“Legacy phosphorus hasn’t been widely discussed”


17 million ton pool of phosphorus affects eutrophication work

Almost half of the phosphorus currently entering the Baltic Sea could leak from an enormous pool of accumulated phosphorus on land. Decision-makers must take this in into account when designing nutrient reduction measures and targets, says researcher Michelle McCrackin.

Text: Henrik Hamrén

– This huge amount of accumulated phosphorus on land could explain why there haven’t been greater reductions in loads to the sea despite all the measures that have been taken on land, says Michelle McCrackin, limnologist and researcher at Stockholm University’s Baltic Sea Centre.

Over the past century, countries around the Baltic Sea have applied more than 44 million tons of mineral phosphorus fertilisers to their agricultural soils. This is ar greater than the amount of phosphorus removed by crop uptake and harvest. The remaining phosphorus has created a “pool” of so-called legacy phosphorus on land. This pool will continue to leak for decades to downstream lakes and coastal areas where it can cause environmental problems.

In a recent policy brief, Michelle McCrackin and her colleagues estimate that about 45 percent of all waterborne phosphorus to the Baltic Sea comes from legacy sources on land.

– That makes legacy phosphorus an important factor in decisions involving setting nutrient reduction targets and implementing programmes of measure to reduce nutrient loads to the sea, she says.

How is that?

– The constant input of legacy phosphorus creates a potential for lag times between the implementation of nutrient abatement measures on land and the reduction in nutrient loads to the Baltic Sea. This lag time is important to consider when environmental targets are set.

Aren´t decision-makers already aware of this?

– Legacy phosphorus is very rarely mentioned in discussions about eutrophication in the Baltic Sea. Focus is more often on the nutrient input from agriculture and sewage, and of course on the so-called internal load*. Legacy phosphorus has been discussed by the scientific community for the past decade, but has not been widely discussed in policies such as the Baltic Sea Action Plan, or the Programmes of Measure in River Basin Management Plans under the Water Framework Directive.

How big is  the legacy pool on land?

– We estimate it to be about 17 million tons of phosphorus. In comparison, the internal load of accumulated phosphorus in the Baltic Sea is about 100 000 tons of phosphorus.

Can the leakage be mitigated?

– It depends on where the legacy phosphorus is located in the landscape. We need more detailed studies to better understand how much legacy phosphorus remains and where it is. For example, if the legacy phosphorus is in agricultural soils, there is a greater possibility to utilize it compared to a situation where legacy P is in lake or stream sediments.

What kind of measures are we talking about?

– In agricultural soils, field-scale tests and soil maps could be used to determine the presence of phosphorus surpluses and insight into site-specific crop phosphorus needs. Once identified, phosphorus surpluses in soil could be used by actively reducing the amount of new fertilizer applied. Buffer strips and sedimentation ponds are examples of other common practices to intercept nutrient loss and reduce phosphorus transport to from crop land to surface waters. However, to be effective, these measures must be adapted to local conditions.

Will we ever get rid of the legacy phosphorus?

– The leaky legacy pool on land will continue to affect the Baltic Sea in the foreseeable future. But the good news is that we believe it is depleting. This means that it is likely the worst has passed and that the contributions of legacy phosphorus will decrease in the future. Our study is just a first step. More work is needed at the river basin scale to identify the location of legacy phosphorus pools and their potential to interfere with other actions taken to reduce phosphorus loads to the sea.

* The accumulated phosphorus that moves back and forth between the sediment and water column

Michelle McCrackin


Legacy phosphorus

Poor sewage treatment and an over-use of fertilisers, mainly from the 1960s to the 1990s, has created an accumulated pool of about 17 million tons of phosphorus in soils and lake and stream sediments in the Baltic Sea catchment.

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, background sources.

Phosphorus is usually attached to particles that move from land to inland water bodies, like lakes and streams, and ultimately to the sea. Most legacy phosphorus likely reaches the sea through rivers, not through groundwater.

Unavoidable delays

Phosphorus inputs to the Baltic Sea catchment stay for an average of about 30 years before entering the sea or long-term storage in soils or lake and stream sediments.

Similarly, once phosphorus enters the sea, it stays in the water or surface sediments for about 30 years before being flushed to the North Sea or buried in deep sediments.

As a result, there are unavoidable delays between when actions are taken on land and when the sea will benefit from those actions.

Source: Key processes in the coupled carbon, nitrogen and phosphorus cycling of the Baltic Sea