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
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.