Rivers    Lakes    Sea

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 waters.

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 years.

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 fleet.

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 year long.

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 on them.

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 fish alike.

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

© 2000 Steen Ulnits
Skytten 116, Fiskergaarden - DK-8900 Randers - Denmark
Tlf. +45 23 32 89 88 · Website: www.ulnits.dk · E-mail: steen@ulnits.dk