sjöarna

Sjöarna, the Swedish term for “the lakes,” refers to natural or artificial bodies of standing freshwater surrounded by land. They vary widely in size, depth, and water chemistry, and they commonly retain water for extended periods, creating diverse aquatic ecosystems. The distinction between a lake and a pond is not fixed, but lakes generally have longer water residence times and more complex habitats.

Most sjöarna form through glacial processes in the last ice age, as bedrock was scoured and depressions filled with meltwater. Other formation mechanisms include tectonic subsidence, volcanic calderas, river damming creating oxbow lakes, landslides that block valleys, and human-made reservoirs. Water inputs include precipitation, inflowing rivers and groundwater, while outputs occur through evaporation or outflow to rivers or groundwater.

Limnology studies lakes' physical, chemical, and biological processes. Lakes often stratify seasonally, developing distinct upper and lower layers, which affect oxygen distribution and nutrient cycling. They are categorized by trophic state—from oligotrophic (nutrient-poor) to eutrophic (nutrient-rich)—and by depth, residence time, and catchment characteristics. Lakes support diverse communities of algae, invertebrates, fish, aquatic plants, and birds and are sensitive to pollution and climate change.

Humans rely on lakes for drinking water, irrigation, hydroelectric power, recreation, and transportation. In Sweden, several large sjöarna—such as Vänern and Vättern—play important roles in the national economy and landscape. Lakes are often protected as part of environmental and watershed management programs, which aim to reduce nutrient loading, preserve habitats, and sustain fisheries.