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  Summary, conclusions and the future  
  Degree of nutrient enrichment  
  Direct effects  
  Indirect effects  
  Effectiveness of strategies and measures  
  The future  
  Nutrient load
  Nutrient concentrations
  N/P ratio

Degree of nutrient enrichment

Detailed nutrient load compilations to Danish waters were initiated in 1989, although the main increase in nutrient loads from land and atmosphere actually took place long before. As an example, the estimated annual riverine load of N to Danish coastal waters in the 1960s was about 60% of that in the 1980s and the P load increased four fold in the Baltic Sea, North Sea regions from the 1940s to the 1970s. A growing number of sewage treatment plants have reduced the Danish point source loads of P to surface waters (fresh and marine) since the late 1980s by nearly 90% and the total land based P load to marine waters has been reduced by 60% from 1990 to 2001. The improved sewage treatment has also reduced the overall N load by about 14%, while decrease in leaching from agricultural soils has reduced the N load with about 21%. All figures are corrected for variations in runoff.

Atmospheric deposition of inorganic nitrogen is important on large sea surfaces as Kattegat and the Belt Sea where atmospheric N deposition makes up about 30% of total N load from surrounding land and atmosphere. During the period 1989–2001 there was a decrease in the air concentration of N bound in particles and a tendency to a decreasing deposition of about 15%.

The assessment of trends in nutrient concentrations in Danish waters is based on indices for mean annual concentrations of DIN, TN, DIP and TP in the upper mixed layer developed for estuaries-coastal waters and the open Kattegat–Belt Sea.

The decreasing nutrient load to Danish waters is reflected in the nutrient concentrations. The nitrogen concentrations in 2001 were the lowest observed during the period 1989-2001 and at the same level as in the very dry years 1996 and 1997, even though the runoff was about normal. In the open waters of the Kattegat and Belt Sea the runoff corrected nitrogen concentrations shows a steady decrease since 1989. In the estuaries and coastal waters, a significant decrease was observed after 1997. In the estuaries and coastal waters, the phosphorus concentrations have stabilised at a low level after significant decreases in the beginning of the 1990s.

The optimal N/P-ratio, the Redfield ratio, for phytoplankton growth is 16:1. In the open Belt Sea and Kattegat the winter DIN/DIP-ratio did not deviate much from the Redfield ratio and there was no general trend in annual mean N/P-ratio during the period 1989–2001.

In estuaries the winter DIN/DIP-ratio is high (>25) to very high (>100). Annual mean N/P-ratios in the estuaries-coastal waters showed an increase from 1989 to 1998 parallel to the reduction in phosphorus load, and then a decrease to 2001 parallel to the decrease in nitrogen load per runoff.

In the North Sea the N/P-ratio was generally high ranging between 25 and 60, except in the saline central North Sea water. Also in the Skagerrak N/P-ratios were high at salinities lower than 33.

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Danish Environmental Protection Agency & National Environmental Research Institute • updated: