Oligochaetes, organic sedimentation, and trophic state: how to assess the biological recovery of sediments in lakes?

Oligochaete communities were used to assess the biological recovery of sediments after the abatement of eutrophication in Lake Geneva (Switzerland). In 1995, mean relative abundance of oligotrophic worm species (OS) was around 30% in areas of low organic sedimentation (LOS) whereas, in areas of high organic sedimentation (HOS), it was below 15%. The abundance of OS observed in LOS areas was close to values predicted from total phosphorus (TP) concentrations in the water. Recovery from eutrophication proceeded at the same pace in the water (TP) and in the sediment (OS) of LOS areas, whereas it was delayed in the sediment of HOS areas. Lake Geneva was mesotrophic according to the oligochaete communities of LOS areas, but it was meso-eutrophic according to those of HOS areas.