By Hans-Heinrich Schuster
With an area of 15 km², Lake Dümmer is the second largest lake in Lower Saxony and, with an average depth of only 1.1 m, an extremely shallow lake. High nutrient inputs from the intensively farmed Hunte catchment area led to a drastic decline in reed beds as early as the 1950s and to the loss of the formerly extensive underwater vegetation and rush
islands in the lake. For more than fifty years, Lake Dümmer has been dominated by planktonic algae, which cloud the lake water heavily all year round. Additionally, for the last ten years, intensive blue-green algae blooms have characterised the summer look of Lake Dümmer, regularly leading to oxygen depletion in the shore areas of the lake and in the lake’s outflows, resulting in strong odour nuisance and even local fish kills. In response to these events, the Lower Saxony Ministry of the Environment therefore commissioned the NLWKN (Lower Saxony State Agency for Water Management, Coastal Protection and Nature Conservation) to draw up an implementation concept for the continuation of the Dümmer remediation.
An indispensable element in reducing phosphorus input into Lake Dümmer is a constructed wetland, covering an area of at least 1 km² in the large reed polders, to purify the nutrient-rich water of the Hunte River before it flows into Lake Dümmer. Further measures – ranging from water protection adisory services to watercourse development and re-naturalisation initiatives in the upper Hunte catchment area –aim to reduce nutrient pollution in lake Dummer in the long term. Systematic success monitoring across the lake’s catchment area is a core element of the current Dümmer restoration concept. It supports the identification of nutrient input pathways and documents the effectiveness of measures to reduce phosphorus loads entering surface waters. To track ecological change, Lake Dümmer is now subject to continuous monitoring of chemical and physical parameters as well as the biological quality elements relevant within the EU Water Framework Directive.
In parallel with restoration efforts across the catchment, in-lake measures are being implemented to improve shoreline structure. These include protecting and promoting the development of reed beds along the lake’s shoreline as well as targeted fishing measures to help establish a fish community typical of the lake.
Near-natural shoreline structures are essential within a complex, interconnected lake ecosystem. Improving habitat conditions along the shore directly enhances biodiversity in the transition from aquatic to semi-terrestrial environments.
These structural improvements particularly benefit key biological quality characteristics such as macrozoobenthos (aquatic invertebrates) and fish. Together with other biological components, including phytoplankton and macrophytes/diatoms, they create the conditions necessary to achieve good ecological status in Lake Dümmer, in accordance with the European Water Framework Directive.
The NLWKN Lake Competence Centre therefore expressly welcomes the Euro-Lakes reed bed protection project at Lake Dümmer.
With the financial support provided by the EU-funded EUROLakes project, the decline of reeds at Dümmer Lake will be stopped and new reed beds will be developed.
Hans-Heinrich Schuster has grauated Biology at the University of Hanover and University of Veterinary Medicine, Hanove, with a specialization in Hydrobiology. He is a Limnologist at the Lake Competence Centre, Lower Saxony State Agency for Water Management, Coastal Protection and Nature Conservation (NLWKN). The Lake Competence Center is responsible for all lakes in Lower Saxony that are larger than 50 hectares (lakes covered by the EU Water Framework Directive). Its main task is to monitor the water quality of the lakes.