Only about 40 percent of all the lakes in Germany achieve the objectives of the EU Water Framework Directive. Its objectives are considered met when a lake breeds the populations of fish, flora and microorganisms which typically occur in it. The Directive defines this status as ‘high’ or ‘good’, which 52 percent of the deep, oligotrophic lakes of northern Germany do. No other type of lake in northern Germany receives such high marks. Its overall positive status is due to a good chemical condition and a comparatively low nutrient load. This above average high ranking as compared to other lakes is due to the favourable balance between the size of its catchment area, water volume, lime content and basin depth. These factors mean that fewer nutrients are deposited into the lake. Phytoplankton growth may be triggered if nutrient input nevertheless occurs, but the quantity remains low enough so as not to overtax the oxygen content of the water as the algae degrades. Phytoplankton and nutrients are then deposited together on the bottom of the lake and thus removed from the nutrient cycle. When this efficient self-cleaning process is functional, the populations of typical flora and fauna such as stoneworts, water insects and fish like the vendace, perch and pike can flourish.
Many of these lakes are also ideal for bathing and diving because of the good water quality. Recreational boaters in canoes and small motor boats take advantage of the passageways to the Baltic and North Seas that many of the lakes in Brandenburg and Mecklenburg-Western Pomerania afford via the rivers Oder, Havel and Elbe.
There is still a need for action, however, in about half of all deep and oligotrophic lakes. This applies to sites where favourable natural conditions are no longer sufficient to offset excessively high inputs of nutrients from the catchment area. As a result, there is heavy algal bloom and water becomes murky. Nutrient inputs from municipal wastewater and farms can be effectively reduced in wastewater treatment plants and through installation of integrated sewer systems around lakes. Another problem is when construction measures to build promenades, beaches and boat docks alter a lake’s natural shores.
Among the more well-known of this water type are: Lake Stechlin, Wandlitzsee Lake and Werbellinsee Lake in Brandenburg, the Breite Luzin and Schmale Luzin lakes and Lake Schwerin in Mecklenburg-Western Pomerania; Schöhsee Lake and Lake Plön in Schleswig Holstein, and the lake Arendsee in Saxony-Anhalt.
Click to enlarge