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.