Difficult to catch nutrients on their way to the sea - and to measure success
The right measure, in the right place, with the right design, is crucial to successfully reduce diffuse nutrient inputs from land into the Baltic Sea. This is shown by the latest study from the Living Coast project. The study also shows that it is difficult to capture nutrients in the short term once they have entered the waterways, and that some measures need maintenance to avoid becoming counterproductive.
In the early 2000s, Björnöfjärden was one of the most eutrophic bays in the Stockholm archipelago. Extensive algal blooms caused turbid water and widespread oxygen depletion in the bottom waters, and plant and animal life was clearly affected. This was due to high nutrient inputs from the land, caused in part by inadequate wastewater treatment from holiday homes built in the 1950s and 1960s and later converted to permanent housing and from a conference centre. Nutrient leaching from agriculture and some horse farms in the area also contributed.
The aim of the Living Coast project was to try to show that it is possible to reverse the trend and recreate a good environment in a bay like Björnöfjärden. In 2011, the BalticSea2020 Foundation hired the researchers Linda Kumblad and Emil Rydin to implement the measures needed to reduce eutrophication, improve water quality and create more natural plant and animal communities in the bay.
"It was an exciting and challenging task," says Linda Kumblad. "We were to use all possible measures to try to show that it is possible to improve the environment in highly eutrophic areas with cost-effective methods and thus stimulate more action."
Successful aluminium treatment
Decades of high nutrient inputs to the bay had also led to oxygen deficiency in the bottom water contributing to phosphorus release from the seabed – known as internal loading –that fuelled large annual algal blooms. To address this part of the problem, the researchers carried out an aluminium treatment in 2012-2013, adding aluminium to the seabed deeper than six metres throughout the bay to bind mobile phosphorus.
"The method had been used for some time in eutrophic lakes, but this was the first time it was tested in the Baltic Sea," says Emil Rydin.
The treatment proved to work well even in the brackish coastal environment. Phosphorus levels in the water were halved and the environment improved significantly in a short time. Subsequent studies have shown that the measure posed no risk to humans or animals in the bay. Over time, the researchers estimate that the treatment results in three tonnes of dissolved phosphorus being bound to aluminium.
"What used to be a source of phosphorus, was reverted to being a phosphorus filter," says Emil Rydin. "The environment in Björnöfjärden has improved, but in addition, the export of phosphorus from the bay to the coast outside has decreased significantly."
Necessary measures on land
However, to maintain a good environment in the bay, nutrient inputs from land must also be reduced. Otherwise, nutrient levels in the water will gradually increase and internal pollution will eventually return.
"The aluminium treatment was intended to be a first step that would stimulate all stakeholders to contribute to further improvements. And it certainly worked. The commitment around and for Björnöfjärden has been enormous," says Linda Kumblad.
Between 2013 and 2016, the researchers, together with local residents, the municipality and other stakeholders in the area, carried out a large number of measures in the Björnöfjärden catchment area, which covers just over 15 square kilometres.
To reduce leakage from agriculture, structure liming was performed at the fields. This means that lime is mixed into the soil, making it more porous so that crop roots can access water and nutrients in the soil more easily, resulting in less nutrient leaching.
Lime filter ditches were also dug to collect nutrient-rich water from the fields, and phosphorus ponds were constructed to allow excess nutrients to settle. At the outlet of the ponds, and downstream of some of the horse paddocks, lime filters were installed to prevent dissolved phosphorus from flowing further into the bay.
Several new practices were also introduced at the horse farms, such as more regular mucking out of the paddocks. New dense manure slabs were installed, the land was drained and stabilised, and the paddocks were fenced off to keep the horses away from the waterways.
At the large conference centre, a new sewerage system was installed, and many of the local residents also chose to improve their sewers. A latrine station was built to facilitate the safe disposal of latrine waste from holiday homes with outhouses, and sealed tanks were installed at the outhouses in the nature reserve.
In addition, a pike wetland was constructed at the far end of the bay to boost pike stocks. Increased predator populations, such as pike, can reduce the effects of eutrophication through a so-called cascade effect: more predators lead to fewer small fish, which leads to more zooplankton and thus less phytoplankton clouding the water.
"We really left no stone unturned, and implemented all the measures we deemed necessary in the catchment," says Linda Kumblad.
In total, the land-based measures were estimated to eventually halve the annual input of phosphorus to Björnöfjärden.
Extensive sampling
The ambitious measures programme also included an equally ambitious sampling programme, including monthly measurements in all major waterways. Some of this unique material, comprising 11 years of data from 65 measuring points, has now been compiled and analysed by researchers from Living Coast together with Faruk Djodjic at the Swedish University of Agricultural Sciences (SLU), among others.
So how much has the nutrient input to Björnöfjärden actually decreased thanks to the measures? The study gives a somewhat disappointing answer.
"The measurements in the direct outlets to the bay show very large variations both within and between different years, but overall for the whole period it is not possible to see a significant change," says Faruk Djodjic. "This may be because the measures have not yet had enough effect, but it is also possible that are not yet detectable, for example because there are still a lot of nutrients stored in the soil. Although 11 years of measurements may sound like a lot, in this context it is quite a short time to detect trends in a system with extremely variable nutrient levels."
The measurements showed that nutrient transport through the landscape is closely linked to the magnitude of water flows, with large volumes of water and thus nutrients flowing during short periods of the year.
"It is very likely that the sampling will miss the flow peaks, resulting in an incomplete estimate of the annual flows," explains Emil Rydin. "We measured frequently and over a decade during this project. Still, in retrospect, it seems that there was not enough to capture all the events."
So, what could be done instead to evaluate the effects of measures?
"This is a difficult question to answer," says Faruk Djodjic. "One possibility is to install instruments that measure water flow at all times and automatically take water samples during peak flow periods. Today, there are also sensors that can measure certain parameters every 15 minutes, which in turn can be related to phosphorus, for example. However, this is quite expensive, and there is a risk that something goes wrong and you have no data at all, even though the sensors are becoming more reliable. But maybe we need to refine or find other ways to evaluate measures than measuring nutrient fluxes, for example by using tracers."
Impact of different measures
By taking a closer look at the measurements in the different parts of the catchment, the researchers have nevertheless gained a good picture of how well the various measures have worked out.
For example, measurements upstream and downstream of the horse farms showed that the battery of measures implemented upstream of the lime filter had a significant effect.
"It is good to take measures right at the nutrient source," says Linda Kumblad. "Measures on horse farms should be a rather 'low-hanging fruit', with focus on collection and recycling of manure. It is also important to minimise trampling damages by the horses in order to maintain as full a vegetation cover as possible. This gives the soil a much better capacity to capture and store nutrients."
Once nutrients reach water bodies, it is much more difficult to capture them, the researchers also note. In addition, measures that involve digging into the ground – such as the construction of ditches and wetlands to capture nutrient-rich water – can initially release nutrients and thus temporarily increase nutrient transport.
"Digging can expose nutrient-rich clays, and when they are eroded by water flows or waves, nutrient levels increase downstream instead of decreasing," explains Emil Rydin. "It may take some time to assess the long-term effect of such a measure, and it also makes it important that wetlands are constructed in the right places."
The measurements around Björnöfjärden indicate that the two phosphorus ponds were not optimally located. The phosphorus levels in the incoming water do not seem to have been high enough for a significant change to be detectable downstream.
"There are many good examples of the ability of wetlands to reduce nutrients, but the effect is easier to measure the higher the concentrations in the incoming water. It is also a measure with a long lifespan, around 20 years, where you would need to measure for even longer to capture the effect," says Faruk Djodjic.
The third wetland – the pike spawning area – was well located, but because its primary purpose was to promote predatory fish recruitment, it was not made deep enough to function as an effective phosphorus trap.
Another aspect to bear in mind when discussing wetlands is that a phosphorus pond needs to be managed so that it does not become overgrown, reminds Emil Rydin.
"If the nutrient-rich sediment that accumulates on the bottom is not taken care of, and if the material starts to move, the pond can become a source of nutrients. There are many things to consider when trying to assess whether this type of measure is cost-effective," he says.
Technical solutions require maintenance
A similar phenomenon was observed for the lime filters installed at the outlet of the phosphorus ponds. The filters were initially very effective in trapping phosphorus, which was reflected in reduced concentrations downstream, but after a few years the effect seemed to diminish. Instead, the trapped nutrients began to leach out and concentrations in the outgoing water rose again. On the other hand, the lime filter installed next to some horse paddocks proved to have no effect at all, probably because the water did not pass through the filter material evenly.
"Passing water through filter material is difficult; water quickly finds the easiest way through the material and the filtration risks becoming ineffective after a while. The filter material then needs to be replaced, which means new excavation with the risk of increased nutrient transport and new costs," says Emil Rydin.
The wastewater treatment plant installed at the conference centre showed to be effective; the levels in the large crown ditch downstream decreased after he installation. However, also when it comes to wastewater management, it should be borne in mind that some technical solutions, such as wastewater treatment plants, require continuous maintenance and the right dosage of precipitation chemicals, which costs money.
"If this is not done properly or if the dosage is too low, the treatment plant risks quickly becoming a major source of nutrients," says Emil Rydin.
However, allowing wastewater to infiltrate into the soil instead means loading the soil with nutrients that may sooner or later leach out.
"Improving the treatment of toilet waste is important and may have a major impact in the long term. The safest way is to collect the nutrients in a tank and transport them to efficient large waste water treatment plants," says Linda Kumblad.
Forest area unexpected source of nutrients
The study also clearly demonstrated that nutrients stored in the soil can have a long-term impact. The measurements showed that a forested area in the north-west of the catchment was leaching an unexpected amount of nutrients - more than the active agricultural land in the catchment area.
"When we got to the bottom of it, it turned out that the forest was planted on old farmland in the mid-20th century. Farmland is more fertile than forest land, and the over-fertilisation of the past probably led to a lot of nutrients being stored in the soil, which are still leaching into the waterways today, decades later," says Faruk Djodjic. "The fact that nutrient levels were lower downstream of the farmland also suggests that the structural liming and other agricultural measures that were implemented were effective in binding nutrients."
Elsewhere in the area, old nutrient build-up from phosphorus-intensive agriculture and non-functioning sewers is likely to continue to leach out and contribute to nutrient inputs to Björnöfjärden, and is likely to continue to obscure the effect of some of the measures taken.
"A lot of patience and detective work is needed, both to get the right measures in the right place and to evaluate the impact of measures on land," concludes Linda Kumblad.
What is then most effective to reduce diffuse nutrient inputs from different sources on land?
"The best way is to avoid releasing excess nutrients. This means applying fertiliser at the right time – when the crop is germinating or growing, collecting and disposing of horse manure from both stables and pastures, and collecting toilet waste in closed tanks. These nutrients can then be used on the fields, which creates recycling of nutirents. Damage to soil layers should also be avoided, for example by not digging unnecessarily.”
Text: Lisa Bergqvist
Läs hela studien i Ambio:
Last updated: March 11, 2025
Source: Östersjöcentrum