Elodea & Co.: Ruhrverband gears up for this year’s aquatic plant season
The Ruhrverband is keeping a close eye on the growth of aquatic plants in its reservoirs on the lower Ruhr. Dives were carried out in the lakes back in the spring to assess the underwater vegetation and make growth forecasts. A further dive is planned for July.
Whilst in recent years the so-called ‘narrow-leaved waterweed’ (Elodea nuttallii) has posed a problem for water sports enthusiasts, at present the comb-leaved pondweed (Potamogeton pectinatus) and curly pondweed (Potamogeton crispus) “seem to be taking over”.
Combed pondweed and filamentous algae dominate Lake
Kemnade In the coming weeks, it may become necessary to deploy the mowing boat “Manati” on Lake Kemnade to cut back the Elodea and pondweed. At present, comb water-milfoil and filamentous algae are the dominant species. Increased natural outflows could, in principle, limit the growth of crested water milfoil and filamentous algae. However, as these outflow conditions depend on the weather, the Ruhrverband’s scope for intervention beyond mowing is limited in this instance.
Current situation at Lake
Baldeney The situation is similar at Lake Baldeney in Essen: there are currently hardly any Elodea populations here; instead, curly pondweed is causing frustration among water sports enthusiasts. Unlike Elodea, curly pondweed has delicate leaves and a slender, rather loose structure. Due to its denser and more robust structure, Elodea is somewhat easier to mow than pondweed. At Lake Baldeney, the mowing boats ‘Kaiman’, ‘Nimmersatt’ and a leased boat from the company BERKY are currently on standby. The Ruhrverband coordinates the technical provision of the boats and equipment. The boats are manned at Lake Baldeney by the City of Essen and the Weiße Flotte respectively. The specific areas to be mowed are determined in close consultation with the Lake Baldeney Interest Group. All parties involved are doing everything they can to make the situation as bearable as possible for water sports enthusiasts.
Elodea at Lake Harkort and Lake
Hengstey By contrast, there is no sign of waterweed at Lake Harkort and Lake Hengstey. The results of the diving surveys carried out in the spring at Lake Harkort have revealed a potential mass growth of Elodea. There, the plant currently stands at around 40 centimetres in height. The outlook for Lake Hengstey is somewhat more optimistic. The current height there is 30 centimetres.
On both lakes, a motorboat equipped with a so-called T-beam is being used. Here, unlike on the other lakes, the cut vegetation is not loaded onto the boat via a conveyor belt, but is carried by the current to the run-of-river power station, where it is ‘dredged’ out of the water. Mowing is coordinated and carried out by the local clubs at both lakes.
Concrete forecasts are hardly possible
The growth of pondweed and elodea is primarily favoured by the very good water quality of the Ruhr reservoirs. Modern sewage treatment plants and the colonisation by the Asian basket clam ensure particularly clear water. This allows sunlight to penetrate right down to the lake bed – ideal conditions for light-loving aquatic plants. Under favourable conditions, Elodea therefore grows at a rapid rate and can colonise large areas of a lake within a short time. The same applies to pondweed, which can also multiply rapidly under favourable conditions. It is currently impossible to make a reliable assessment of which species of aquatic plant will dominate the lakes in future. As far as water sports are concerned, it seems as though the ‘Elodea devil’ has been driven out by the ‘water milfoil Beelzebub’. And so the Ruhrverband and the City of Essen have no choice but to mow.