Oligotrophication of acidified, nitrogen-saturated softwater lakes after dredging and controlled supply of alkaline water

Abstract

After acidification of shallow softwater lakes as a consequence of atmospheric sulphur and nitrogen deposition, the concentrations of nitrogen and carbon dioxide in the water layer are raised and the cation reserves in the sediment are depleted. Liming can counteract acidification, but can also lead to further nutrient mobilisation. Controlled supply of calcareous groundwater is an alternative method to restore the pH and alkalinity of the water layer. Sediment removal and subsequent restoration of pH leads to a reversal of the nutrient status towards pre-acidification levels. However, rapid re-acidification may occur as a consequence of supplying water from the catchment which is acid and rich in nitrogen. After three years of repeated groundwater supply, the base saturation of the sediment was higher than that of adjacent reference lakes where the sediment had been removed simultaneously. This base saturation is possibly a key factor in the prevention of re-acidification. During a five-year period after the start of groundwater supply, CO 2 and nitrogen concentrations in the water layer decreased and were not higher compared to adjacent reference lakes. Characteristic softwater macrophytes returned, but not in the reference lakes.