| Summary: | Acid sulfate soils occur worldwide and can cause serious ecological damage by releasing acidity and toxic metals into watercourses. This study focused on short- and long-term changes to the water quality of a lake in western Finland. Here, a decline in water quality took place in the late 1960s. Since then, events concerning poor water quality have frequently occurred. The annual load of some potentially toxic metals varied from hundreds of kilograms to thousands of tons, depending on the metal. The proportion of low-order streams draining the nearest field to the lake is only 7% of the catchment area, while the share of the total metal load was estimated to be over 30%. This points to the importance of monitoring small coastal catchments. High runoff conditions were proven to be more important in terms of water quality than occasional summer droughts. During the past decade, acidic conditions have become more prolonged in the autumn, which is in line with a predicted scenario relating to climate change.
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