A specially designed vessel is working during September to bind phosphorus in Bergshamraviken's bottom sediments. Norrtälje Municipality says 392 tonnes of aluminium solution will be distributed over 47 hectares. The treatment aims to reduce eutrophication, but does not replace efforts to limit new phosphorus entering the bay.
What happened?
The municipality says the work covers parts of the bay deeper than five metres. The solution is delivered through around thirty pipes into the sediment, approximately ten centimetres below the bottom surface. The vessel tows tanks and its crew monitors the application on screens.
The amount applied is calibrated for each square metre of the treatment area. Cameras and map monitoring help the crew see the position of the pipes and which areas have already been covered. The 392 tonnes refers to the solution being used, not 392 tonnes of pure metal.
The work is financed through LOVA funding for local water conservation projects, awarded to the municipality by the County Administrative Board. Treatment is expected to take several weeks, meaning the specialised vessel will be visible in the bay for much of September.
Location
Norrtälje, Stockholm
Impact and significance
The problem is not limited to nutrients entering the bay today. Phosphorus has also accumulated in bottom sediments over a long period and can be released back into the water. Municipal investigations identify this leakage as an important factor in the bay's current condition.
The process can reinforce itself: phosphorus encourages algal blooms, decomposing dead algae consume oxygen, and oxygen depletion contributes to further phosphorus release from the bottom. Treatment is intended to interrupt that cycle by binding phosphorus in the sediment.
Background
The target is for Bergshamraviken to reach good ecological status by 2039. The municipality describes clearer water and improved conditions for underwater vegetation and fish as desired outcomes. Pike and perch are cited as examples of predatory fish that benefit from an improved environment.
This is a long-term environmental target, not an announcement that good status has already been achieved. The municipal water strategist emphasises two parallel tasks: reducing new phosphorus entering the bay and addressing the material already stored in its sediments.
Latest update
In the municipality's September report, the crew says visibility around the equipment has already improved. That observation is not equivalent to a completed assessment of water quality across the whole bay. The practical advice to boat traffic is to keep clear of the working vessel while treatment is under way.
Sources
Related analysis
Reviewed: 2026-09-07. Independent editorial content. WUA Sverige is not affiliated with any political party.