WILL AN INCREASED GREENHOUSE IMPACT IN FENNOSCANDIA GIVE RISE TO MOREHUMIC AND COLORED LAKES

A large-scale increase in the discharge of humic material in rivers, a s well as a darkening of Swedish lakes during the last 15 years, are i llustrated and discussed. Increased precipitation was responsible for the greatest changes, but a marked part (10-40%) of the increase in ri ver transport of coloured material was independent of discharge. Possi ble explanations for this increase may be: increased ground water leve l leading to increased runoff in superficial soil levels, a flow in di rect contact with the organic soils; humus fractions in these layers m ay also be rich in iron resulting in more coloured water; increased pr imary production giving rise to increased amounts of litter and humic material; reduced capacity of soils to absorb organic matter because o f acidification; and effects of modern forest management. Models of gl obal climate predict a doubling of atmospheric-CO2, an increase in mea n annual temperature of up to 1.5 to 4.5-degrees-C, and in Fennoscandi a possibly an increase in precipitation. The factors responsible for t he observed large-scale change in humus flux may be strengthened by th e increasing greenhouse impacts, and may result in more humic and colo ured lakes in Fennoscandia during the next century.