Special Area of Conservation Mühlenteich

Problem: Siltation, sludge accumulation, oxygen depletion, fish death

Project highlights

Eco-friendly and low-impact application in a nature reserve (Special Area of Conservation)
Long-term support of measures through intensive monitoring over a decade
Successful sludge reduction contributed to sustainable preservation of the water – without the treatment, the mill pond would no longer exist in its current form

The starting point 

 An idyll at risk of disappearing 

The lake Mühlenteich at Kloster Oesede in the town of Georgsmarienhütte is located in the middle of a European nature reserve. It is surrounded by wetlands and the diversity of its animal and plant species makes it subject to the Habitats Directive (it is what's known as a Special Area of Conservation, or SAC for short). Increasing periods of drought and the growth of sediment were causing the water level to sink steadily, which eventually led to entire areas silting up. For several years in a row, fish were regularly dying due to the smaller water volume and the consequent lack of oxygen in the water. To preserve the lake as a nature reserve in accordance with the Habitats Directive and also preserve its value as a recreational area, this process needed to be stopped with minimal impact and lasting effect. 

Avoiding further damage

After careful examination, the first idea of dredging the lake was rejected because the use of heavy machinery would have damaged the land too much. It was feared that nature and its biodiversity would no longer be able to self-regenerate. The aim was therefore to find an effective but low-impact solution that would preserve the entire biotope of the Mühlenteich – that's where OASE came in.

The solution

OASE lake therapy: Step by step to success 

The solution was obvious to the OASE team: Lake therapy using SchlixX Plus, which ensures natural degradation of the biomass that would otherwise continue growing indefinitely. After an intensive environmental impact assessment, SchlixX Plus was first applied to selected places in September 2012, initially in order to observe changes in the properties of the sediment and any possible effects on the overall biotope. Further applications followed in 2013, 2014, 2018, 2019 and 2020 with the aim of further reducing the sludge layer and compensating for new siltation that naturally occurs outside the times when SchlixX Plus is applied. 

Acting responsibly means observing closely

At the same time, an extensive monitoring system was set up to document the results in depth over the long term. Since 2013, samples have been taken continuously at ten separate measuring points in order to detect changes in the sludge thickness and examine whether the biocomponents (plankton, macrophytes and macrozoobenthos) have been affected in any way. 

The successes

Preserving the valuable biotope 

The successes after a decade are clear to see: After just the first application of SchlixX Plus, the measured water depth rose to above 1 m again in several places. The monitoring measures in 2013 estimated the sludge layer to be 83 cm, but by 2020 it was only 53 cm. Despite new sediment formation of about 3 cm each year – due in part to the decomposed plant material of the yellow water lilies covering the surface of the pond – the sediment has been reduced by a third overall with just five applications.

Having less sludge means that there is a greater volume of more valuable water available. The sustainable water therapy using SchlixX Plus has therefore not only successfully stopped the siltation process – the increasing depth of the water has also created a more oxygen-rich habitat for the fish. There have been no excess fish deaths since and the fish population has recovered very well thanks to the significantly improved conditions. Furthermore, studies have shown that there have been no negative effects on the biocomponents of plankton, macrophytes and macrozoobenthos. During their on-site visits, most recently in 2020, experts were able to observe a species-rich biotope with higher distribution of amphibians and dragonflies. No wonder SchlixX Plus has now won the German Award for Sustainability. 

Acting responsibly in the long term 

Without initiating OASE's water therapy, the rapid pace of re-siltation would undoubtedly have led to the Mühlenteich's biotope as we know it today being lost. As a naturally occurring process, siltation will remain a challenge in the long term. For this reason, OASE is continuing to assist the project managers as an advisory partner. This will ensure that the success will be sustained for years to come and that the important biotope will be preserved for the future. 

Project fact sheet

Project nameMühlenteich
City/region/areaGeorgsmarienhütte/Lower Saxony
Type of waterFreshwater
Type and sizePond in a conservation area; 10,700 m2
ProblemSiltation, sludge accumulation, oxygen depletion, fish death
Product appliedSchlixX Plus
Period and type of applicationFirst application in September 2012
Other applications:
Result of the application
  • Prevention of further fish deaths by making the water over 1 m deep after just the first application 
  • Sludge reduced by an average of 34 cm from 2013 to 2020 despite an average of 3 cm of re-siltation per year 
  • Preservation of the species-rich biotope 

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