Nutrients in coastal waters
Assessment made on 01 May 2004
ClassificationWater (Primary theme)
Coasts and seas
- WEU 004
Policy issue: Are nutrient concentrations in surface waters decreasing?
Nutrient concentrations in Europe's seas have generally remained stable over recent years, though some coastal stations in the North Sea, the Baltic Sea, the Mediterranean Sea and the Black Sea have demonstrated a decrease in phosphate and nitrate concentrations. A small number of stations in the Baltic Sea and the North Sea also showed an increase in nitrate or phosphate concentrations
Generally, the available time series show no clear changes in winter surface concentrations of nitrate. The nitrate concentrations appear variable, due to the fact that the nitrogen loads are related to the runoff, which in turn is highly variable from year to year. A decrease is found at a few stations in the Baltic Sea, the North Sea and the Mediterranean Sea. An increase is observed at a few stations in the Baltic Sea and the North Sea.
The time series show a decrease in winter surface phosphate concentrations at 33% of the coastal stations in the North Sea and at 22% in the Baltic Sea. Increasing trend is observed at 1 Finnish and 1 Swedish station in the Baltic Sea, and at 1 Belgian North Sea station. No general trend is observed at the majority of the coastal and marine stations. Measures have been taken in the catchment areas and a reduction in the use of fertilisers has occurred. However, recent research has indicated that phosphate concentrations in open Baltic waters including the Kattegat are strongly influenced from internal processes and transport due to variable oxygen regimes in the bottom water (Rasmussen et al. 2003). For example in the Gulf of Finland, a recent increase in phosphorus has been observed, due to leaching from sediment during hypoxic conditions (HELCOM 2001). In the North Sea major reductions in phosphate concentrations are found in the Dutch and Belgian coastal zone. This is probably due to a reduction in riverine P loads.
The time series show an increase in winter surface N/P-ratio at a number of stations in the Dutch, Belgian and Danish coastal waters of the North Sea, and in the Danish, German, Lithuanian and Swedish waters of the Baltic Sea. A decreasing trend is observed at 2 Finnish coastal stations in the Gulf of Finland, at 1 Swedish Baltic Sea station, at 1 German coastal North Sea station and at 2 Italian Mediterranean stations. No general trend is observed at the majority of the coastal and marine stations. This mirrors the generally unchanged nitrate concentrations but decreasing phosphate concentrations at a number of stations in the North Sea and Baltic Sea and decrease of nitrate at a few Italian stations.
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