Eutrophisation du Léman indiquée par les communautés d'oligochètes: campagnes 1982–1985

Oligochaete communities (tubificid and lumbriculid worms) were studied between 1982 and 1985 in 23 areas of the mesoeutrophic Lake Geneva (Switzerland). Species numerically dominant in eutrophic lakes (mostlyPotamothrix hammoniensis), in mesotrophic lakes (mostlyP. vejdovskyi) and in oligotrophic lakes (mostlyStylodrilus heringiamus) constituted 52%, 36% et 12% of the worm communities, respectively (means across the 23 areas). The relative abundance of eutrophic species increased (15 to 94%) according to depth or to external organic inputs. In this case, the mean biomass (wet weight) of oligochaetes per area was higher than 15 g·m⁻² (up to 210 g). Mesotrophic species decreased (65 to 1%) along the same gradients of depth or of sedimentation. Oligotrophic species increased (0 to 31%) only in areas where the organic sedimentation was lessened by steep bottom slope and by currents. Thus, the structure of oligochaete communities reflected the patterns of sedimentation and the trophic state of Lake Geneva.