If underwater plants become a plague:
A Practical Invention: Immersible Opaque Foils (IOF) - sustainable and without detriment to the environment
Natural or excavated lakes often show an unwanted spread of tall aquatic plants and filamentous algae, that hinder and prevent swimming or boating.
With the help of Immersible Opaque Foils (IOF) the biologist Alexandra Hoesch - La Rosée combats unwanted aquatic plants and gives rise to an active restoration of the lake vegetation.
The effect of the IOF is to eliminate underwater vegetation with their roots permanently as well as sustainably. An essential idea behind the concept of the IOF`s is, that the plants are not mechanically or chemically destroyed, but eliminated through the lack of light. Since the IOF is impenetrable for light, the plants under the IOF decay because photosynthesis cannot take place.
Technology and Function of the Immersible Opaque Foil (IOF)
The IOF is provided with a hose pipe, that can be filled with either air or water. Is the IOF being filled with air, it unfolds and floats on the water surface. If water is being pumped into the hose pipe, the IOF becomes heavier than water and sinks, covering the proliferating vegetation under it. The IOF has a size about 25 m2. This size is the most convenient regarding the handling of the sinking and lifting of the foils and maximum cover of the ground.
After about 4-6 months the vegetation under the IOF has disappeared through deprivation of light. The IOF can be lifted to the surface by pumping air into the hose pipe. Where it was lying, the ground is without vegetation. The IOF is reusable and can be placed onto different proliferating vegetation areas for several times.
Planting of a Low Growing, Perennial Vegetation
This vegetation free area can either be left to itself or be planted with a vegetation, that is of low growth and perennial, like the Charales. This is a special genus of green algae, which has a limited growth between 10 cm up to 100 cm (sometimes 150 cm). The Charales never grow up to the water surface and like a lawn they cover up the ground and do not decay in winter. The planting of the Charales is very easy, since they are heavier than water, being thrown into the water, they sink to the ground and spread rapidly. But if the planting of Charales is possible, depends on the water quality of the lake. The trophical tolerance of the different Charales species range from a nutrient poor, low trophic level, i.e. oligotroph to a medium trophic level, i.e. mesotroph to eutroph. In nutrient rich waters some species of the Charales (like Chara globularis, Chara contraria, Nitella flexilis) do grow, but their proliferation is reduced and they can be displaced by unwanted high growing vegetation (link to photo gallery).
Schema for the application
The schema shows a small lake, overgrown with proliferating aquatic plants. In the center of the little lake IOFs are being placed over the vegetation. The area above the IOFs can be used for swimming and boating as soon as the IOFs are placed.
After a few months the vegetation under the IOFs has decayed. The IOFs are transported to the water surface by pumping air into the hose pipes and are placed onto the adjacent vegetation by pumping water into the pipes. The sediment, which was under the IOFs can now be planted with low growing vegetation.
This procedure can be continued until the entire ground of the lake is free of the disturbing aquatic plants and the desired vegetation has become established.
Conventional Methods
1. Physical removal - Mowing and Racking
Physical removal of the plants, like mowing or racking if done manually or with machinery is time consuming and expensive.
Mowing brings only short-term success, because after mowing the plants grow up to the surface again. Mowing can even enhance the growth of the plants. Some of the most annoying groups like Elodea and Myriophyllum can also be spread by mowing, since the regrowth of stem fragments, that result from cutting operations, is rapid. In any case, the cut material must be removed from the water to avoid deoxygenation. The cut weed has to be disposed from the lake`s bank, to avoid seepage back into the water.
When the underwater plants are racked, they are pulled out with their roots. This mobilizes a lot of sediment and brings back nutrient into the water. So racking can lead to a eutrophication of the water and lead to plankton blossoms. When too many of the underwater plants are racked, the lake can become turbid due to Plankton blossoms.
2. Grass Carp Stocking
Grass carps (Centopharyngodon) can´t reproduce in Central Europe, because they need flowing water with temperatures above 20°C for reproduction. Nevertheless they are rampant in Europe, because in the sixties grass carps were stocked very much. Their maximum age is unknown, but they can grow very old and can reach lengths up to 1,6 m. The stocks from then are still living in European waters. In the sixties Grass carps were stocked, because they were known as `plant eaters´, that feed on unwanted underwater vegetation. Nowadays in many German provinces it is forbidden to stock grass carps or a official permission is necessary.
In densely stocked lakes they can eat up all the plants and the water becomes turbid and blue algae blossoms appear.(link zu Olchinger Badesee, Grass carps) Also the fish species can change due to grass carps, because many fish need underwater plants as spawning grounds.
It is extremely difficult to remove grass carps out of a lake, because they don´t go for baits.
3. Sediment removal: Dredging und Suction
The total dredging of a lake is very expensive and elaborate. In shallow lakes with drainage a dredger will be used. Before the lake is drained, the fish fauna has to be taken out. Also big mussles like Unionidae, that live in the sediment, have to be gathered. Because with dredging the complete underwater vegetation is eliminated, also precious communities consisting for example of Charales are destroyed. Also the complete macrozoobenthos, like insect larvae and worms, is killed. The dredged sediment has to be deposited. This measure is only advisable, when large amounts of sediment have accumulated, that lead to deoxygenation of the water and fish kill.
A suction of sediment is less disturbing to the ecosystem of a lake. Usually only in chosen parts of a lake a suction is applied, for example where the sediment disturbs the boating.
In both cases after the removal of the sediment it is indicated to apply an initial planting of low growing underwater plants like Charales, to avoid high growing spices to proliferate.
Under Projects you will find several documentations about lake restoration and river regulation