Since the Danish law on Environmental Protection
was launched in 1973, counties and municipalities have worked
hard to improve conditions in Danish rivers, lakes and coastal
In the county of Aarhus everything started
with the mighty River Guden - with some 150 km the longest river
system in all Denmark. As a consequence of the research work
and findings of the 70's, a lot of other rivers and lakes in
the county were included in the effort.
Measures to improve the water quality have
led to better aquatic habitats in most of the rivers in County
Aarhus - way better than in the 70's where it all began.
At that time most major rivers were severely
polluted with organic and chemical waste from cities and industry.
The construction of several and very costly sewage treatment
plants have now led to wastewater low in oxygen consuming content.
As a result of these investments most rivers
improved significantly over the 80's and 90's. Major rivers like
the River Guden and the tributary Lilleåen blossomed and
even smaller cities and societies started building their own
sewage treatment plants
At the same time a different river management
was undertaken. Weed cutting was reduced thus leaving rivers
in much better shape for plant, fish and insect life.
Since then increased focus has been put
on the restoration of rivers around dams and impoundments. Fish
ladders have been built or new river passages constructed to
negotiate the dams, thus allowing river fish to migrate to their
spawning grounds. In some places new spawning grounds have been
made by dumping gravel in suitable places.
Despite all these efforts a number of small
rivers and brooks still suffer from diffuse organic pollution
and heavy-handed weed cutting. But increased focus on these matters
will improve conditions here too in the years to come.
Contrary to rivers that react promptly
to reduced loads of organic waste, lakes are much slower to show
signs of improvement.
The reason for this being that a lot of
phosphorous material has settled on the bottom of the lakes from
where it is slowly leaked back to the water column. In this way
algal blooms may continue for a long time even after all waste
water has been cut off or cleaned. And we are talking tens of
This has been the case in many of the lakes
in County Aarhus. In some lakes the situation has improved significantly
- with water getting clearer and clearer every year resulting
in better conditions for plant, fish and insect life. Still a
number of smaller lakes are not showing the improvement needed.
The county of Aarhus has 130 lakes larger
than 3 hectares - lakes for which maximum measures of phosphoropus
waste has been established. In more than half the lakes these
measures alone will reduce the phosphorous content of the lake
bed over the years.
This goal has been reached already in a
number of lakes but in some lakes improvements will not show
up without further help.
This is where "biomanipulation"
comes in. When organic waste water has been cut off or the water
cleaned sufficiently, this method can be very helpful in restoring
the natural balance of lake fisheries.
Basically, it is the majority of small
fish like roach and bream that prevent the lake from getting
clearer. They eat the zooplankton - small animals barely to be
seen - that would otherwise eat the algae. Now the algae flourish
and cause the water to turn turbid. Light does not penetrate
so higher plants cannot live on the lake bottom. And because
of the turbid water predatory fish like pike and perch cannot
see to catch their prey and thus keep the population of small
roach and bream down. A bad circle has established itself.
To change this situation the surplus of
small fish are being fished out using nets while at the same
time predatory fish - primarily pike - are being stocked. In
many cases the turbid lake will stabilize quickly after such
measures and the water will turn clear again. In some lakes the
transparency of the water has already trebled - with higher plants
back on the lake bottom.
Since the late 80's there has been a significant
improvement in the water quality along the 600 km long coastline
of the county, sewage treatment from the bigger cities being
the prime factor.
On the other hand, nutrient influx from
agriculture - nitrogen and phosphor - has not been reduced markedly
and thus we still live with the risk of sudden algal blooms in
bays and in the open sea.
When wind and weather cooperate, the research
vessel "M/S Tyrfing" goes to sea to monitor the aquatic
environment in the waters surrounding County Aarhus.
"Tyrfing" on the lookout
The county stresses the close monitoring
of the salty environment and built a specialty vessel for this
purpose only in 1988 - a 47 foot long aft trawler named "Tyrfing".
Since then two further research vessels have been added to the
In the Bay of Randers research is now being
done by "M/S Maren", based in Udbyhøj. In nearby
Bay of Mariager the monitoring work is being carried out by "M/S
Maria" of Mariager.
The tasks of the research vessels are many
and very different. In the Bay of Aarhus, the Bay of Randers,
the Bay of Hevring and in the Bay of Mariager water is being
sampled and its quality monitored every one or two weeks - all
Water samples gathered are analyzed with
regard to nitrogen, phosphor and plankton algae. In the latter
case focus is on those algae that are known to excrete toxins,
deadly for fish and potentially dangerous to human beings. Warnings
may be given in case of poisonous algal blooms. Last time this happened was in the autumn of 1997
when poisonous algae showed up in the waters around Djursland
and in the Bay of Aarhus.
Anglers will know that waters often get
turbid in the months of September and October. The reason being
the bloom of various non-toxic algae that may never the less
be harmfull to the environment. Due to their massive oxygen consumption
during night time they may cause fish kills in limited areas.
Fauna of the sea
"Tyrfing" also takes samples
of the ocean bed. The composition of animals found here tells
a lot about the oxygen conditions down deep. If oxygen is plentiful,
life is good for bottom living animals and the fish that feed
In the Bay of Aarhus living conditions
for the fauna have improved significantly. Especially the building
of the large and highly technological sewage treatment plant
at Marselisborg has accounted for improved water quality.
In April and early May you may sometimes
- depending on the prevailing wind - witness large numbers of
tube worms drifting ashore. What may initially look like some
disaster caused by pollution is an absolutely natural phenomenon.
It is caused by the swarming and mating of theses worms that
die after mating and thus get carried ashore by the currents.
In this way these worms become a major food item for birds and
Flora of the sea
Seagrass and kelp forests are valuable
feeding grounds for various fish species. Along stony beaches
and rocky piers the clean waters often abound in beds of brown,
green and red algae.
If the shoreline waters are polluted with
organic waste, these algae get covered by quickly growing threadlike
annual algae. These algae absorb all the light and thus choke
their host, be it sea grass or kelp. They often wash ashore during
summer and autumn.
Finally, every second year professional
divers monitor the sea grass beds and the kelp forests of the
county in the Bay of Aarhus and the Bay of Randers. Every fourth
year the remaining waters are checked.
Department of Nature and Environment,
County of Aarhus