Response of oligochaete (Tubificidae and Lumbriculidae) and Diptera Chironomidae communities to the decrease of phosphorus concentrations in Lake Geneva (Little Lake)

A survey carried out in 1999, in the same way as in 1994, indicated that th e composition of oligochaete and chironomid communities changed during this period in response to the decrease of phosphorus concentrations in the wat er of the western basin (the Little Lake) of Lake Geneva (Switzerland and F rance). Chironomid larvae were present in 66 % of 159 sites in 1999 against 40 % in 1994 whereas the abundance of oligochaetes decreased. In addition, the relative abundance of taxa indicator of oligotrophic conditions (Micro psectra and Spirosperma velutinus) was greater in 1999. According to these changes, the biological recovery of deep sediment was in 1999 mid-way betwe en the meso-eutrophic state prevailing around 1980 and the oligotrophic sta te prevailing before 1950. However, the extent of recovery was not the same in the three basins of the Little Lake: it varied according to the intensi ty of organic deposition.