Biological lake restoration with microbes
In less than a year, two heavily loaded fishing lakes near Hvide Sande were markedly improved through biological restoration with microorganisms, a bioreactor and aeration.
The same lake before and after treatment
The project was carried out at Klittens Put & Take from July 2025 to spring 2026. Here are the measurable changes in the lake system.
From a few centimetres to up to 85 cm in spring 2026.
A stabilised oxygen profile throughout the water column, even under ice.
A sharp reduction in algae growth and visible green discolouration.
Thanks to the improved water quality, fish mortality has ended.
Klittens Put & Take consists of two connected lakes with a total area of about 1.2 hectares. The lakes were established on former mink farm land, which is still considered to contribute nutrients from the surrounding soil. In addition, fjord water with elevated phosphorus levels was added periodically, meaning the lake system was under constant external nutrient load. For several years the lakes had been marked by severe biological imbalance.
The combination of high nutrient levels, organic bottom sludge and unstable oxygen conditions created optimal conditions for algae growth and degraded water quality. Especially during warm periods the lakes developed massive green algae growth, which reduced visibility depth to a few centimetres. The unstable conditions affected both the fish stock and the general experience of the facility. During certain periods fish mortality was observed as a result of oxygen deficiency and biological load. Previous measures had only produced short-lived improvements and had not created a stable biological cycle in the lakes.
In July 2025 an integrated restoration strategy was implemented, consisting of specialised bacterial cultures, a bioreactor and aeration and circulation. The aim was not only to improve the appearance of the water, but to re-establish the biological processes that naturally stabilise an aquatic environment. The technique is based on bioaugmentation biological reinforcement – where targeted microorganisms support the breakdown of organic material and the turnover of nutrients. The added bacteria helped reduce the accumulation of organic sludge and lower the internal nutrient load in the lakes
The bioreactor served as a stable environment for the bacteria and ensured high biological activity even under changing temperature conditions. At the same time, aeration and circulation were used to improve oxygen levels throughout the water column and prevent bottom turnover and oxygen deficiency. The combination of microorganisms, bioreactor and oxygenation gradually created a more robust biological cycle, in which both water quality and biological conditions stabilised.
At the start of the project in summer 2025, visibility depth was measured at only 0–10 centimetres. Drone images from the period clearly showed strongly green and murky water with a high biological load. After just a few weeks of treatment, the lakes gradually began to change character. The water became less green, and more stable oxygen conditions and improved water chemistry were observed. As the biological processes stabilised, visibility depth rose markedly.
During autumn 2025 the improvements became clear. Bottom oxygen rose from around 0.6 mg/L to between 9 and 11 mg/L, while the algae index fell markedly. The water appeared clearer, and ammonium and nitrite levels became more stable. Visibility depth developed from a few centimetres to around 70–80 centimetres during the autumn, and in spring 2026 visibility depths of up to 85 centimetres were measured. At the same time the owner observed fewer dead fish, less accumulation of loose sludge and more stable conditions in the lakes.
The winter of 2025–2026 became an important test of the lakes' biological stability. Periods of prolonged ice cover normally increase the risk of oxygen deficiency, accumulation of ammonium and biological collapse in heavily loaded lake systems. Despite the winter conditions, high oxygen levels and relatively high visibility depth continued to be recorded throughout the winter period. In March and April 2026 oxygen was measured at 13 mg/litre both at the bottom and at the surface. Thanks to the improved water quality, fish mortality has ended
The results indicated that the lake system had achieved a more stable biological balance rather than merely a temporary cosmetic improvement. At the same time it showed that the biological restoration strategy was robust against seasonal loads and temperature fluctuations.
| Parameter | July 2025 before treatment | September–October 2025 | Spring 2026 | Development |
|---|---|---|---|---|
| Visibility depth | 0–10 cm | 45–80 cm | Up to 85 cm | Markedly improved |
| Bottom oxygen | Approx. 0.6 mg/L | 9–11 mg/L | Approx. 13 mg/L | Stabilised oxygen profile |
| Algae index | 6–8 | 1–2 | 1–3 | Sharply reduced |
| Water colour | Strongly green and murky | Darker and clearer | Clearer and more stable | Visible improvement |
| Fish mortality | Periodically high | Reduced | Substantially reduced | More stable conditions |
| Sludge conditions | Loose organic sludge | Reduced accumulation | Firmer bottom observed | Signs of biological turnover |
| Ammonium | Elevated and unstable | More stable | Falling levels | Stabilised nitrogen turnover |
| Winter stability | Risk of oxygen deficiency | – | Stable under ice cover | Robust biological balance |
As part of the assessment, the development in Klittens Put & Take was compared with Klegod fishing lake north of Hvide Sande. At the start of the project the two lake systems had comparable challenges with high algae load, low visibility depth and unstable biological conditions. While Klittens Put & Take was treated with biological restoration, the reference lake was not treated in the same way. During autumn 2025 and spring 2026, field measurements and drone images showed a clear difference between the two lakes.
The treated lake developed markedly clearer water, lower visible algae growth and more stable biological conditions, while the reference lake continued to appear greenish and murky with very low visibility depth. The comparison supported that the improvements in Klittens Put & Take could not be explained by seasonal variation or climatic conditions alone, but were closely linked to the restoration measures carried out.
The project is particularly interesting because the results were achieved under continued load from historical nutrients, organic sediment and periodic input of phosphorus-rich fjord water. At the same time, the improvements were achieved without excavation, draining the lakes or extensive use of chemicals.
The case demonstrates the potential of biological restoration as a nature-based method for gradual stabilisation of loaded aquatic environments. The technique can potentially be used in recreational lakes, put & take facilities, stormwater basins and other eutrophic lake systems where gentle and sustainable solutions are desired. By supporting natural biological processes, the technique can contribute to improved water quality, more stable ecosystems and a reduced need for invasive restoration methods.
We help with nature-based solutions for lakes and aquatic environments, focusing on biological balance, improved water quality and long-term stabilisation.
Contact us for advice, project dialogue or an assessment of your lake environment.
Back to Natural treatment