Since 2000, the Ruhr's impounded lakes at Lake Hengstey, Lake Harkort and Lake Kemnad have been subject to massive occurrence of waterweed (elodea nuttallii), at times covering more than half of the lakes' surface in summertime. In 2008, the waterweed species also appeared on large parts of Lake Baldeney for the first time.
Elodea nuttallii is a so-called neophyte, i.e. a non-native plant. It originated in America and first appeared in European waters in the 1930s.
In the River Ruhr, the plant was only able to flourish thanks to the considerable improvement of water quality, primarily as a result of the expansion of wastewater treatment installations in the Ruhr river basin, which reduced the phospor load to a tenth of the readings from the 1980s. As a result, phytoplankton density decreased, and light was now able to reach the bed, enabling the growth of hydrophytes.
The ecological improvement of the Ruhr's impounded lakes has a positive impact not only on the growth of various hydrophyte species, but also on the development of fish populations, as well as the biodiversity and population of invertrebrates, such as crustaceans, bivalves and insect larvae.
However, these positive ecological developments create conflicts with regard to use, limiting watersports activities in the weedy lakes during the vegetation period, and creating potential obstacles to hydropower generation. Therefore, the Ruhrverband takes measures to try to reduce the elodea population within means and responsibilities.
While continued unobstructed use for watersports is not part of the Ruhrverband's tasks, it does try to act in the interest of watersports enthusiasts by enabling recreational use within means.
In the process, on the one hand, the Ruhrverband has taken long-term, sustainable measures to control biological elodea population. For this purpose, a native fish species called rudd has been planted in the waters since 2002. It feeds on elodea, even at low water temperatures. As soon as a sufficiently large fish population has been established in the Ruhr's impounded lakes as a result of natural progeny, it should be able to prevent massive elodea occurence by feeding on the plant's buds already in the springtime.
On the other hand, the Ruhrverband tries to at least partially enable local use for water sports by mowing and other mechanical measures. The aim - supported by the water sports enthusiasts themselves - is to keep the passages between berths and unobstructed parts of the impounded lakes weed-free, or to enable regattas by freeing planned courses from the weed.
Due to legal obligations, the Ruhrverband is unable to keep the Ruhr's impounded lakes entirely weed-free for recreational use. Therefore, it takes the combined effort of all bordering towns and private users to harmonise the requirements of the various interest groups while respecting ecological requirements.