Eutrophication occurs when a body of water becomes oversaturated with nutrients. In lakes and many coastal waters, the main culprit behind eutrophication is the nutrient phosphorus, from for example water treatment plant or agricultural emissions, stormwater, or individual sewers. These emissions can cause a great deal of phosphorus to accumulate in the bottom sediment. When this phosphorus begins to leak from the sediments into the water, it results in something known as “elevated internal loading”. To remedy eutrophication and determine what type of measure is appropriate, we therefore need to understand the significance of internal loading.
LIFE IP Rich Waters has developed a handbook to support the assessment and decision-making process regarding measures to combat the internal loading of phosphorus in lakes. The handbook was published in April 2023 and is Sweden’s first comprehensive decision-making tool for addressing internal loading. It presents a four-step method for assessing whether a body of water is internally loaded. It also includes descriptions of possible measures and information regarding costs and funding opportunities.
The full version of the Handbook is available in English:
Handbook – a decision support tool for measures against internal phosphorus loading in lakes
Other publications:
Removal of fish – a potential management measure to increase water quality in Lake Hjälmaren (13 March 2025)
Investigation of internal loading in lakes affected by eutrophication in Örebro county (28 January 2025)
Lake Norrviken is located in the municipalities of Sollentuna and Upplands Väsby. For several decades, nutrients (mainly phosphorus) from industry and agriculture and other pollutants have flowed into the water and been stored on the lake bottom, causing eutrophication. The lake has long been cloudy, green and unhealthy. If nothing is done, it could take more than a hundred years for it to recover.
To help the lake break this negative spiral of eutrophication, Sollentuna Municipality and Upplands Väsby municipality have taken joint action. By injecting the bottom with aluminium chloride, a salt used in the purification of drinking water, the stored phosphorus is bound to the lake bottom. Thanks to the aluminium treatment, the amount of phosphorus in Lake Norrviken has decreased and its nutritional status has gone from poor to good. The amount of phytoplankton has decreased, the water has become much clearer, and the depth of visibility has improved.
Publications:
Assessment of internal loading of phosphorus in Lake Norrviken, before and after aluminium treatment (16 January 2025)
Stopping internal phosphorus leaking – a full lake treatment in Norrviken (12 August 2022)
Lake Öljaren has long had major problems with eutrophication caused by internal loading and resultant algal blooms. To help the lake with this problem while simultaneously infusing nutrients back into the cycle, Katrineholm municipality has tested the low-flow dredging method. It might be described as vacuuming the lake. With hoses from a manned pontoon out on the lake, the top layers of the bottom sediment are sucked up and then dewatered in several steps up on land. The nutrient-rich sediment that is sucked up can be used as fertiliser and thus reenters a natural cycle. The water is then pumped to a ditch and flows back out into Lake Öljaren.
Publication:
Environmentally friendly removal of phosphorus-rich sediment from lake Öljaren and re-using it as a fertilizer in agriculture (6 November 2024)
The non-profit association Ecopelag has tested and developed a method for mussel farming as a water improvement measure in the Stockholm Archipelago. In their work, they have identified suitable areas, developed techniques for cultivation and harvesting, and found uses for the harvested mussels.
The mussel farms are submerged below the surface of the water. The farming works by placing a cultivation substrate – a type of rope or net – during the spring, when the water is teeming with mussel larvae. The larvae attach to the ropes and remain there while they grow.
Mussels eat algae and organic particles and store nitrogen and phosphorus from the food they eat. When the mussels are then harvested, the nutrients stored within their shells follow them up on land. Every part of the mussel is used, for example as animal feed and fertilizer in agriculture. The blue mussels create a circular system between sea and land in which important nutrients are preserved and put to use.
Ecopelag’s work contributes to increased knowledge about mussel farming and how cultivation can best be carried out in the prevailing conditions of the North Baltic Sea Water District.
Publication:
Evaluation report on mussel farming in the NBWD as a remedial measure from a socio-economic and an environmental cost-benefit perspective (31 October 2024)
Film: Low-flow dredging in Lake Öljaren (English subtitles available)
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