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The water

Lake Stechlin is one of the deepest and cleanest lakes in the north of Germany. © M. Feierabend
Lake Stechlin is one of the deepest and cleanest lakes in the north of Germany. © M. Feierabend

Lake Stechlin is famous for its clear water. The depth of visibility often reaches eight to ten metres. This is because of the low nutrient content of the water. With so few nutrients, phytoplankton – photosynthesizing algae and cyanobacteria that float in the water – cannot proliferate to any great extent. The transparency of the water, in turn, favours the growth of plants on the lake ground. Stoneworts, with their low light requirements, extend furthest into the depths, where they form extensive underwater fields.

Algae and other types of plankton occur at relatively low densities in Lake Stechlin. Nevertheless, the diversity of life (biodiversity) is especially high here. Around 1,200 species have been detected. Together with environmental factors such as temperature and food availability, these organisms determine the relationships in the food web and the cycles of matter in the lake ecosystem.

Also typical of Lake Stechlin, aside from its clarity, is the high oxygen saturation of its deep waters. This promotes rapid degradation of dead biomass in the lower water layers and on the ground – even if the summer stratification (see info box) lasts for a relatively long time. Down in the sediment, however, the oxygen becomes completely used up already in the first few centimetres. Going deeper, fermentation processes dominate and methane gas forms, some of which rises to the surface, where it can escape into the atmosphere.

Lake stratification in summer

During the summer, in deep lakes such as Lake Stechlin, surface water heated by the sun forms a layer above the permanently cold deep water. This is due to the large temperature-related density differences of the water. Water reaches its highest density at 4 °C, at which point it sinks to the lake ground. Warmer water remains on the surface. In autumn, when the temperature equalizes throughout the entire lake due to cooling at the surface, the wind introduces enough energy of its own to completely mix the upper and lower water masses. This distributes oxygen and nutrients evenly throughout the entire water body so that these elements will be available again for the production and degradation of biomass during the next summer's stratification phase.

Lake Stechlin in figures

Surface area: 425 ha (4,25 km2)
Water volume: 97 Mio m3
Shoreline: 16.1 km
Average depth: 24 m
Maximum depth: 69.5 m
Average depth of visibility in summer: 8.4 m
Untere Wasserpflanzengrenze: 4-12 m