DISTRIBUTION OF DISSOLVED ORGANIC-CARBON AND BACTERIA AT THE INTERFACE BETWEEN THE RHONE RIVER AND ITS ALLUVIAL AQUIFER

To understand the efficiency of interstitial habitats in the eliminati on of organic matter as it moves from surface water to groundwater (ba nk filtration), we studied spatial and temporal variations of sediment organic matter concentration, biodegradable (BDOC) and refractory (RD OC) fractions of the dissolved organic carbon, bacterial abundances, a nd microbial enzymatic activity in the first metre of sediment of the Rhone River immediately downstream of a large city. The study area was fed most of the year by the surface water inflow (downwelling area), because of groundwater pumping wells located similar to 80 m from the river. Decreasing gradients from surface water to deep sediments and f rom the river to the shore were observed in most of the cases for the tour variables. The decrease in RDOC concentrations did not vary seaso nally (this decrease is probably due to physical process, such as adso rption on fine mineral particles), whereas decreases in BDOC concentra tions only occurred when microbial enzymatic activities were high; BDO C is rapidly assimilated by microbial communities. Physical and biolog ical processes together make this first metre of sediment an efficient filter for organic matter.