WaterSubject

Anthropogenic acidification seriously hit Swedish lakes and watercourses from the early 1960s and onwards following the increased emissions of sulphur oxides in Europe. A national remediation programme through large-scale liming of inland waters was launched in 1976. Although the acid deposition now continuously declines, an improved liming programme is still in action treating ca. 6000 lakes. The aim is to detoxify the water by restoring pH and alkalinity thereby making it available for acid-sensitive or Al-sensitive biota. Effects and experiences of acidification and the first decade of liming were summarized by Henrikson and Brodin (1995). However, long-term changes among the different communities and at an ecosystem level still needed study, in particular the question of reversibility. Such aspects are the objectives of a programme for Integrated Studies of the Effects of Liming in Acidified Waters (ISELAW) initiated and financed by the Swedish Environmental Protection Agency from 1989 onwards. The programme comprises 15 lakes limed during on average 25 years (here: ISELAW lakes). These lakes are nutrient-poor, from ultraoligotrophic to mesotrophic and the productivity is generally regulated by phosphorus but nitrogen may also interfere at some sites. Parallel studies with similar methods are carried out in seven acid lakes and in eight circumneutral reference lakes.

The primary task of the present study is the question to which extent zooplankton communities in the limed ISELAW lakes have recovered. In the study, two approaches are used: (i) comparison of present-day communities with pre-liming communities (ii) comparison of present communities in the group of ISELAW lakes with those of the circumneutral and acid reference lakes. In addition to detection of differences, the influence on the zooplankton communities of food (phytoplankton), predation (pelagic fish and Chaoborus), pH, toxic inorganic Al and trace metals is addressed. Thanks to this multifactorial analysis both general regulation patterns and exceptional patterns in some lakes could be revealed.