196
P.H. Nienhuis
varied a great deal, but showed no clear trend, showing that the
increased level of phosphorus concentration during this period cannot
be accounted for by the changes in the hydrographic conditions.
The composition of plant species found on the sea bottom around
Funen and the areas they occupy have changed significantly since the
turn of the century. The eelgrass-covered areas in coastal waters have
been reduced by 50% owing to the reduced penetration of sunlight. The
depth limit for eelgrass in open waters has been reduced from 9-10 m in
1890 to 5-6 m today and in fjords from 6-7 to 2-3 m. For seaweeds the
depth limit has been reduced from 30 - 35 m to 10 -12 m during the same
period. Filamentous algae and sea lettuce, Ulva species, increase, particularly in enclosed waters with high nutrient loads. In many areas
perennial macrophytes have disappeared completely.
Large areas of the coastal waters of Funen, where the depth exceeds
15 m, are often affected by oxygen depletion in the bottom waters. Every
year some coastal water areas experience oxygen depletion during
August to October; in 1988 and 1989 this occurred from June to October.
From 1976 to 1989, oxygen concentrations in the bottom waters of the
accumulation basins around Funen and in the deep channels of the Great
Belt have shown a downward trend. These changes in the oxygen content
of the bottom waters are the result of a combination of increased
eutrophication in the coastal waters and "unfortunate" variations from
year to year in the meteorological and hydrographic conditions (Funen
County Council 1991).
In all deep basins in the Little Belt, the benthic animal communities
have been reduced, and in one case completely eliminated between the
1930s and today. During the last couple of years, the catches of the most
important commercial fish species in the Belt Sea have declined
significantly. The reduced catches reflect a significant decline in the
stocks of plaice and cod, which is possibly the result of higher local
organic loads.
In many nearshore areas around Funen, the natural vegetation is
reduced to a few, pollution-tolerant species, typically filamentous algae
and sea lettuce. Around the islands of the Southern Funen Archipelago
the vegetation in large areas is dominated by floating filamentous algae
while sea lettuce (U[va spp.) completely covers the inner part of Odense
Fjord (Seden Strand). Assuming a doubling time of 3 days, a net primary
production of more than 10000 tons dry weight Ulva spp. can be
calculated for an area of 2 x 2 km 2 in Odense Fjord (Frederiksen 1987).
In the inner areas of Helnaas Bay filamentous algae are common. During
the warm summer of 1989, the growth of filamentous algae together with
anoxia and blooms of sulphur bacteria increased exponentially in the
P.H. Nienhuis
varied a great deal, but showed no clear trend, showing that the
increased level of phosphorus concentration during this period cannot
be accounted for by the changes in the hydrographic conditions.
The composition of plant species found on the sea bottom around
Funen and the areas they occupy have changed significantly since the
turn of the century. The eelgrass-covered areas in coastal waters have
been reduced by 50% owing to the reduced penetration of sunlight. The
depth limit for eelgrass in open waters has been reduced from 9-10 m in
1890 to 5-6 m today and in fjords from 6-7 to 2-3 m. For seaweeds the
depth limit has been reduced from 30 - 35 m to 10 -12 m during the same
period. Filamentous algae and sea lettuce, Ulva species, increase, particularly in enclosed waters with high nutrient loads. In many areas
perennial macrophytes have disappeared completely.
Large areas of the coastal waters of Funen, where the depth exceeds
15 m, are often affected by oxygen depletion in the bottom waters. Every
year some coastal water areas experience oxygen depletion during
August to October; in 1988 and 1989 this occurred from June to October.
From 1976 to 1989, oxygen concentrations in the bottom waters of the
accumulation basins around Funen and in the deep channels of the Great
Belt have shown a downward trend. These changes in the oxygen content
of the bottom waters are the result of a combination of increased
eutrophication in the coastal waters and "unfortunate" variations from
year to year in the meteorological and hydrographic conditions (Funen
County Council 1991).
In all deep basins in the Little Belt, the benthic animal communities
have been reduced, and in one case completely eliminated between the
1930s and today. During the last couple of years, the catches of the most
important commercial fish species in the Belt Sea have declined
significantly. The reduced catches reflect a significant decline in the
stocks of plaice and cod, which is possibly the result of higher local
organic loads.
In many nearshore areas around Funen, the natural vegetation is
reduced to a few, pollution-tolerant species, typically filamentous algae
and sea lettuce. Around the islands of the Southern Funen Archipelago
the vegetation in large areas is dominated by floating filamentous algae
while sea lettuce (U[va spp.) completely covers the inner part of Odense
Fjord (Seden Strand). Assuming a doubling time of 3 days, a net primary
production of more than 10000 tons dry weight Ulva spp. can be
calculated for an area of 2 x 2 km 2 in Odense Fjord (Frederiksen 1987).
In the inner areas of Helnaas Bay filamentous algae are common. During
the warm summer of 1989, the growth of filamentous algae together with
anoxia and blooms of sulphur bacteria increased exponentially in the