IMPACT OF A FLOOD EVENT ON THE BIOGEOCHEMICAL BEHAVIOR OF A MESOTROPHIC ALPINE LAKE - LAKE BOURGET (SAVOY)

A flood event which occurred during the onset of thermal stratificatio n and of algal growth (March 1996) was studied in mesotrophic Lake Bou rget (France). Both physical and biogeochemical processes occurring du ring this episode were assessed. The dominant effect was a decrease of nutrient concentrations along the river-lake ecotone. This phenomenon seemed mainly linked to biological factors: stimulation of the spring algal and bacterial growth. After the flood, the algae aggregated wit h allochthonous particles brought by the river and were rapidly elimin ated by sedimentation. The effect of this early spring flood, seemed s patially restricted and rather neutral as regards lake water quality. In the lake region crossed by the river plume, the overall effect afte r the flood was a decrease of P available for the phytoplankton. In th e open water, the P concentration was probably not modified by the loa d supplied by the hood.