Trend lakes

The subprogramme provides an annual overview of the environmental status of Sweden’s lakes and serves as a foundation for official statistics. The lake status assessments are used to follow up on national environmental quality objectives and for international reporting and agreements, such as the UN Convention on Long-range Transboundary Air Pollution (CLRTAP).

In addition to supporting water management reporting and official statistics, the Trend Lake Stations Subprogramme provides an important foundation for evaluating and further developing assessment criteria used to determine the environmental status of Swedish lakes, including under the EU Water Framework Directive.

Sampling

The subprogramme includes both chemical and biological sampling according to a defined schedule. Surface water chemistry is sampled in all lakes every season, i.e. four times per year. Benthic invertebrates are sampled every autumn, and phytoplankton is sampled anually in August. Fish monitoring is carried out in a selection of lakes, although only a few are surveyed each year. Macrophyte are sampled in all lakes at six-year intervals. 

Ten of the lakes, selected along a climate gradient, are monitored more intensively; for example, water chemistry is analysed eight times per year. Zooplankton is also included in the programme, and together with phytoplankton is sampled four times annually in these lakes.

Scope of the Subprogramme

The Trend Lake Stations Subprogramme includes just over 100 lakes across Sweden. These trend lakes are chosen because they are unaffected by local emissions or intensive land use, making them suitable as reference sites for assessing environmental status and diffuse impacts.

The subprogramme includes lakes from the most common limnic types in Sweden. Recently, three reference lakes from the completed IKEU programme were incorporated into the subprogramme. In addition, ten alpine lakes above the treeline will be gradually added, starting in 2026.

The lakes monitored in the subprogramme are medium-sized, ranging in area from 0.02 to 52.6 km².

The Trend Lake Stations Subprogramme is closely related in design to the River Mouths Subprogramme.  Together, they provide the most comprehensive environmental monitoring of Sweden’s inland waters, covering a wide range of physical, chemical and biological parameters.

History of the Subprogramme

The Trend Lake Stations Subprogramme began in 2007. Prior to that, similar monitoring was conducted through the former Reference Lakes Programme ("referenssjöar"), which started in the mid-1980s. Between 1995 and 2006, the programme was known as the Time Series Lakes Programme ("tidsseriesjöar"). Additional lakes have been added over time, and time series of up to 40 years are available for some sites.

Responsibilities Within the Subprogramme

The Department of Aquatic Sciences and Assessment is responsible for conducting the programme and also hosts the generated data—except for fish data. The Trend Lake Stations Subprogramme is part of the Freshwater programme area within Sweden’s national environmental monitoring system and is funded by the Swedish Agency for Marine and Water Management.