Nationwide monitoring of nutrients and their ecological effects: state of the Danish aquatic environment

In 1987, the Danish Government passed the Action Plan on the Aquatic Environment, the main objectives of which were to reduce nitrogen and phosphorus discharge to the aquatic environment by 50% and 80%, respectively, before 1994. At the same time a harmonized nationwide monitoring program was established to monitor the reduction in nutrient load from different sectors and the ecological effects in the aquatic environment. The monitoring program covers all aquatic media and provides spatial and temporal information on the land-use related distribution of nutrients and their effects on the aquatic environment. Results obtained during the first year are presented together with long-term trends in nutrient loading and surface-water quality. Nitrogen has been documented to be the limiting nutrient for phytoplankton production in Danish marine waters, except in many inshore waters for a short period in spring where phosphorus is the limiting nutrient. One of the main objectives of the Danish Action Plan is therefore to reduce nitrogen loading of the aquatic environment by 50% before 1994. Preliminary model estimates show that a similar reduction in the nitrogen input from Sweden and Germany to the Kattegat and the Belt Sea will lead to an increase in autumn bottom-water oxygen concentration of 0.5-1.0 ml L-1.