsh to the intertidal flat. The spatial distribution of this organic matter over the tidal-flat area was controlled by the tidal cu rrents and the presence of mussel beds, Lipid markers of diatoms (20:5 omega 3 and brassicasterol) and bacteria (18:1 omega 7 and odd, linea r and branched, fatty acids) were also found in the surface sediments. Diatoms and benthic bacteria as well as organic matter From the salt marsh were the significant food sources available to the macrozoobenth os on the intertidal Bat. The ingestion of these food types by the dom inant species of the macrozoobenthos was confirmed by the presence of their respective lipid markers in the animals, The presence of these m arkers in animals subjected to a starvation experiment confirmed that these food types are really assimilated. The lipid composition of the starved animals indicated that the species studied were able to accumu late the fatty acid 20:5 omega 3 (considered to be a diatom marker), a nd that the annelid Nereis diversicolor supported an internal bacteria l population.