Riverine inputs of nitrogen and phosphorus
Total discharges from point sources and losses from diffuse sources into surface waters within the Baltic Sea catchment area decreased for phosphorus between 1985 and 2000. Reductions in nitrogen discharges/losses over this period were lower.
In 2000, about 660,000 tonnes of nitrogen and 28,000 tonnes of phosphorus entered the Baltic Sea via rivers. Four large rivers – the Neva, Nemunas, Vistula, and Oder – together accounted for the majority of the nutrient loads entering the Baltic Sea. More than half of the total waterborne phosphorus load and nearly one third of the total waterborne nitrogen load originated from Poland (Figures 1 and 2).
Figure 1. Annual average riverine runoff (m3/s) and riverine inputs of nitrogen(Ntotal )in t/a into the different sub-basins of the Baltic Sea 1994-2000. Different scales have been used in the graphs for the various sub-basins.
Figure 2: Annual average riverine runoff (m3/s) and riverine inputs of phosphorus(Ptotal) in t/a into the different sub-basins of the Baltic Sea 1994-2000. Different scales have been used in the graphs for the various sub-basins.
During the period 1994-2000, riverine loads of nitrogen and phosphorus closely followed variations in freshwater runoff. In years with high precipitation and high runoff, more nitrogen leached from cultivated areas than during dry years, resulting in higher riverine nitrogen loads.
In 2000, anthropogenic point and diffuse sources comprised the main part of the total riverine nitrogen and phosphorus loads entering the Baltic Sea. In most of the HELCOM countries 50-80% of the riverine nitrogen originated from diffuse sources. Diffuse loads were also the most important source of phosphorus in all areas except the Gulf of Finland, where point sources are more significant. The only area relatively unaffected by human activity was the catchment area around the Gulf of Bothnia.
Due to incomplete data sets for the whole time period, Russian nutrient data for the Baltic Proper (Kaliningrad Region) and the Gulf of Finland has been omitted from Figures 1 and 2. However, the total Russian nutrient riverine load in 2000 is estimated as 8,566 t/a nitrogen in the monitored part of the Baltic Proper catchment area, and 61,105 t/a nitrogen and 5,807 t/a phosphorus in the monitored part of the Gulf of Finland catchment area.
Source: HELCOM PLC Database
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