Biodégradabilité de la matière organique dans le bouchon vaseux et la crème de vase de l'estuaire de la Loire

The organic matter trapped in the turbidity maximum zone of the Loire estuary, as a support of bacterial activity, constitutes a basic element in the biogeochemical cycle which permits us to understand the causes of the anoxia observed in September. Measurements of the main physical and chemical parameters and of bacterial biomass were carried out in the water column and in the fluid mud in September 1990 and 1991. The anaerobic fluid mud is presumed to be the centre of fermentation and/or hydrolysis processes; the formation rate of dissolved organic carbon (DOC) in this layer reaches 3 g C.m(-3).d(-1). When particles are suspended in the water column, the organic products formed in the fluid mud are integrated into the aerobic metabolic cycle and contribute to the decrease of dissolved oxygen concentrations in the water column. The fluid mud participates in ammonia regeneration in the turbidity maximum zone; the formation rate of this component ranges from 3 to 10 nmol N-NH4.(g of particulate matter)(-1).h(-1). Causes of the anoxia cannot be studied without a thorough knowledge of the transformation processes of organic and mineral components in the fluid mud.