Photo: Michelle McCrackin.

Humans alter the landscape by activities such as clearing land to grow crops, building cities, or altering the flow of rivers. These activities can leave “footprints” that last for centuries. Think of the discoveries of archeologists, who uncover ruins of ancient cities or the canals that delivered water to the cropland of prehistoric settlers. Other legacies are more recent, like the disposal toxic chemicals from industries (e.g.Superfund Sites in the US or HELCOM Hot Spots).

In the past decade, scientists have become aware of a different kind of legacy, one that results from how we have handled the nutrients (nitrogen and phosphorus) in fertilizer and sewage. This situation can be described as “too much of a good thing” because nitrogen and phosphorus are critical for life, which is why they are important crop fertilizers. But nutrients from cropland and livestock farms can leak to nearby water bodies, where they have a fertilizing effect and contribute to eutrophication.

Additionally, our food generally contains more nitrogen and phosphorus than our bodies need and the excess ends up in sewage systems. In the early days of flush toilets, wastewater was disposed of, untreated or poorly treated, in nearby surface waters. The nutrients from this sewage likely remain in the sediments of rivers and lakes and can be flushed out during flood events, causing problems for downstream areas.

Legacy phosphorus could play a role in the Baltic Sea region. Even though many actions have been taken to reduce phosphorus inputs to the sea from sources like agriculture and sewage, phosphorus inputs must be further reduced by 31% (about 10 thousand tonnes) to meet the goals of the Baltic Sea Action Plan.

To understand phosphorus dynamics in the Baltic Sea drainage basin, we developed a phosphorus “budget” going back to 1900 and a computer model to explore the fate of this phosphorus. The model included a “fast” pathway that represented sources such as runoff from cropland and a “slow” pathway that represented leakage from legacy sources. Our work was recently published in a peer-reviewed scientific journal.

The model suggests that about half of current phosphorus inputs to the sea are from old, legacy sources. As a result, it could be difficult to make substantial, near-term reductions to the sea. That said, the model also suggests that the worst has passed: the contribution of legacy sources could decrease in the coming decades because of all the actions that have been taken to manage phosphorus. This situation calls for patience and the need for realistic expectations. It takes time for management actions to have an effect, and these lags can cause frustration in society.

It is still possible to reduce phosphorus inputs to the sea today. For example, the “fast” transport pathway can be reduced through the amount, timing, and method of manure and fertilizer application. The model suggests that 17 million tonnes of phosphorus has accumulated around the sea in, for example, soils and lake and stream sediments and could leak for decades. This is nearly the same amount of phosphorus that is in 30 years’ crop harvest! It is possible to “mine” at least some of the accumulated phosphorus in cropland by reducing fertilizer application.

While sewage treatment capabilities have improved substantially under the EU Waste Water Directive, HELCOM has recommended even stricter standards. Phosphorus-removal efficiencies in sewage treatment plants vary greatly among countries in the region, between 63% and 97%. Further improvements in sewage treatment capabilities, especially for cities on large rivers that drain to the sea, could result in relatively quick reductions in phosphorus inputs.

Legacy phosphorus cannot be readily measured, so computer models are currently the best way to understand this phenomenon. Our large-scale model is a first step and should be followed up with more detailed, smaller-scale models and soil testing to identify where phosphorus has accumulated in the landscape.

There is also the potential for a nitrogen legacy in the Baltic Sea region, similar to what has been found for the Mississippi River. Stay tuned – I am thinking about a computer model for legacy nitrogen.

Text: Michelle McCrackin

Further reading

Peer-reviewed paper about legacy phosphorus in the Baltic Sea region

HELCOM report on progress towards nutrient reduction targets

Phosphorus-removal capabilities on sewage treatment facilities around the Baltic Sea

Legacy nitrogen in the Mississippi River