NORTHERN EUROPEAN LAKE SURVEY, 1995 - FINLAND, NORWAY, SWEDEN, DENMARK, RUSSIAN KOLA, RUSSIAN KARELIA, SCOTLAND AND WALES

In the autumn of 1995, Finland, Norway and Sweden initiated a joint No rdic Lake Survey using standard protocols for lake selection criteria and analytical procedures. Subsequently, the project was expanded to i nclude Denmark, Russian Kola, Russian Karelia, Scotland and Wales. The survey covers a geographic area of ca. 1 300 000 km(2); this is about 13% of Europe's total area and 28% of Europe excluding European Russi a. The total number of lakes (> 0.04 km(2)) in the study area is about 155 000 and of these 5690 lakes (3.7%) were sampled in this survey. T his is 3.7% of the total lake population (> 0.04 km(2)). The lake dens ity is about 1 lake per 9 km(2), and the sampling density is 1 lake pe r 244 km(2). The first common evaluation of lake chemistry in northern Europe showed that the chemistry of these lakes, except those in Denm ark, are characterized by low ionic strength (dilute) waters, with low concentrations of the nutrients nitrogen and phosphorus. There are ge neral differences in the lake water chemistry between the countries, d ue to differences in hydrology, precipitation chemistry, hydrology, so il cover and vegetation. From western Norway, Scotland and Wales to ea stern Finland and Karelia, there is a gradient from high to low precip itation (5000 to 300 mm yr(-1)) and from mountain areas with thin and patchy soils to forested areas with thick soils. These two factors are reflected in lake water chemistry with low concentrations of base cat ions (Ca, Mg, Na, K), alkalinity (HCO3-) and total organic carbon (TOC ) in the western areas to higher concentrations in the eastern areas. An exception is high TOC levels in lakes in Scotland and Wales. Median values for base cation concentrations are three times higher in Swede n, Finland, Scotland, and Wales than in Norway, and TOC concentrations vary even more. The lakes in Denmark are influenced by a different be drock mineralogy than in the other countries, and agricultural activit ies are much more intensive in their catchments. The Danish lakes gene rally have much higher ion concentrations and high concentrations of n itrogen and phosphorus. In Finland, Norway, Sweden, Russian Kola and K arelia, more than 50% of the lakes have low critical load (CL) values for sulfur acidity (CL < 60 meg m(-2) yr(-1)) while Scotland, Wales, a nd Denmark have very few lakes with low CL. The highest percentage (27 %) of lakes with exceedance of critical loads for sulfur acidity (at p resent S-loading) is found in Norway. In Russian Kola, Sweden and Finl and 17%, 9% and 9%, respectively, of the lakes are exceeded. In Scotla nd, critical loads for sulfur acidity are exceeded in only 1% of the t otal lake population, while 24% of Welsh lakes are exceeded. In total, this adds up to approximately 22 000 lakes, corresponding to 14% of t he total lake population in northern Europe, where CL for sulfur acidi ty is exceeded.