The Karelian Isthmus, including a lake district of about 15 000 kilometers=BF52=BF0 in area, is situated between the Baltic Sea and Ladoga Lake (northwestern Russia). Hydrochemical examination of the small lakes in this region have indicated extensive anthropogenic acidification.

The impacts of acidification on lake ecosystems have been characterized, mainly based on the bacterial components aquatic food webs and the balance of primary production and destruction. In several acid lakes, decreasing density of bacterial population and bacterial activity (bacterial productivity and destruction, respiration per cell) were noticed. The ratio of primary production to destruction decreased with decreasing pH.

An in situ enclosure experiment was performed in a Krasnoye lake to examine the effect of acidification on density of bacteria, diversity of phytoplankton, several bacterial rates and balance of primary production and destruction. The diversity of phytoplankton, the density of bacterial population as well as the ratio of primary production to destruction decreased with increasing acidity. There were no differences in the morphological type or mean volume of the bacterial cells. Bacterial production and destruction were at the same level as at pH 6.4, but at pH 4.0, 5.4 bacterial activity was lower. Respiration per bacterial cell was lower in all acidified enclosures than in the control.

In addition, phosphatase activity of bacteria was determined. The estimation of activity of bacterial phosphatase permits the estimation of biochemical activity of lake water connected with bacteria. Acidification was the primary cause for the strong depletion biochemical activity of lake water. It is possible that the decrease in activity of bacterial phosphatase is one of the mechanisms potentially leading to oligotrophication of acidified lakes as a result of phosphorus deficiency.