SEASONAL DYNAMICS OF NUTRIENT AND BIOFILM IN INTERSTITIAL HABITATS OF2 CONTRASTING RIFFLES IN A REGULATED LARGE RIVER

To describe nutrient and biofilm dynamics in interstitial habitats of riffles (in a downwelling upwelling sequence), oxygen, nitrate, dissol ved organic carbon (DOC) contents, bacterial abundances, hydrolytic an d deshydrogenasic activities of biofilms were studied for 18 months in two geomorphologically contrasting riffles of a regulated channel of the Rhone river (a 7th-order stream). The first one was located in an section of the channel affected by river bed incision, whereas the oth er section has only been slightly impacted by river bed incision. Duri ng the transit of surface water through river bed sediments, oxygen co nsumption, DOC immobilization, and nitrate production were observed es pecially during the warm seasons when biofilm activity was high. Organ ic matter decomposition by interstitial biofilms may induce nitrate re lease by mineralization and nitrification. When the oxygen content dec reased strongly during summer, exceptional nitrate depletion occurred in the sediments due to denitrification. A comparison with the Maple R iver, a 3rd-order stream (Hendricks, 1993) highlights that differences were mostly linked to the origin of groundwater, to differences in st ream order (high quantities of nutrients), and to local geomorphologic al characteristics (heterogeneity in fine sediments and total organic matter - TOM - deposition). River bed incision might induce (1) weak b iofilm development and activities in the sediment, (2) changes in vert ical distribution of bacteria and biofilm activities, (3) marked tempo ral variations of biofilm activities, and (4) decrease of sediment eff iciency in oxygen consumption, DOC immobilization, and nitrate product ion.