Area characterisation:
The Bordesholmer See is a shallow lake (0.71 km², max. depth 8 m) located in a moraine landscape in northern Germany. It is fed by the Kalbach stream, draining a 13.88 km² catchment dominated by intensive agriculture. The lake is currently in a weakly polytrophic state due to excessive phosphorus inputs (~1,100 kg/year), primarily from diffuse agricultural runoff. This leads to toxic algal blooms, oxygen depletion below 4 m depth, and degraded aquatic vegetation. As a central recreational site, the lake is vital for local residents, particularly youth, making its ecological condition closely linked to public health and social well-being.
Objective:
The project aims to restore the ecological status of Bordesholmer See by reducing phosphorus concentrations and mitigating eutrophication. The primary objective is to achieve a target concentration of 0.06 mg/l phosphorus, improving water quality, oxygen conditions, and biodiversity. At the same time, the project seeks to restore the lake’s function as a safe, accessible recreational space, particularly for children and youth. The approach focuses on intercepting and transforming diffuse agricultural nutrient inputs through nature-based solutions, while complementing these measures with targeted in-lake management to address legacy phosphorus and ensure long-term ecosystem stability.
Financing:
The project combines public funding, policy instruments, and cost-effective maintenance strategies. Initial implementation is supported through regional and EU funding mechanisms aligned with the EU Water Framework Directive. Agricultural measures are incentivized through payments under the Common Agricultural Policy, compensating farmers for environmentally beneficial practices. Operational costs are relatively low (approx. €900/year), primarily covering maintenance such as reed harvesting and monitoring.
Potential impacts/benefits:
The project delivers multiple environmental, social, and economic benefits. Ecologically, it reduces phosphorus inputs by 50–100 kg annually, improves water clarity, stabilizes oxygen levels, and enhances habitats for fish and waterfowl. Socially, it restores safe swimming conditions, supporting public health, environmental education, and equitable access to free recreational space, particularly for youth. Economically, it protects local tourism and reduces reliance on costly technical interventions. The intervention contributes to long-term ecosystem resilience and aligns with EU environmental targets. Overall, it demonstrates how nature-based solutions can provide integrated benefits across ecological restoration, climate adaptation, and social well-being.
Actions:
The core actions include the creation of a 6,500 m² constructed reed bed at a major inflow and the restoration of the lake’s shoreline reed belt using Phragmites australis. These measures intercept and biologically sequester nutrients before they enter the lake and enhance in-lake self-purification. Maintenance includes selective harvesting of reed biomass to ensure permanent phosphorus removal. To address internal loading, targeted low-energy aeration stabilizes oxygen conditions and limits sediment phosphorus release. The project is supported by a Living Lab approach, engaging stakeholders, particularly farmers and local communities in co-creation, monitoring, and adaptive management.
Contacts:
Paula Thomsen
This is a conceptual academic project and is not currently planned for real-world implementation. Comments and suggestions are very welcome.