Aerial view of a coastal landscape with several interconnected islands surrounded by blue sea. Narrow roads and bridges connect the islands, which are covered in green vegetation and have small buildings and jetties. Smaller islands and islets are visible scattered in the surrounding water.
Biotest lake outside Forsmark nuclear power plant. Photo: Göran Hansson

Lake Biotestsjön

Page reviewed:  18/03/2025

The Biotest Lake is an approximately 90-hectare enclosed water reservoir that receives heated cooling water from the Forsmark Nuclear Power Plant. This creates a unique environment about 8 degrees warmer than the surrounding sea, but otherwise entirely natural.

Biotest Lake can be compared to a time machine to future seas and provides an exciting environment for research on what happens when water temperatures rise.

History of the Biotest Lake

The Biotest basin was completed in 1977 and warm water from the nuclear power plant began flowing through the basin in the spring of 1980. Two years later, in 1982, the warm water discharge from the plant reached its maximum, at 90,000 liters per second. Today (2020), just over 100,000 liters of cooling water are discharged per second, and the water in the Biotest basin is approximately 8°C warmer than the surrounding coastal water.

Free passage for fish since 2004

Until spring 2004, the discharge point of the Biotest basin was equipped with a grid, with the aim of preventing fish from moving between the basin and the surrounding coastal areas. In 2004, the grid was removed and since then fish can move freely between the areas.

Biological investigations and climate research

In order to investigate how the warm water affects the marine environment in the Biotest basin and in the Bothnian Sea, we at the Department of Aquatic Resources carry out regular checks. We carry out test fishing with gillnets, we monitor benthic animals and we study coastal birds in the area. We compile the results of the investigations in annual reports and in-depth reports approximately every five years.

In the Biotest basin, we carry out research on fish growth in warmer water. Please read more in the news "Experimental evidence of gradual size-dependent shifts in body size and growth of fish in response to warming".

Results from our biological surveys

We report the results from the biological surveys annually. Every five years, we also conduct a more comprehensive and in-depth evaluation of the results from the monitoring programs and summarise this in a so-called five-year report. All reports can be found in SLU's publication database (SLUpub).

Questions about the Biotest basin

Do you have any questions or would you like to use the Biotest basin for scientific projects? Please contact Anders Adill.

Contact