The impact of acidic precipitation and eutrophication on the freshwater pearl mussel Margaritifera margaritifera (L.) in Southern Norway

In Southern Norway, there are strong indications that acidic precipitation is the main reason for the extinction of 94% of all known populations (n = 47) of the vulnerable pearl mussel Margaritifera margaritifera (L). An example is the famous pearl mussel river, the River Audna, where the death of both young and old mussels took place in the period 1930 - 50. When the pH is in the range 5.0 - 5.5 or lower for prolonged periods of time, the mussel can no longer tolerate the acidity. The only three known localities remaining in this part of the country, with ageing populations of the pearl mussel, are all lowland streams that are only very little acidified. The main threat there is eutrophication.