A new study shows that almost half of the phosphorus currently entering the Baltic Sea could derive from accumulated phosphorus on land, so called legacy phosphorus. This legacy originated from sewage and agriculture sectors, mainly during the 1960s to 1990s. The amount of legacy phosphorus that leaks to the Baltic Sea has decreased over the last 20 years, and will decrease further over time. But the size of this reduction will depend on continued actions to reduce the accumulation of phosphorus on land.
The accumulated pool of phosphorus on land – called legacy phosphorus - receives less attention than the pool of phosphorus in the Baltic Sea, despite being much larger and the ultimate source to the sea. The “legacy” results from how phosphorus was handled in the sewage and agriculture sectors in the past. The sources of legacy phosphorus include fertiliser, manure, and human sewage.
A legacy from the past
Over the past century, countries around the Baltic Sea have applied more than 44 million tons of mineral phosphorus fertilisers to agricultural soils. Throughout the years, farmers been advised to apply large amounts of mineral fertiliser and manure to increase crop yields. As a result, more fertiliser has been applied than has been removed with harvested crops, and this has led to the accumulation of phosphorus in soils. A more balanced fertilisation is recommended nowadays, but there are still areas with large surpluses. For the Baltic Sea catchment as a whole, the amount of phosphorus that has accumulated in agricultural soils is the same order of magnitude as three decades of crop nutrient needs.
Phosphorus from human sewage wastewater has also accumulated on land. In the early days of flush toilets, wastewater was untreated or poorly treated and disposed of in nearby surface waters. Some phosphorus from this wastewater likely remains in the sediments of rivers and lakes that flow to the Baltic Sea.
The legacy can be described as too much of a good thing, because while phosphorus is critical for all life, it can leak to water bodies and have an unintended fertilising effect that contributes to eutrophication. Even if no more phosphorus is applied to land, legacy phosphorus will continue to leak for decades to downstream lakes and coastal areas where it can cause environmental problems.
Over the past century, countries around the Baltic Sea have applied more than 44 million tons of mineral phosphorus fertilisers to agricultural soils. Farmers have been advised to apply large amounts of mineral fertiliser and manure to increase yields. As a result, more fertiliser has been applied than has been removed with harvested crops, and this has led to the accumulation of phosphorus in soils and sediments, lakes and streams. In addition, untreated or poorly treated wastewater was discharged into surface waters prior to modern sewage treatment capabilities.
A new study estimates that 45% of waterborne phosphorus to the Baltic Sea is from legacy sources; 14% is from rapidly transported sources, 8% is sewage from coastal cities. The remaining 33% is from natural, backgound sources.
A new study estimates legacy phosphorus magnitude
For the first time, legacy phosphorus dynamics have been estimated for the entire drainage basin of the Baltic Sea. Research carried out at the Stockholm University Baltic Sea Centre suggests that leakage from old, legacy sources contributes a substantial amount of phosphorus to the sea. Today, about 45 percent of the phosphorus entering the sea could derive from this source.
Another 33 percent comes from natural, background sources, 8 percent from direct inputs of sewage from coastal cities, and the remainder from rapidly transported sources, such as the runoff of fertiliser from cropland.
A reason for being patiently realistic
The computer model used for estimations suggests that phosphorus leakage from legacy sources will continue for decades, making it difficult to achieve large reductions from diffuse sources like agriculture in the near term. The potential for time-lags is not an excuse for inaction but a reason for being patiently realistic about the time-scale for observing results.
However, the model also suggests that the amount of legacy phosphorus that leaks to the Baltic Sea has decreased by about ten percent in the past 20 years. This decrease is due, in part, to the measures that have already been taken on land to reduce over-fertilisation and remove phosphorus from wastewater.
According to the model, we can expect that the amount of legacy phosphorus that leaks to the Baltic Sea will decrease even further over time, perhaps by as much as one-third compared to today. But the size of this reduction will depend on continued actions to reduce the accumulation of phosphorus on land. In other words, reducing the accumulation of phosphorus in the catchment today reduce future long-lasting inputs from legacy sources.
Total phosphorus loading to the sea has decreased by more than 50 percent since the 1980’s. Much of this reduction has been attributed to improved sewage treatment. During the past decades, many actions have also been taken to improve phosphorus management in agriculture. The lack of larger reductions in phosphorus loads to the sea from diffuse sources (i.e. not point sources) is partly due to the leakage of legacy phosphorus.
Actions taken today create tomorrow’s legacy
Mineral phosphorus fertilisers derive from phosphate rock, which is a finite resource, like fossil fuels. Unlike fossil fuels, however, there is no substitute. Today, 85 percent of phosphate rock reserves are controlled by three countries: Morocco, China, and Algeria. The European Commission has added phosphate rock to the list of 20 Critical Raw Materials for which supply security is at risk and economic importance is high.
In fact, society currently operates outside the planetary bound-aries of the phosphorus cycle. This is not sustainable. Therefore, recycling existing phosphorus within agriculture better and reducing the overall import of new phosphorus to the Baltic Sea region will not only mitigate eutrophication but also lead to the more sustainable use of a finite resource.
Therefore, it is critical that we continue to reduce the accumulation of phosphorus on land because actions taken today create tomorrow’s legacy. There is potential to make progress towards the goals of the Baltic Sea Action Plan by increasing the recycling of phosphorus in agriculture, reducing over-fertilisation, and improving wastewater treatment. Policies should aim to reduce the build up and transport of phosphorus already in the landscape.