Liming of Acidified Swedish Lakes and Streams and its Consequences for Aquatic Ecosystems

Acidification of poorly buffered lakes, with negative effects on fish populations as well as the functioning of whole aquatic ecosystems, prompted an extensive program for neutralization of surface waters in Sweden. The biological response to lake liming has been mainly positive, especially in lakes with water retention times of more than 1–2 years, and in lakes where the deleterious effects of acidification have been moderate. Liming promotes decompositional processes. Whether it results in a temporary or permanent increase in nutrient recycling, is not yet known. The number of phytoplankton species generally increases in limed lakes. Zooplankton species which are sensitive to acidification reappear often spontaneously in limed lakes within a few years. The benthic fauna, established during the acid period, changes over several years following liming. New species invade during the neutral conditions and populations of acid sensitive species, previously present at very low levels, expand. The biomass of chironomids often decreases after neutralization. Reproduction and recruitment of fish have generally been successful in lakes where satisfactory water quality has been achieved. Toxicity to fish due to aluminum after liming has been reported. Liming techniques for lakes with short water retention times and especially for running waters, have not been fully developed in the Swedish liming program. The neutralized aquatic ecosystem is not a restored “unpolluted” ecosystem. It is a new stage in the lake history, where organisms survive but are stressed by the continued input of acid drainage waters with elevated concentrations of metals.