A hot summer’s wake up call to the City of Bruges: A blue-green algae crisis and what comes next.

Hot summers + nutrient-rich waters = toxic algae bloom

Toxic algae bloom is becoming a widespread problem in the summer in many cities with open, slow-moving bodies of water, due to climate change. The culprits are cyanobacteria, informally known as blue-green algae. These microorganisms live in fresh and brackish waters and reproduce explosively when the conditions are favourable: warm water and an abundance of nutrients. Their “food” comes primarily from the nitrogen and phosphorus in the water. To a minor extent, those nutrients originate from the excrements of animals in and around the water, but the main source is human-made. As a by-product of the algae bloom, certain species of cyanobacteria produce toxins, which are released into the water when they die or become stressed. 

Even small concentrations can be harmful. Toxic algae blooms have far-reaching effects on the health of humans and animals, ranging from skin irritation to nausea and organ damage in severe cases. 

It also disrupts the entire aquatic ecosystem and causes a chain reaction that weakens biodiversity and the life in and around the canals: from oxygen levels that plummet and suffocate fish to the loss of sunlight that water plants rely on.

The postcard canals of Bruges and a hidden threat

In the canals of Bruges, the blue-green algae find ideal ground for their own well-being: The canals are a closed system with a very slow stream (if any). Nutrients reach the water from illegally connected sewers. Owners of centuries-old houses might not even be aware they are connected to the canals. Also, occasional stormwater surges flush pollutants into the water system.

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But there is another reason why toxic algae are an increasing problem in Bruges and around the world: Climate change.

Rising temperatures in summer, and the heat island effect (when cities become hotter than surrounding areas because buildings and paved surfaces trap heat) lead to an increase in surface temperature of the water, which accelerates algae growth.

The summer of 2025: A turning point for the City of Bruges?

The blue-green algae plague of 2025 was particularly heavy, started early in the season, and affected urban life.

Those Bruggelians who were looking forward to enjoying a dip were disappointed from 11 July onwards, when the public swimming zone had to close due to the poor water quality (find a video here on how much fun it was just before, in June of the same year). The swans, an important symbol of Bruges’ wellbeing and power (read more about the legend here) had to be kept away from certain areas of the canals. Although there is never a good time for an algae crisis, in 2025 the City of Bruges and University of Ghent (UGent) were already looking into installing a nature-based water filtration system in the Augustijnerei, near the spot where the urban swim will be located from 2026 onwards. 

However, due to the algae plague in the summer of 2025, the City of Bruges, together with Prof. Peter Goethals and his team from UGent, decided to change things around, use the pontoons of the urban swim to install a water treatment infrastructure that should help improve the water quality. Innovation requires flexibility, even if it means putting previous plans on hold  (for now) and addressing the issues at hand.

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Pilot testing in action – from the left: Alderman Franky Demon, the Blue4Green team of Stad Brugge - Magalie Mahieu, Eva Gheselle, Astrid Stroobandt; Professor Peter Goethals. Photo by Stad Brugge.

Peter Goethals made it his mission to turn the crisis into an opportunity to collect important data for the future of the water quality in the canals of Bruges and build a temporary infrastructure. A quickly built tiny house next to the canals housed parts of the infrastructure. The system isolated a specific quantity of water from the canals, which was then pumped into a filtering system inside the tiny house. There, the water passed through filters with activated carbon, which bound harmful substances. Each hour, tens of thousands of litres of water could be cleaned. 

The plague of 2025 highlighted the importance of addressing the root causes of the algae bloom, to ensure a robust and liveable Bruges. The experiments at the urban swim have only been the beginning of the improvements Blue4Green is aiming to achieve. Marie-Laure Machiels, a doctoral researcher at Waterwijs is currently conducting the laboratory tests for her research and to see how much of the water purification system had on the water quality. In the Blue4Green project, an EUI Innovative Action, the City of Bruges is testing new approaches to tackle the challenges of water quality.

What’s next? From emergency actions to long-term measures

The crises in water quality in the canals of Bruges in the summer of 2025 underlined how important addressing challenges relating to nutrients in the water is, especially in times of the climate crisis and the connected increase in temperatures. Already in the first year of Blue4Green, the project gained in significance and provided flexible approaches and solutions to address the water quality issues.

“We turned a necessity into a virtue, and the swimming pontoon became an open lab for the professor. Many Bruges residents were very curious about the research, as it suddenly became very visible,”

Astrid Stroobandt, Blue4Green project manager of the City of Bruges.

From 2026 onwards, Professor Goethals and his team from UGent, together with the City of Bruges will experiment with nature-based water filtering methods in the Augustijnenrei. The location has been strategically chosen, as it lies close to the Langerei, a canal where the urban swimming zone will be located in the future (why the swimming zone had to move from is a story for another time). Another important argument has been that during heavy rainfalls, ‘dirty’ water (Prof. Goethals might call it “nutrient-rich”) pours into the canals close-by.

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From 2026, a filtering system will be installed at Augustijnenrei. Photo: Johannes Riegler.

At the new location, the UGent will experiment with a filtering system made of different natural fabrics and assess their impacts on the water quality. Parts of the pontoons are already there (see photo). The installation will also provide green areas and habitats, and eventually nesting grounds for the local water birds.

The Blue4Green project will not stop there. In the next year, the team will expand the measure to tackle the toxic algae blooms before it occurs. The team will work on:

• An AI-supported predictive model for blue-green algae blooms, providing the city’s services with information for evidence-based policy decisions.
• An integrated monitoring system for water quality, including chemical, physical, and microbiological aspects, using remote sensing as well as citizen science.
• A dashboard providing real-time information on water availability, water flow and quality.
• … and much more. 

The summer of 2025 brought an early and heavy blue-green algae bloom and a corresponding harmful decrease of water quality to Bruges’ Reien. Although the blue-green algae plague killed the joy of many this summer, the relevance and importance of the Blue4Green project only increased. The City of Bruges is aiming to move from crisis management to long-term transformation through smarter planning and environmental stewardship. As such, Bruges could become a frontrunner showing how heritage and innovation can be combined to address climate challenges of the 21^st century. The knowledge and experience of Bruges could become a blueprint and inspiration for other cities and urban areas facing similar climate-driven water quality issues. In the coming years, the city’s swans, the symbolic ambassadors for clean water, can hopefully glide through clearer waters for many summers to come, unaware of all the innovations working along the canals and on local public administration’s screens. 

Curious about what’s next for the City of Bruges and Blue4Green?

Make sure you do not miss any news about the quest to hydrate the city by combining heritage, natural and high-tech solutions:

• More about Blue4Green’s background: From smart sensors to medieval cultural heritage: A new chapter in Bruges' water identity with Blue4Green. 
• Follow Blue4Green’s progress on Portico.