Möhne Dam: The deep water will need to be aerated again this year

As oxygen levels in the Möhne Dam have been on a downward trend since the beginning of August, the Ruhrverband is expected to begin precautionary aeration of the deep water from next week onwards to prevent fish deaths. The necessary perforated hoses are currently being laid.

Consequently, the ban on fishing in the area of the oxygenation plant also comes into force with immediate effect. The imaginary line between the two deep bays (on the north bank, the first bay as seen from the dam, near the Möhnesee Yacht Club; on the southern shore, the second cove as seen from the dam) may only be crossed in the direction of the dam without engaging in any fishing activity. Mooring boats at the orange-yellow buoys, which mark the end and intermediate sections of the oxygenation plant, is strictly prohibited.

These are purely precautionary measures which the Ruhrverband has also implemented at various reservoirs in recent years. Based on several years’ experience, it is anticipated that aeration will be required for around four to six weeks. The fishing ban will remain in force for this duration. The drop in oxygen levels is a natural phenomenon that has nothing to do with water quality – which remains consistently high at all of the Ruhrverband’s reservoirs, even during this exceptionally hot and dry summer.

Background: Between spring and autumn, the water in the reservoir forms distinct temperature layers. The coldest and densest water remains at the bottom of the lake, whilst the water at the surface can warm up significantly, particularly during warm and sunny summers. Between the surface water and the deeper water lies the so-called thermocline. This stable temperature stratification is only broken down again by autumnal storms, cooler air temperatures and increased inflows into the reservoir.

Oxygen-consuming decomposition processes involving dead phytoplankton and zooplankton can cause a virtually oxygen-free ‘barrier’ to form within the thermocline, effectively trapping the cold-loving fish species in the cold deep water. If the oxygen content drops too low even in the deepest water layer, there is a risk of fish mortality. The situation becomes critical when oxygen levels fall below three milligrams per litre. The Ruhrverband’s reservoir operations, joint research laboratory and fisheries management teams therefore monitor these levels very closely from late summer onwards and, in recent years, have been able to effectively prevent fish from being harmed by low oxygen levels through targeted aeration at various reservoirs.