Lakes in the Finnish Eurowaternet: status and trends

The Eurowaternet is a monitoring network designed to cover all European countries, with the overall objective of obtaining timely, quantitative and comparative information on the status of inland waters so that their key environmental problems can be defined, quantified and monitored. Finland launched the Eurowaternet for lakes in 2000. The network was set up according to guidelines of the European Environment Agency (EEA) and was based on national, regional and local monitoring programmes. It includes 253 lake sites, divided into the following groups by, for example, their loading background and size: (i) reference sites, (ii) point-source loading (impact) sites, (iii) agricultural sites, (iv) representative sites, i.e. lakes <100 km2 with typical water quality in the catchment and (v) lake deep sites in large lakes (>100 km2). We examined the water quality and its development during 1976–2001 in these groups, using data from surface and near-bottom water layers at summer and winter stagnation. In addition, the representativeness of the network was evaluated. Summer mean concentrations of total P (TP) during 1990–2001 in surface water were (i) 6.8±1.8 μg 1−1 in reference lakes, (ii) 24±5 μg 1−1 in lakes affected by point loading, (iii) 58±32 μg l−1 in agricultural lakes, (iv) 14±2.2 μg l−1 in representative lakes and (v) 11±2.1 μg l−1 in large lakes. Comparison with reference lakes revealed major impact of human activities on TP and total N (TN)—and consequently on chlorophyll a (a-chl) and transparency—in all other groups, especially agricultural lakes. Decreasing TP and increasing O2 trends were found at impact sites, reflecting water protection measures taken in pulp and paper mills and municipalities. More surprisingly, we found increasing alkalinity trends in all but agricultural sites and decreasing NO2–N+NO3–N (NOx–N) trends particularly in pelagic areas of large lakes. Comparison of water quality in Eurowaternet lakes with those in randomly sampled lakes (Finnish Lake Survey) revealed that the Eurowaternet covers the entire range of concentrations of TP, TN and conductivity in Finnish lakes, but <1-km2, eutrophic and/or brown water lakes are underrepresented. Only 0.2% of a total of 13 114 small lakes (0.1–1 km2) are included in the annual monitoring program of the Eurowaternet.