Sjöer

Sjöer are inland bodies of standing freshwater. In Swedish, the plural term sjöer refers to lakes of varying size, from small basins to large inland waters. They occur in diverse climates and landscapes and are shaped by geological and hydrological processes.

Most natural lakes in temperate regions are of glacial origin, formed when retreating ice left depressions that filled with meltwater. Other formation mechanisms include tectonic rifts, volcanic craters, oxbow lakes along rivers, landslides that create basins, or damming by moraines. Humans also create lakes through reservoirs and impoundments.

Lakes vary widely in depth, water chemistry, and productivity. Many temperate lakes experience seasonal stratification in summer, developing a warm upper layer and a cooler bottom layer, separated by a thermocline. In spring and autumn, lakes may mix when density differences disappear. Water residence time, nutrients, and light influence primary production and the composition of the aquatic community. Nutrient-rich lakes tend to be more productive (eutrophic), while nutrient-poor lakes are oligotrophic and clearer.

Ecology and hydrology: Lakes receive water from precipitation, inflowing rivers and groundwater, and lose water through outflow, evaporation, and seepage. They support diverse communities, including algae, aquatic plants, invertebrates, fish, and birds, and they provide habitats for migration and breeding. Lakes are important for drinking water, irrigation, fishing, and recreation, but are sensitive to pollution, invasive species, climate change, and land-use changes.

Notable examples include Vänern in Sweden, Lake Onega and Lake Ladoga in Russia, and Lake Geneva on the France–Switzerland border, illustrating the geographic and ecological variety of sjöer.