MACROINVERTEBRATE-INFERRED STREAM-WATER ACIDITY IN NORTH EASTERN FINLAND ALONG A SULFUR DEPOSITION GRADIENT

Recently developed weighted averaging (WA) models and widely used tole rance-limit approaches for inferring stream minimum pH from macroinver tebrates were tested in northeastern Finnish Lapland. Surface waters t here are threatened by large sulphur emissions in the Russian Kola Pen insula. The modelled sulphur deposition increases from west to east ap proximately parallel to longitude. The effect of deposition on stream minimum pH was assessed by relating the macroinvertebrate-inferred str eam minimum pH to longitude. In a test set of 17 streams, the minimum pH inferred from pooled invertebrate samples of three seasons by WA mo dels showed a strong correlation (r = 0.67-0.72) with the minimum pH ( range 6.3-7.1) observed during the spring snowmelt period, The relatio nship was slightly weaker (r = 0.59-0.69) using autumn samples only. T he tolerance-limit approaches assigned the streams into the correct ac idity class, but the inferred pH was not related to the observed minim um pH. This further demonstrates the superiority of the WA approach, e specially in detecting early signs of acidification. The minimum pH in ferred by WA from autumn assemblages of 37 streams along the depositio n gradient showed a significant negative correlation with longitude, s uggesting that the emissions from the Kola region contribute to low pH events in streams of northeastern Finnish Lapland, The results demons trate the potential of bioassessment and monitoring using invertebrate s in detecting impacts and changes that could remain unnoticed by conv entional water quality-analyses.