Restoration of shallow lakes: possibilities and methods
Still water can be turbid or clear, with all possible gradations. If the water is very turbid, there will be no aquatic plants, because the sunlight does not reach the bottom of the lake. The water is then usually highly charged with nutrients. Planktic algae are responsible for the green color and turbidity of the water.
The Mieminger Swimming Lake (Tyrol)
The Mieminger lake is in a restoration program since 2011, because public use was inhibited by the proliferating underwater plants growing to the surface of the water, which hindered bathing and boating. This lake has about 2 hectare and a maximum depth of 5,5 m.
The results of the underwater vegetation mapping shows, that the lake is changing from an nutrient poorer (mesotrophic) to an nutrient richer level (eutrophic). The lake has still large areas with Charales (mesotroph vegetation), but already high growing plants like pondweeds are proliferating. Especially around the entry of the ditch, that is fed by the surrounding pastures. Here the proliferation of the pondweeds is very strong. The main species of the lake Chara intermedia, has disappeared in this area. With the eutrophication process the growth of the Phytoplanktic algae is enhanced and the water become less clear. That is why in the deeper parts of the lake, the pondweeds are more frequent, because in spring they grow very fast to the surface. The Charales have an limited growth, so they have an disadvantage in the deeper zones.
To prevent dredging, that is very expensive and that destroys also the large fields of charales, in the year 2012 250 sqm of pondweed vegetation were covered with IOFs.
In 2013 the IOFs were lifted and the vegetation free areas were planted with Chara contraria. The IOFs were placed onto other high growing pondweed areas.
The construction of a sedimentation basin for the ditch is planned to reduce the nutrient input.
Restoration of the Steinhoeringer Swimming Lake
The video shows the situation of the Steinhoeringer Lake in the district Ebersberg (Bavaria) before the restoration. In the year 1997 the lake was excavated and already in 1999 public usage was limited by filamentous green algae, which covered the water surface especially when the sun was shining. In addition phytoplankton blossoms appeared due to over fertilization of the lake`s catchment area. These blossoms covered the underwater vegetation with a white film.
After dredging the lake in 2000, the lake was planted with Charales which spread over the entire bottom of the lake within one year. They cover the ground similarly to lawn, and do not grow to the surface of the lake. The Charales replaced the filamentous green algae and so public use of the lake was reestablished. In 2003 residents stocked the lake with carps and within several months the Charales had disappeared. The carps muddy the water and so the light couldn´t reach down to the Charales.
The Underwater Field Mapping of the Olchinger Swimming lake and the expertise
The Olchinger lake is an excavated lake, which is about 14 hectares and has a maximum depth of 6 m. It is polymictic, i.e. the water body has no stable stratification in the summer, and therefore the water is completely intermixed several times a year.
About 30 years ago the lake, was stocked with grass carp (Ctenopharyngodon idella).
With increased occurrence of blue algae during the summer months, the swimming quality deteriorated significantly. The town of Olching authorised Alexandra Hoesch-La Rosée to examine the lake and write an expertise on it.
As the expertise and Data of the trophic level showed, that the main reason for the large quantities of blue algae are the nutrient charges that are transported into the lake by a ditch. This ditch is responsible for poor water quality and mass extinction of Charales in the Olchinger Lake.
Another reason for the blue algae blossoms are the grass carps (link grass carps under conventional methods).
They eat about once to twice their own body weight. They do not digest the food very effective, so a lot of nutrients are deposited in the water and will cause blue algae blossoms.
The ditch flows into the lake in the south east. There on the map the trophic level shows an orange area, that signals poor water quality. (Link zu Trophiekarte)
The following is an abstract from the expertise:
1. Restoration of the Ditch
As the study has shown, the Zitzstauden ditch is the main source of nutrient input. This input has existed for many years, because the underwater vegetation, that changes only slowly, shows many indicator plants for high nutrient input in the vicinity of the ditch entry. However, the development of the algae and the decline in the Charales happen quickly. This is why it is necessary to restore the Zitzstauden ditch as quickly as possible. A study of the catchment area should be made in order to locate the detrimental nutrients. In a second step these nutrient inputs should be restricted, for example through the construction of sedimentation basins. Wider buffer zones can also, in some cases, hold back the nutrients from agricultural land.
2. Removal of the grass carps
The grass carps had been stocked several years ago. They feed on the Charales and leave behind stubble fields. On these stubble field the blue algae can grow easiliy. Therefore the grass craps have to be removed. (s. IOF/Grasskarpfenbesatz).
zu den Video und Trophiedaten >>>
Lake-Mapping with supporting video, example Eibsee lake
The Eibsee lake below the Zugspitze mountain. An example of a video mapping of transects of the shown lake. Click on the dots to view the videos (QuickTime required).
Photo Gallery
The photo gallery shows aquatic plants (macrophytes) taken in several Bavarian lakes by Alexandra Hoesch-La Rosée. These species are mainly indicators, which are used by specialists for plant identification and for judging the water quality of lakes.
The gallery is not complete yet and will be updated.
Photo Gallery >>>