| Summary: | Changes in fish population and water quality in response to the decline in sulphur deposition in small waterbodies in north-eastern Finland were studied by using data from two consecutive surveys of fish and water chemistry at 20 sampling sites in 1993 and in 2000. The study area consisted of three separate catchment basins near the Norwegian–Russian border area, 40–50 km west of the nearest large source of SO2, the Nikel industrial complex in the Kola Peninsula. The most common fish species caught were minnow (Phoxinus phoxinus), burbot (Lota lota) and brown trout (Salmo trutta). Special attention was focused on minnow because of its sensitivity to acidification and its frequent occurrence in the study area. At all sampling sites, the alkalinity values of the sampled brooks and lakes were significantly higher and the sulphate concentrations significantly lower in 2000 than in 1993. The increased densities of minnow and changes in the length distribution of the sampled fish indicated that reproduction of the species was successful at most of the sampling sites in the late 1990s. The results suggest that the decrease in acid emissions from the Kola industrial centres has resulted in noticeable chemical and biological recovery of the most acid-sensitive surface waters in north-eastern Finnish Lapland.
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