Eutrophication is a global problem that affects many small and large lakes. Every year, the Baltic Sea receives 30,000 tonnes of phosphorus and almost one million tonnes of nitrogen, and it now has the world’s largest dead zone. Lake Mälaren is Sweden’s largest source of drinking water. The situation was at its worst in the 1960s and now the condition is better, but not good.
One reason for the eutrophication of Lake Mälaren is the large number of farms in the region. Agricultural fertilizers that contain phosphorus and nitrogen stimulate the growth of crops. The soil naturally releases different amounts of phosphorus and nitrogen depending on the season, and there is a lot of leaching in the spring. During heavy rains when the plants do not absorb nutrients very well, the nutrients flow to the lake and stimulate the growth of algae and aquatic plants. If eutrophication continues, there will be a lack of oxygen and this will eventually lead to a full-scale lack of oxygen, with the extinction of fish and plants as a possible result.
In 2019, Menhammar Farm built a phosphorus pond. It captures phosphorus from horse manure in pastures and fields, which would otherwise be transported by the rain to ditches and streams. By directing surface water to this pond, the amount of phosphorus that reaches Lake Mälaren is reduced by 40-60 percent.
“In the pond, phosphorus binds to clay particles that sink to the bottom as sediment, while the nitrogen settles in the shallow areas of the pond. Aquatic plants at a depth of about half a meter absorb nitrogen and use it as a nutrient” says Ulf Segerström, operation manager at Menhammar Farm.
The investment in the pond was made with a grant of SEK 200,000 from the County Administrative Board as part of the rural development program.
“The pond has several benefits for the environment. In addition to reducing the amount of phosphorus that reaches Lake Mälaren, it also contributes to greater biodiversity. In many areas, there is a shortage of wetlands that amphibians, insects and thus also birds depend on” says Emma Lennmo, wetland manager at the County Administrative Board in Stockholm County.
Sweden’s first phosphorus pond was built in 2009. A doctoral dissertation from SLU, Swedish University of Agricultural Sciences in 2014 showed that these ponds are effective in reducing the amount of manure that flows into watercourses.