Nitrogen compounds
Nitrogen is a gas that makes up 78% of the air. Nitrogen compounds are essential building materials for everything that lives: without nitrogen, plants could no longer build proteins. Nitrogen is taken up in the form of nitrate or ammonium salts. However, when there is an excess of these salts in the environment, not all of the materials are taken up and one speaks of eutrophication. Nitrogen compounds in the air contribute to the greenhouse effect.
- Sources
- Average nitrogen levels, graph by Michel Vloon, based upon data from the Yearbook Monitoring of National waters from RIKZ
The prominent sources of nitrogen are from agriculture (emission in the air in the form of ammonia, emission in the ground and surface water in the form of nitrate), motorized vehicles (emission in the air in the form of nitrogen oxides), industry and power stations (likewise in the air in the form of nitrogen oxides). Nitrogen and phosphorus enter the marine environment via the air and surface water.
The total annual nitrogen burden on the North Sea flows primarily in via the English Channel and the northern Atlantic Ocean. However, the present concentrations in these influx currents hardly deviate from the natural background concentrations. The amount of nitrogen in sea coming from the atmosphere is significant.
Water in tidal regions, estuaries and along the Dutch coast is very rich in nitrogen. This is due to the increased supply of nitrogen compounds via the rivers. This nitrogen comes primarily from agriculture where it is used excessively as fertilizer: two-thirds of the nitrogen in the rivers comes from the fertilizer used by farmers. Between 1980 and 1995, the supply of nitrogen from the river basins have declined barely if at all. The present supply is a multiple of the natural nitrogen load.
The Dutch government is striving for a level of nitrogen equal to the natural background concentration of 0.15 mg per liter seawater. With the exception of the central North Sea, the concentrations are still too high. Between 1975 and 1997, the Westerschelde showed a clear declining trend, while the trends in other tidal and coastal waters were only slightly declining. The concentrations stabilized after 1997.
The consequences of the high nitrogen concentrations in coastal water and the policy that is directed at decreasing the emissions are dealt with in the chapter on eutrophication.
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