søer

Søer are inland bodies of standing freshwater, typically found in Denmark, Norway, and other parts of Scandinavia. They vary widely in size, depth, and in the landforms that host them, and they are distinct from rivers, streams, and coastal seas. The term sø (singular) and søer (plural) are used in Danish and Norwegian to describe these lakes.

Most søer in northern Europe were formed by glacial processes during the last Ice Age. Retreating ice sheets carved basins and left moraines that dammed valleys, creating long, shallow or deep lakes. Other søer have formed through tectonic faulting, karst dissolution, volcanic activity, or as oxbow lakes when rivers changed course. Human activity has also created lakes through damming for water supply, hydroelectric power, or flood control, and in some regions drainage and land-use changes have altered natural water levels.

Hydrologically, søer receive water from precipitation, surface inflows, and groundwater, and they release water via outflow streams, evaporation, or seepage. Turnover and mixing depend on depth and climate; shallow søer often mix seasonally, while deeper ones may stratify into an epilimnion and hypolimnion with a yearly turnover pattern in temperate zones. The chemistry of søer ranges from oligotrophic (clear, nutrient-poor) to eutrophic (nutrient-rich with abundant plant and algal growth), with nutrient status heavily influenced by surrounding land use.

Ecologically, søer provide habitat for freshwater fish such as perch, pike, roach, and trout, as well as waterfowl, amphibians, and diverse aquatic plants. Biodiversity and productivity are closely tied to water quality and nutrient input. Human use includes drinking-water supply, fishing, recreation, and tourism, while protection efforts focus on water quality, habitat conservation, and ecosystem health, guided by regional and national frameworks for freshwater management.