Europe, covering about 10 400 000 km2 or 2.0% of the Earth's surface, is a continent with a large variety of inland waters. Water resources are abundant compared with other continents, but the resources are unevenly distributed. Water resources in Europe generally decrease towards densely populated warmer regions. These regions are mainly located in the geographical region defined as Southern Europe by the United Nations. The three additional corresponding European geographical regions are Northern, Eastern, and Western Europe (Figure 1).
Lakes are most abundant in Northern Europe. Finland is the country in Europe that counts most lakes, i.e., almost 190 000 lakes that are larger than 500 m2. Despite the high abundance, these northern lakes have captured little international attention. Northern Europe is relatively sparsely populated and water of sufficient quality is abundant. Therefore, most of these lakes are of limited economic, social, and political concern. Often lakes become of general interest only when acute water quality problems arise. A typical water quality problem of Europe has been the human-induced increase of nutrient concentrations, i.e., eutrophication. Especially the strong increase in phosphorus concentrations during the middle of the twentieth century, due to untreated sewage waters, resulted in heavy algal blooms. The algal blooms were visible for everyone, awakening strong pressure from the public to find solutions. In the 1970s and the 1980s, many European countries finally began to use and to improve sewage water treatment plants. At first phosphorus concentrations remained on a high level because of internal phosphorus loading from the lake sediments, but subsequently a decrease in phosphorus concentrations could be observed in many systems, a process known as reoligotrophication. As a consequence of this process, the occurrence of heavy algal blooms decreased in a variety of lakes but the problem became apparent again during recent exceptional warm summers, and debates are ongoing in how far climate change can cause heavy algal blooms.
Climate change is only one of multiple stressors affecting lakes of Europe. Well-known other stressors are intensive land use, increasing industrial activities, overfishing, invasive species, and changes in the atmospheric deposition of acids, metals, and organic pollutants. Consequences of stressed lake ecosystems can be, for example, water shortage, flooding, water pollution by various substances, harmful algal blooms, changes in biodiversity, extinction of fish species, and smell and taste problems of drinking waters. The challenge is to face these water problems in time by adequate water management.
The responses of lakes to stressors can vary widely, depending on the lake characteristics. The most important factors characterizing lakes are climate, geological history, land use and to some extent atmospheric deposition. In the following an overview is given about the main factors characterizing lakes of Europe. A description of common European lake characteristics follows and finally examples of lakes of special interest are given.
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