Riverine inputs of nitrogen and phosphorus

	Authors: HELCOM PLC - Group Editor: Seppo Knuuttila, Finnish Environment Institute

Key message

In 2003 the total riverine (including coastal areas) nitrogen load entering the Baltic Sea amounted to 460 000 tonnes, and the total phosphorus load to 20 000 tonnes. Both figures were lowest during the period 1994-2003. Also the total runoff in 2003 was extremely low, 10 300 m³/s.

Results and assessment

Relevance of the indicator for describing developments in the environment

Eutrophication of the Baltic Sea is mainly caused by the excessive nitrogen and phosphorus loading from land-based sources. About 75 % of nitrogen and at least 95 % of phosphorus enters the Baltic Sea as waterborne (i.e. via rivers or as direct discharges). The total catchment area of monitored rivers cover 97 % of the total Baltic Sea catchment area. Together with unmonitored rivers and coastal areas they comprised 93 % of the total waterborne N load entering the Baltic Sea in 2003 and 88 % of the total P load respectively. The rest originates from direct point (and direct diffuse) sources.

Policy relevance and policy references

Information on the waterborne inputs, including riverine inputs of nitrogen and phosphorus is of key importance in order to follow up the long-term changes in the nutrient load into the Baltic Sea and to be able to, together with information from land-based sources within the catchments, determine the priority order of different sources of nutrients for the pollution of the Baltic Sea as well as assessing the effect of measures taken to reduce the pollution load. Quantified input data is a prerequisite to interpret and evaluate the state of the marine environmental and related changes in the open sea and coastal waters.

Assessment

Riverine nutrient load consists of discharges and losses from different sources within a river's catchment area, including discharges from industry, municipal waste water treatment plants, scattered dwellings, losses from agriculture and managed forests, as well as natural background losses and atmospheric deposition. According to the PLC-4 report diffuse load (mainly agriculture) contributed almost 60 % of waterborne nitrogen inputs and 50 % of phosphorus, respectively. As a result of improved treatment on industrial and municipal wastewater treatment plants nutrient discharges from point sources has reduced significantly between 1985 and 2000. However, the reduction targets for diffuse sources such as agriculture have not yet been fulfilled. Despite of that both figures for nitrogen and phosphorus load were in 2003 the lowest during the whole period 1994-2003, it cannot be concluded that the development is a result from effective implementation of measures for reducing nutrient load in the catchment. The N and P loads vary considerably from year to year depending mainly on hydrological conditions. In the periods of high runoff nutrients are abundantly leached from soil increasing the loads originating from diffuse sources and natural leaching.

The annual freshwater inflow (riverine runoff) during the period 2000-2003 was quite low compared to years before, and the year 2003 being exceptionally dry. The lower runoff has been measured only 3 times before, e.g. the runoff to the Bothnian Sea was lowest since 1921. To evaluate the change of pressure on nutrient supply to the Baltic region it is necessary to know the variability of runoff and normalize for this natural variability. To be able to evaluate the reductions of nutrient load made in the different sectors, comparable long-term data on riverine loads (since early '80s) should be available in order to prepare a proper trend analysis.

References

PLC-4 report

Click image to enlarge!

Figure 1. Riverine load of Nitrogen (N_total, NH₄-N, NO_2,3-N) to the Baltic Sea during 2003 for the 9 HELCOM countries. Units: tonnes. 

Click image to enlarge!

Figure 2. Riverine load of Phosphorus (P_total, PO₄-P) to the Baltic Sea for the 9 HELCOM countries. Units: tonnes.
 

Time series of riverine inputs to the Baltic Sea and coastal flow in 1994-2003.

Figure 3. Time series for riverine load of Nitrogen (N_total, NH₄-N, NO_2,3-N) in t/year to the Baltic Sea and the river and coastal flow in m³/s for the period of 1994-2003 of the 9 countries. (Note variable scales in the graphs) CLICK IMAGE TO ENLARGE.

Figure 4. Time series for riverine load of Phosphorus (P_total, PO₄-P) in t/year to the Baltic Sea and the river and coastal flow in m³/s for the period of 1994-2003 of the 9 countries. (Note variable scales in the graphs) CLICK IMAGE TO ENLARGE.

Table 1. Riverine, coastal and point source flow to the Baltic Sea of 9 countries in 1994-2003, m³/s.

Table 2. Riverine and coastal inputs of N_total of  9 countries in 1994-2003 as totals, t/year.

Table 3. Riverine and coastal inputs of P_total of  9 countries in 1994-2003 as totals, t/year.

Meta data

Technical Information

1. Data source:

Data have been collected by the Contracting Parties of HELCOM and submitted to the Pollution Load Compilation database (PLC database). The data base is located in Finnish Environment Institute (SYKE).

2. Description of data: 

The data are based on annual average concentrations (µg/l) of nutrients and their fractions (P_total, PO₄-P, N_total, NH₄-N, NO₂-N, NO₃-N and NO_2,3-N), and mean flows (m³/s). The Contracting Parties have calculated the annual loads (t/a) of monitored and unmonitored rivers and coastal areas. Monitored river loads are based on measurements, unmonitored river loads, coastal area loads on estimates, respectively.

3. Geographical coverage:

Drainage area of the Baltic Sea, 9 countries; Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia and Sweden, including the drainages of the Baltic Sea in Belarus, Czech Republic, Norway, Slovakia and Ukraine. 

4. Transboundary rivers:

To avoid double data reporting of the transboundary rivers, their loads have been included in as follows:

River Tornio (Finland)/Torne älv (Sweden): in the Swedish data

River Narva (Russia and Estonia): in the Estonian data

River Oder (Poland): in the Polish data

5. Temporal coverage:

Nutrient loads in 1994-2003.

6. Methodology and frequency:

Variable, basically as agreed on the PLC-4 guidelines 

Quality information

1. Strength and weakness:

The total catchment area of monitored rivers, unmonitored rivers, and coastal areas cover 97 % of the total Baltic Sea catchment area. They comprised 93 % of the total N load entering the Baltic Sea in 2003 and 88 % of the total P load respectively. To be able to evaluate the reductions of nutrient load made in the different sectors, comparable long-term data on riverine loads (since early '80s) should be available.

2. Reliability, accuracy, robustness, uncertainty:

Heterogeneous data in space and time may cause some variation between years. Due to robust analysing methods in some countries reported loads might be too high.

Missing data

1) Monitored rivers

Denmark: Heavy metals

Latvia: All data of 2003, Hg

Lithuania: Hg

Russia: all nitrogen fractions in 1994-1999 and all data of 2003

2) Unmonitored rivers and coastal areas

Latvia: All data of 2003

Russia: all nitrogen fractions and part of the phosphorus data

Denmark: heavy metals 

Table 4. Missing data on monitored rivers, unmonitored and coastal areas in 1994-2003 by parameter.

(DE = Gemany, DK = Denmark, EE = Estonia, FI = Finland, LT = Lithuania, LV = Latvia, PL = Poland, RU = Russia, SE = Sweden, years indicated after abbreviation)