Marine områder - Status over miljøtilstanden i 1998 - Faglig rapport ...

Summary
The environmental status of the marine environment around Denmark
is determined by interactions between climate and anthropogenic
influence, in particular the load of nutrients from the land and
the atmosphere. The two previous years to 1998 were characterised
by low rain fall and long periods with warm, quiet weather during
summer. This low rain resulted in a reduction in the load of nutrients
in 1996 and 1997 near those of the goals outlined in the Action Plan
on the Aquatic Environment. The national monitoring of marine
waters (Ærtebjerg et al. 1998) and reports from the counties released
in 1998 have documented an improvement of the environment for a
number of criteria up until 1997. However, important problems still
occur, including oxygen depletion in shallow waters and estuaries
especially during warm, quiet summers. The most spectacular example
of oxygen depletion occurred in Mariager Fjord in 1997.
The climate in 1998 was characterised by a mild, wet winter and a
cold, windy summer. After two extremely dry years, the freshwater
input in 1998 was 18% over the average for the period 1989-98 and
near to the level observed in 1995. The N load from Denmark to the
marine environment was 10% over average for the period 1989-98,
but the P load was still low, 35% under the average for the same time
period due to construction of advanced sewage treatment plants.
There was a decline in atmospheric nitrogen deposition for the period
1989-98, but nitrogen deposition was still relatively high due to
the higher than average rainfall.
The surface water temperature was a few degrees lower than normal,
and together with the enhanced wind, allowed for fewer instances of
thermoclines in shallow waters. Wind mixing also allowed for benthic
filtrators to have access to water column plankton algae. Secchi
depths were generally good during the summer period. The salinity
in the surface was during the spring months markedly higher than
normal in the inner Danish waters. A subsequent outflow of water
from the Baltic established a strong halocline in April in the SE Little
Belt, where the bottom water was first exchanged in October.
There continues to be declines in phosphorus concentrations and a
weak tendency of a decline in nitrogen concentrations. This has resulted
in P playing a more important role as a potential limiting factor
for primary production, but has resulted in few ecological
changes. The most important changes are smaller and fewer blooms
of Ulva in certain estuaries. A possible reason for the lack of response
is that there still is a large pool of P in the sediments.
Oxygen depletion in shallow estuaries and the coastal areas in 1998
was limited in distribution, in length and in intensity due to higher
than normal winds and lower than normal water temperatures. In
the deep stratified areas oxygen depletion was more developed than
during the two previous years. Only in the southern Belt Sea and in
the Arkona Basin in the Baltic Sea developed strong oxygen depletion.
The Little Belt and the deep adjacent estuaries had an unusually
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