Foto: Free image in pxhere

Every sixth year HELCOM carry out a pollution load assessment for the Baltic Sea.

The latest assement, Pollution Load Compilation (PLC6),was released recently and shows that nitrogen (N) and phosphorous (P) inputs to the Baltic Sea continue to decrease.

Between 2006 and 2014, N decreased by 16% and P decreased by 13%, showing progress towards the nutrient reduction targets in the Baltic Sea Action Plan (BSAP). Reductions were seen across all pathways of nutrient delivery to the sea.

Comparison of total nitrogen and phosphorus inputs to the Baltic Sea by delivery pathway between PLC5 (2006) and PLC6 (2014).

"Important to understand the sources of nutrients"

While there has been progress towards BSAP targets, it is also important to understand the sources of nutrient to the sea in order for the HELCOM countries to develop appropriate measures for continued nutrient reductions. The main sources are:

• Direct point sources include the discharge of sewage effluent from coastal cities.
• Atmospheric deposition originates from fossil fuel combustion and agriculture.
• Rivers are not only the largest source of nutrients delivered to the Baltic Sea, but also the most difficult to understand.

Out of the total nutrient input to the sea, 70% of the N and 95% of the P are from rivers. But while it is possible to measure the amount of N and P in river water using laboratory equipment, there are currently no similar ways to measure  the source of the N and P. For example, P from sewage cannot be distinguished from P from background or agricultural sources. As a result, computer models are used to estimate the contribution of different sources, based on information from the countries around the Baltic Sea. This is also called source apportionment.

According to the recent estimates, the natural background sources of N and P are about one-third of total river inputs to the sea; the remainder is attributed to diffuse sources, human sources (like agriculture), point sources (like sewage), and transboundary sources (inputs from countries that are not contracting parties to HELCOM, like Belarus).

Methods to estimate different sources are not harmonized

Understanding how the sources of river nutrients have changed over time is vital for monitoring and improving the efforts to decrease nutrient inputs to the Baltic Sea. But, because of changes in source apportionment methods, the information is inconsistent between years, which makes it difficult to make comparisons. For example, the amount of P attributed to natural background sources in PLC6 is about double what was reported in PLC5 because of changes in methodology.

"Some countries simply don’t provide the information"

In addition, methods to estimate nutrient source apportionment are not harmonized among HELCOM countries, and not all countries provide this information. Only Germany, Denmark, Finland, Lithuania, Poland, and Sweden provided detailed source information. As a result, it is not possible to quantify the total magnitude of specific sources, such as agriculture, for the sea as a whole and its sub-basins. For example, we know part of the agriculture sources, but not the total amount – because some countries simply don’t provide that information.

Ideally, the results of multiple different models could be compared, much like is done for climate and weather predictions. But in the absence of such a process, the information reported by the HELCOM countries in PLC6 is the best that is available for the sea as a whole.

"Agriculture is the largest human-related source of river nutrients"

Based on data reported to HELCOM, agriculture is the largest human-related source of river nutrient, followed by sewage, urban and industrial sources. PLC6 shows that there is opportunity to make further reductions in river nutrient loads by reducing losses from agriculture and improving sewage treatment capabilities in upstream (non-coastal) areas. But the size of the opportunity varies between country and sea sub-basin.

Sources of N and P delivered by rivers to the Baltic Sea for 2014 estimated from country-reported data in PLC 6. Not defined or unknown sources could include agriculture in Russia and Latvia; detailed source apportionment was not available for these countries. Transboundary is from non-HELCOM countries.

There is a need for a common, catchment-wide modeling approach

Nutrient reduction targets identified in the BSAP are based on what is needed to achieve good environmental status in the sea. The reduction targets were allocated to the HELCOM countries in proportion to their inputs to the sea for the reference period, 1997 to 2003. This target is called the Country Allocated Reduction Target (CART). In some cases, a country might not be able to meet its CART, for example, in cases where CART is greater than human-related sources. This would mean that natural background sources would need to be reduced, which is not feasible or practical.

"Each HELCOM country uses a different computer model"

Thus, as HELCOM prepares to update the BSAP beyond 2021, there is potential to use source apportionment to evaluate the feasibility of nutrient reduction targets by country and sea sub-basin. Doing so, however, would require a consistent approach to source apportionment for the entire catchment. Currently, each HELCOM country uses a different computer model to estimate their sources of nutrients to the sea (and several countries lack such models entiretly) and there is no common, catchment-wide modeling approach.

First we need a common approach across all countries to develop source apportionment. Then, ideally, we need multiple common approaches to really improve the accuracy in the estimates.

"We need a better understanding of the sources of nutrients"

Despite the inaccuracy and the lack of a common approach, the estimates in the PLC6 still shows, without doubt,  that nitrogen and phorphorus inputs to the Baltic Sea continue to decrease. Still, additional redutions are needed to meet the BSAP targets. And the greatest opportunities for reductions appears to be from agriculture and sewage-related sources (which includes urban runoff and industries and households).

Today we have a fairly accurate picture of how much nutrients that reach the sea. To be able to develop and implement effective nutrient reduction measures, we need a better understanding of where the nutrients come from.

Text: Michelle McCrackin