BERmon18 Responses of headwater lakes to air pollution changes in Finland
Mannio Jaakko, 2001.
Monographs of the Boreal Environment Research No. 18, p. 48.
URN:ISBN:9521108843. The publication is available also in printed form (ISBN 952-11-0884-3).
The present work provides a national scale assessment of the trace metal contamination of small headwater lakes and the recent development of acidified lakes in Finland. The information is needed as a scientific basis for further actions in air pollution policy. The study is based on observations in a national monitoring network of lake acidification.
Anthropogenic, atmospheric deposition is primarily responsible for the increase of Cd, Hg, Pb and As in headwater lake sediments. However, a decline of 20 to 40% of the accumulation of these elements within the last decades was observed, indicating a relatively fast response to the decline in the atmospheric deposition, and that the accumulated stores of atmospheric trace metals in the catchment soils are not dominating the supply of trace elements to lakes.
Lake waters reflected atmospheric trace metal pollution as well, but it was not as clearly quantifiable. Acidity controls in particular the level of Cd and Zn, while organic matter (humus) controls more the level of Cr, Fe, Cu and Ni in headwater lakes. Lead, Mn and Al concentrations are affected by both these factors. Humus acts as a carrier for trace metals from catchments soils to surface waters, irrespectively of their original source.
Based on comparable chemical data sets, the risks of biological effects in lakes due to trace metals are lower in Finland than in Sweden and Norway. Trace metal levels in lake waters are less critical for the biota than acidity and inorganic (labile) aluminium levels.
Due to acidification, there where estimated to be 2200-4400 damaged fish populations in southern and central Finland. Most of these populations are roach in lakes smaller than ten hectares. Sulphate concentrations have declined in all types of small lakes throughout Finland in the 1990s, indicating a clear response to the sulphur emission reductions. Base cation concentrations are still declining in lakes especially in southern Finland, but to a lesser extent than sulphate.
There are presently no indications of elevated nitrate levels in forested headwater lakes. The increase in buffer capacity (chemical recovery) was relatively uniform throughout the country, except that the changes were not as significant statistically and by magnitude in the dilute lakes in northern Finland.
Nearly 5000 headwater lakes larger than four hectares were estimated to be recovering from acidification at present. The chemical conditions were found to be improving throughout Finland, and first perch population recoveries in southern Finland were observed.
The monitoring and survey results presented here are an example of an approach, where both spatial and temporal data from several sources are aggregated. This facilitates the estimation of regional changes and quantifies the changes on national scale. The consistent monitoring provides also sound basis for further modelling of recovery processes and scenario assessment.
A new challenge is the interaction of acidification/recovery processes and trace metals with possible trends in temperature and hydrology due to global climate change. This should be taken into account when assessing long-term surface water quality and developing future monitoring networks. Empirical data in space and time is needed to judge, whether the emission reduction measures have been efficient.
Jaakko Mannio, Finnish Environment Institute, tel +3589 403 000, firstname.lastname@example.org [jaakko mannio]