L'acidification des torrents vosgiens. Mise en évidence. Impact sur les populations de macroinvertébrés

Les auteurs donnent les résultats d'une étude physico-chimique et biologique portant sur quatre torrents situés sur le versant lorrain des Vosges. Les stations étudiées sont placées en amont de toute agglomération et dans des secteurs non perturbés par l'activité agricole. L'assise rocheuse de nature siliceuse est granitique. La différence dans la composition des granites et leur altération pourraient être à l'origine des différences physico-chimiques observées entre torrents situés dans un même secteur.La présence permanente ou temporaire d'eaux acides appartenant au crénon et au rhitron a été établie. Cette acidité s'accompagne de concentrations élevées en aluminium (Al total > 200 µg/l). Une enquête piscicole et la réalisation de pêches électriques ont révélé l'absence de salmonidés dans les cours d'eau acidifiés en permanence, alors que leur présence est connue antérieurement. L'existence d'épisodes acides lors de la fonte des neiges et des pluies printanières a été démontrée; les auteurs retiennent les retombées acides comme cause de l'acidification des eaux de surface. Ces événements d'ordre hydrométéorologique sont caractérisés par l'augmentation des concentrations en H+, NO3-, SO42- et aluminium, ainsi que par une baisse des ions HCO3-, dans les eaux torrenticoles. Les macroinvertébrés benthiques intègrent ces épisodes et l'étude comparative de leurs populations montre des modifications apparaissant dans les secteurs acidifiés par rapport aux secteurs non perturbés. L'intégration par les invertébrés benthiques des conditions physico-chimiques se traduit notamment par la disparition des Ephéméroptères, et par un appauvrissement de la diversité taxonomique des Plécoptères, Trichoptères et Coléoptères.A biological and physico-chemical survey of four streams has been conducted in the Vosges Mountains. The areas investigated have granitic bed rock and soils in the process of podzolisation; they are regarded as sensitive to inputs of strong acids. The streams drain forested catchments that are affected by decline of the forests Coniferous trees (Abiea alba, Picea abies) are dominant.The sampling sites were located above constructed and agricultural areas (645-685 metres above sea level) thus avoiding organic pollution. Samples of invertebrate fauna in the streams were collected using a surber-type sampler. The presence of fish (Salmo trutta fario) was determined by electro-fishing.To determine water chemistry, pH, Ca2+ , Mg2+ , K+, SO42-, NO3-, total dissolved aluminium, alcalinity (T.A.C.) and HCO3-/SO42- were measured. The permanent or occasional presence of acidic streamwaters was established. Acidification occurred in all streams during snow melting and rainstorms.During these hydrometeorological events, characteristically low pH levels (pH < 5.5), low alkalinity associated with high concentrations of aluminium, nitrates and sulfates, occurred in the water. These are critical periods for aquatic organisms owing to the toxicological effects of low pH and high concentrations of aluminium.Two streams (the "Rouge-Rupt" and the "Grand-Rupt") did not contain trout.Diversity and abundance of benthic macroinvertebrates are reduced with increasing acidity. Only 17 taxa were found in the most acidic stream (mean pH : 4.68), whereas 51 taxa were found in the one with less acidity. Likewise, in the less acidic river, abundance was more than three times higher (933 ind/m2) than in the one with most acidity (295 ind/m2). Ephemeroptera were absent when the pH was below 5.9. This order of aquatic insects seems to be the most sensitive to acidification.The benthic macroinvertebrate community also contained less Trichoptera. Only some Limnephilidae and the genera Plectrocenemia and Rhyacophila were common in acidic streamwaters and seemed to tolerate the acidification process.The Plecoptera dominated benthic communities in acidified streams. Nemouridae, Capniidae and Leuctridae were common and abundant. However, apart from Chloroperla sp., the Setipalpia were absent.A decrease in the diversity and abundance of Coleoptera was observed when the pH was lower than 5.9. The only beetle found in all the streams was Heldes sp.Diptera, Oligochaetes and Flatworms occurred in all four streams and did not present any difference in their distribution when the pH was low.Knowledge of the biology and ecology of the species appears essential in order to understand the impact of acidification on macroinvertebrates.The possibility that some species can carry out their development cycle in periods of no acid stress, thus being protected from the harmful effect of acidification, is suggested. In contrast, other species are undoubtedly partly or totally eliminated, owing to their presence at a sensitive development stage during periods of intense acidification