Water is a unique substance and a vital part of life. Today, freshwater ecosystems are considered as one of the most vulnerable ecosystems on Earth. Degradation of lakes and rivers caused by water pollution, climate change, eutrophication and habitat destruction and fragmentation is a major concern in a world where fresh and clean water is unevenly and scarcely distributed. Acidification of lakes and rivers caused by long-range transported air pollutants has been considered as one of the biggest threats to biodiversity in freshwater ecosystems in Norway. Although the water quality has significantly improved after decades of legislations and efforts to reduce the emissions of long-range transported air pollutants, acidification is still considered as an environmental concern. The natural recovery of surface water chemistry is a slow process, and further emission reductions is needed to avoid the possible case of long-term acidification of acid-sensitive lakes. Long-term studies of lakes are invaluable, and crucial to understand how freshwater ecosystems are impacted/ respond to changes in the environment. This Master thesis traced the footsteps of a nearly 50-years old regional survey, containing data on the chemical and biological conditions of 100 lakes and ponds in South-East Norway. The present study had the following aims: (1) Conduct a regional survey of the chemical and biological status in Vassfaret, in accordance with the methods and the approach of the regional survey from 1968, (2) Assess water quality changes for selected parameters over these nearly 50-year time span, 3) Assess whether diversity and community compositions have remained the same over this time span, 4) Establish casual links between the eventually observed changes in water chemistry with those of the zooplankton community. Facing multiple stressors, knowledge on the stability of freshwater ecosystems is heavily needed.