Liming of surface waters in Sweden — a synthesis

Summary

A trial programme on liming of acidified lakes and watercourses in Sweden was conducted during the period 1976 to 1981. The positive experiences acquired resulted in the initiation of a large-scale operative activity from 1982 onwards, which is mainly financed by governmental subsidies. Approximately 6,300 lakes and 6,000 km of running water have been limed in Sweden, and the activities are among the greatest which have been applied in order to restore or preserve environments at risk in Sweden. Considerable resources have been assigned to research and monitoring of effects.

In order to summarize the experiences acquired, the Swedish Environmental Protection Agency initiated a comprehensive evaluation project. The results are described here, providing a synthesis mainly based on data from other chapters of this book.

Acquired experience shows that liming results in a considerably improved water quality and biological diversity. Liming is also generally favourable to fisheries. The water quality and organism communities following liming are usually rather similar to that of non-acidified water bodies, but liming will not rehabilitate the ecosystems completely. Substantial reductions in acidifying emissions and healthy forest practices are the only long-term strategies applicable to attain full restoration of aquatic ecosystems suffering from acidification.

Complementary measures are often needed to obtain the desired effects, e.g. elimination of obstacles for passage of fish and other organisms, reintroduction of species, and addition of nutrients, particularly phosphorus, in some surface waters. Accurate liming practices often result in a decreased mercury content in fish. Certain undesirable effects may occur after liming, of which deterioration of limed wetlands is the most controversial.

The knowledge of the effects of liming on human society and national economy is sparse in Sweden, but there are indications from some studies that liming may be profitable with respect to economy.

Liming in freshwater to retain biological diversity and fisheries will probably be needed for at least the next 50 years, even if substantial reduction in acid emissions of sulphur and nitrogen in Europe were to occur in the coming years.

Limestone (CaCO₃) is the best and most frequently used neutralizing agent for treatment of acidified surface waters all over the world. To achieve desirable results, appropriate planning is essential, making use of a strategy counteracting episodic acidification and resulting in minimal variability in water quality. A combination of different strategies, i.e. direct lake liming, wetland liming and liming by means of dosers will produce the most adequate ecological results. Due to the potentially severe effect on vegetation, liming on wetlands should only be conducted after well-balanced evaluations regarding desirable and undesirable consequences taking into account the conservation value of different environment categories.

Lime dosages which cause a considerably higher pH than under natural conditions, should generally be avoided. Reacidification of limed water bodies is also to be avoided.

Knowledge of the effects of freshwater liming is considerable, but far from complete. Long-term effects, i.e. more than 20 years, are unknown for natural reasons. It is not yet possible to generalize the effects of liming on a number of aquatic organisms or groups associated with water, e.g. micro-organisms, birds, amphibians and mammals. The long-term development of nutrient level and availability is ambiguous, and further studies are needed to answer the question whether nutrients should be added to limed lakes.

Further studies are also needed particularly on;

• production and decomposition processes,

• interaction between species (especially in running water),

• natural recolonization of species or reintroduction by man,

• effects of liming on micro-organisms,

• the significance of parasites and dieseases,

• mechanisms governing mercury content in fish,

• socio-economic effects.

The national strategy is not to lime naturally acid surface waters, In northern Sweden, however, it is not sufficiently well known to what degree surface waters are naturally acid or acidified because of man’s impact. This constitutes a major problem since several waters of the region are already being limed or planned to be limed.

Strategies must be developed regarding liming practices in lakes having a short water retention time and to counteract episodic ecidification in watercourses experiencing considerable water-flow variations. Measurable biological goals, which can be quantified, are needed to adequately evaluate the effects of liming.

In conclusion, liming practices applied to Swedish surface waters have mainly produced desirable results. Similar conclusions have been drawn from less extensive aquatic liming activities in other countries. Current knowledge is sufficient to recommend continued large-scale operative liming activities in Sweden, to be run parallel with scientific and applied studies to increase knowledge and to optimize liming operations.