Evidence of streamwater acidification in the Vosges Mountains (Northeastern France): impact on macroinvertebrate communities
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.