Scientists work to convert muddy fish pond into clear lake

In the high summer months, many water bodies are plagued by surface water bloom, a layer of organisms called cyanobacteria or blue-green algae, which thrive in water rich on phosphorus. Not only is water bloom unsightly but it can be potentially dangerous to people as the bacteria produce various toxins. Hydrobiologists from the city of Plzen are now trying to cut the bacteria off from their phosphorus supply and turn the largest fish breeding pond in Plzen into a crystal clear lake rife with predator fish and rich vegetation.

The pond 'Bolevak' in Plzen, photo: Google Maps

For this edition of Czech Science Radio Prague spoke to the man behind the project, hydrobiologist Jindrich Duras and asked him how a muddy fish pond can be turned into a clean lake.

"The project concerns biomanipulation. It is a special treatment of an aquatic ecosystem. We employ natural relationships between the main components of a lake or a reservoir. It means we try to modify food webs, mainly between plankton and fish or in the fish community itself. Also, we influence the availability of phosphorus, the key nutrient for planktonic algae. That means we try to support submersed aquatic vegetation that is capable to outcompete blue-green algae."

What are you actually doing in this specific case in the city of Plzen?

"Bolevak is a relatively large fish pond, about 43 hectares and its mean depth is about 2 metres.

The grass carp

The whole biomanipulation project represents several steps that must be realised during three years. The first step was sediment treatment. The aim of this step was to lock phosphorus in the sediment. Parallel to that, fish stock reduction is going on. Fish need to be reduced by 70-80 percent. And the third basic step is enhancement of aquatic vegetation."

What particular kinds of fish live in Bolevak?

"We need to eliminate silver carp and grass carp because especially grass carp eliminated all the submersed vegetation. Then we need to substantially reduce planktivorous and benthivorous fish, for instance, breams, roach and perch and so on. (Benthivorous fish eat benthic animals, that means for instance the larvae of midges and organisms living on the reservoir or fish pond bottom.)"

How does this whole project of the change of the ecosystem concern people? As far as I know Bolevak is a popular area where people go swimming and bathing and so on...

Cyanobacteria

"Of course, the reason why we started this biomanipulation was to improve the water quality in Bolevak pond. The water quality in Bolevak was regularly deteriorated by cyanobacteria water bloom. This water bloom is considered to be potentially toxic and it has caused remarkable nuisance in the locality."

So now you are trying to improve the quality of the water but you are using chemical substances for that purpose. Are those harmless to people?

This chemical is used to lock phosphorus in the sediment. When phosphorus is locked on the bottom it is not available for the growth of planktonic algae, especially blue-green algae. It is safe for people and for organisms living in the fish pond, too."

As far as I know, phosphorus gets into ponds either from sewage or from fields. Even if you use this chemical to lock the phosphorus on the bottom of the pond, I guess more phosphorus will be coming from the fields and other sources. Will you need to repeat this procedure regularly?

"Bolevak is a very nice exception among our water bodies because the input of phosphorus via inflow is very low. So we have to manipulate only the phosphorus cycling in the ecosystem itself. That's why we need and we try to modify the fish stock. That's why need to enhance aquatic vegetation and why we try to treat the sediment to lock phosphorus in it."

How long do you think it will take before Bolevak turns into a clean and clear lake?

Blue-green algae in water, photo: Barbora Kmentová

"Now we are in the half of our first year of the project and I think the project will last in an intensive way for another two years."

And then I believe you will still need to maintain the balance, replenish the fish stock - simply look after the pond turned into a lake...

"I would like to say that shallow reservoirs or lakes are able to exist in two stable states. One stable state is dominated by fish and planktonic food webs which means common appearance of water blooms and so on. The other stable state is called 'littoral state'. That means that in this ecosystem a very important role is played by aquatic vegetation and the role of fish and planktonic food webs is weakened. That means the aim of the project is to reach a stable state of Bolevak. We don't need to create an artificial aquarium or pond but a natural ecosystem that was widespread in Europe in the past."