AN ENVIRONMENTAL INDEX BASED ON LACUSTRINE MOLLUSCA

Lymnaea and small mussels can live in the profundal zone of lakes. However, as the unfavourable conditions build up (increase of dissolved O2 deficit in the hypolimnion and accumulation of organic matter in deep sediments), we observe progressive decrease in bathymetric distribution of molluscs and a progressive disappearance of Gastropoda then small mussels in the profundal, sublittoral then littoral zones. In this pattern of increasing simplification of mollusc populations several states were observed. An index of lake quality (IMOL) ranging from 0 to 8 is proposed from these observations. Only presence-absence and generic criteria are used to determine the index. The purpose of the index is to assess easily and quickly the biological quality of lakes having a relatively small area (less-than-or-equal-to 500 hect.), with depth greater than 10 metres (Zmax > 10 m) and situated at an altitude not exceeding 1 000 metres. However, the use of this index for larger deep lakes on the one hand and for very shallow lakes or ponds on the other hand is discussed. Data obtained for thirty lakes are presented.