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  Summary, conclusions and the future  
  Degree of nutrient enrichment  
  Direct effects  
  Indirect effects  
  Effectiveness of strategies and measures  
  The future  
  Oxygen concentrations

Indirect effects

Analyses of the development in bottom water oxygen concentrations during late summer/autumn in the Kattegat-Belt Sea from the 1970s to late 1980s/1990s showed significant decreases in all areas with a stratified water column. The decrease was especially pronounced from the mid 1970s to the late 1980s. There was no general development in the summer/autumn bottom water minimum oxygen concentration in the period 1989–2001. However, there was a tendency for an increase in minimum oxygen concentrations in spring (April–June). Nevertheless, at the end of August 2002 an unusually widespread and serious oxygen deficiency was observed in large areas of the inner Danish marine waters and common Danish-Swedish and Danish-German waters. In many areas of the southern Kattegat, the Sound and the Belt Sea oxygen levels in bottom waters were reduced to a level seldom or never seen before.

General trends in abundance of macrozoobenthos in the Sound, Kattegat and Belt Sea follow a bimodal pattern over the last 20 years with peaks in the beginning of the 1980s and in the middle of the 1990s. During the period 1998–2001 biomass and essentially total abundance of macrozoobenthos in the open waters showed the lowest values since the measurements started two decades ago. Some evidence indicates that reduced N-nutrient concentrations, and possibly reduced diatom abundance, when corrected for runoff, may have reinforced the decrease in zoobenthos stocks in recent years.

Correlation analyses between biological variables and the North Atlantic Oscillation index and runoff of freshwater from Denmark, showed significant positive correlations with 1 or 2 years time lag, thereby indicating that climate also influences variations in benthic macrofauna. In particular winter nutrient input, and likely the spring phytoplankton bloom, did influence benthic abundance.

The number of coastal areas with increasing species diversity were about the same as those with decreasing diversity during the period 1998–2001. Local factors like oxygen deficiency is a possible reason for these differences. During the same period there was no general change in abundance, biomass or number of species in coastal areas.

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