THE MACROINVERTEBRATE COMMUNITIES OF STREAMS DRAINING A SMALL GRANITIC CATCHMENT EXPOSED TO ACIDIC PRECIPITATIONS (VOSGES MOUNTAINS, NORTHEASTERN FRANCE)

The physico-chemical characteristics and the macroinvertebrate communi ties of six streams draining a small granitic catchment located in the Vosges mountains (northeastern France) have been studied. Among the s treams, five were affected by acidification and characterised by low p H (mean pH: 4.90 to 5.90), low HCO3- concentrations (mean concentratio n: 0-16 mu eq l(-1)) and elevated aluminium concentrations (mean conce ntration: 154-278 mu g l(-1)). Only one stream was typical of waters d raining granitic bedrock (mean pH: 6.90; mean aluminium concentration: 79 mu g l(-1); mean HCO3-; concentration: 152 mu eq l(-1)). The macro invertebrates communities of the acidified streams were strongly affec ted compared to the non-acidified one. Richness, abundance and diversi ty (Shannon index) were significantly lower when acidification occurre d. All the major taxonomic groups were affected, but Molluscs and Ephe meroptera were the most acid-sensitive organisms, whereas Plecoptera a nd Oligochaetes were dominant. However, recolonization of acidified st reams was possible during summer low flow when the global water qualit y slightly increased.