The North Sea Coasts
195
(Denmark) frequently becomes anoxic during warm summer periods.
The water may be stagnant for one to several weeks due to stratification
in temperature and salinity. The anoxia stimulates the anaerobic
metabolism in the sediment and H 2 S accumulates. Benthic animals react
to lack of oxygen by moving out of the mud and may survive lying on
the mud surface. Mussel beds increase the benthic respiration per m 2
tenfold and thereby enhance oxygen depletion of the bottom water. Their
high metabolic rate may thus regulate the size of the mussel beds to the
limit at which animals in the center die off. Frequent anoxia lead to mass
mortality of the bottom fauna in Limfjorden. Due to the seasonal oxygen
depletion, the composition of the benthic community is in some areas
regulated by alternating sequences of extinction and recolonization.
In 1991 an important and comprehensive report appeared (Funen
County Council 1991) on the eutrophication of the Danish coastal waters
between the Kattegat and the western Baltic Sea. The area of the County
of Funen is 3538 km 2 or approximately 8% of the total area of Denmark.
The population on January 1989 was 458000 equal to about 9% of the
population of Denmark. Sewage from homes and industry accounts for
approximately 18% of the nitrogen load and 78% of the phosphorus
load. The non-point runoff (primarily from agriculture) is responsible
for 82% of the nitrogen load and 22% of the phosphorus load.
Measurements of the nitrogen concentration in the coastal waters
during the years 1976 to 1989 show that nitrogen concentration follows
the variations in runoff from the land, not just on an annual basis, but
also on a monthly and even weekly basis. Because of the variability in the
nitrogen discharge, the IS-year period, 1976 to 1990, is too short to
conclude whether there has been a general trend in the increase in
nitrogen concentrations in the sea. The instantaneous nitrogen reservoir
in farm soils, however, is so large that even a small peak in precipitation
can trigger a massive nitrogen discharge to the coastal waters. These
peaks result in an increase in nitrogen concentrations and a burst of
algal production followed by oxygen depletion in the coastal waters.
The time sequential development in the phosphorus concentrations in
the open coastal waters does not follow the variations in runoff from the
land. Concentrations of phosphorus in the coastal waters were actually
higher after the period of oxygen depletion in 1981, even though the
phosphorus runoff after 1981 was less than in the years before. This may
be the result of internal loading, the release of phosphorus from marine
sediments under deoxygenated conditions. Winter concentrations of
phosphorus at the surface of the coastal waters are to a large degree
controlled by the rate of mixing and therefore by wind conditions.
During the years 1976 to 1989, the wind conditions in the winter months
195
(Denmark) frequently becomes anoxic during warm summer periods.
The water may be stagnant for one to several weeks due to stratification
in temperature and salinity. The anoxia stimulates the anaerobic
metabolism in the sediment and H 2 S accumulates. Benthic animals react
to lack of oxygen by moving out of the mud and may survive lying on
the mud surface. Mussel beds increase the benthic respiration per m 2
tenfold and thereby enhance oxygen depletion of the bottom water. Their
high metabolic rate may thus regulate the size of the mussel beds to the
limit at which animals in the center die off. Frequent anoxia lead to mass
mortality of the bottom fauna in Limfjorden. Due to the seasonal oxygen
depletion, the composition of the benthic community is in some areas
regulated by alternating sequences of extinction and recolonization.
In 1991 an important and comprehensive report appeared (Funen
County Council 1991) on the eutrophication of the Danish coastal waters
between the Kattegat and the western Baltic Sea. The area of the County
of Funen is 3538 km 2 or approximately 8% of the total area of Denmark.
The population on January 1989 was 458000 equal to about 9% of the
population of Denmark. Sewage from homes and industry accounts for
approximately 18% of the nitrogen load and 78% of the phosphorus
load. The non-point runoff (primarily from agriculture) is responsible
for 82% of the nitrogen load and 22% of the phosphorus load.
Measurements of the nitrogen concentration in the coastal waters
during the years 1976 to 1989 show that nitrogen concentration follows
the variations in runoff from the land, not just on an annual basis, but
also on a monthly and even weekly basis. Because of the variability in the
nitrogen discharge, the IS-year period, 1976 to 1990, is too short to
conclude whether there has been a general trend in the increase in
nitrogen concentrations in the sea. The instantaneous nitrogen reservoir
in farm soils, however, is so large that even a small peak in precipitation
can trigger a massive nitrogen discharge to the coastal waters. These
peaks result in an increase in nitrogen concentrations and a burst of
algal production followed by oxygen depletion in the coastal waters.
The time sequential development in the phosphorus concentrations in
the open coastal waters does not follow the variations in runoff from the
land. Concentrations of phosphorus in the coastal waters were actually
higher after the period of oxygen depletion in 1981, even though the
phosphorus runoff after 1981 was less than in the years before. This may
be the result of internal loading, the release of phosphorus from marine
sediments under deoxygenated conditions. Winter concentrations of
phosphorus at the surface of the coastal waters are to a large degree
controlled by the rate of mixing and therefore by wind conditions.
During the years 1976 to 1989, the wind conditions in the winter months