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  Effects and consequences  
Figure 1.1
The nitrogen cycle in the aquatic environment. All relevant sources are shown. The major difference between the nitrogen and phosphorus cycles is that phosphorus is not emitted to the air and that there consequently is no anthropogenic deposition. The main anthropogenic sources of organic matter are discharges from wastewater treatment plants and industries as well as losses from aquaculture. From Danish EPA (2000).


The essential nutrients causing eutrophication are nitrogen in the form of nitrate or ammonium and phosphorus in the form of phosphate. In addition, inputs of bio­available organic phosphorus and nitrogen can cause eutrophication, as bacteria can mineralise the organic phosphorus to phosphate and the organic nitrogen to ammonium, which is further oxidised to nitrite and nitrate.

Marine waters receive dissolved and particulate nutrients and organic matter from land via rivers and direct discharges, from the atmosphere and from adjacent seas (see Figure 1.1). In Denmark the most important sources are:

  1. agriculture,
  2. discharges from urban wastewater treatment plants, and
  3. separate discharges from industries,
    the first being the most important diffuse source.

Discharges from point sources, losses from agriculture and atmospheric deposition are monitored as an integrated part of the National Aquatic Monitoring and Assessment Programme and reported annually (Conley et al. 2002).

The inputs from adjacent seas are also monitored within the national monitoring programme. There are three major avenues of advective transport of nutrients that must also be considered:

  1. Inputs from the German Bight to the waters along the west coast of Jutland.
  2. Inputs of inorganic nutrients from Skagerrak to the Kattegat bottom water.
  3. Inputs of surface waters from the Baltic Sea to the Danish Straits and Kattegat.

All three sources have influence on the marine environment by supplying additional nutrients. The waters from the German Bight and the Baltic Sea have already received high inputs of nutrients. The input from the Baltic Sea differs from the two firsts because it actually dilutes bioavailable nitrogen concentrations in the Danish Straits and the Kattegat. The median winter surface concentrations of nitrate (1986–1993) in the water from the Baltic is 4.6 µM compared to the median concentrations in the waters in the Danish Straits and Kattegat, which are 7.3 (6.5–8.8) µM and 7.4 (6.0–9.7) µM, respectively.

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