Why did the Brussels Canal turn green and leave fish dead?

Brussels Environment

Organisation

Brussels Environment

Regional authority that confirmed the algae, announced an inspection and monitors surface-water quality.

Why it matters

Regional authority that confirmed the algae, announced an inspection and monitors surface-water quality.

About

Leefmilieu Brussel is de openbare instelling die op het vlak van milieu, dierenwelzijn en energiebeleid een uitgebreid gamma aan bevoegdheden beheert.

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The episode matters directly to people living, working or spending time beside the canal, particularly around central Brussels and Anderlecht. It is also a warning about the condition of a waterway that serves simultaneously as an industrial port, wildlife habitat and increasingly prominent public space. The visible green tint does not by itself identify which algae are present or prove that toxins are in the water. Even so, Brussels Environment generally advises avoiding contact with canal and pond water during very hot weather because some cyanobacterial blooms can present risks to people, dogs and other animals.

Pieter Elsen, founder of the Brussels non-profit City to Ocean, offered one explanation for the deterioration. He told Belga that fertilisers and organic matter accumulated in the canal’s sediment can nourish algae. Regular sewer overflows may add to that burden: when Brussels’ combined sewer network is overwhelmed by rain, stormwater and wastewater can spill into the Senne and canal. Elsen said such overflows normally occur between one and three times a month during rainy conditions and that the latest one known to him had taken place at the end of July.

His account is a plausible mechanism, not a settled diagnosis. Elsen explicitly said the overflow could not be identified unequivocally as the principal cause of this bloom. Brussels Environment confirmed the algae but, in the first public response, did not attribute them to a particular discharge, company or incident. No named Brussels minister or Port of Brussels representative was quoted in the reports announcing the episode, and the absence of published sampling results leaves open questions about nutrient levels, dissolved oxygen and the species involved.

The institutional evidence nevertheless shows that this is not an isolated type of event. Brussels Environment describes the canal as vulnerable to recurring summer ecological crises because nutrient enrichment and slow-moving water encourage algal and cyanobacterial blooms. Algae and decomposing organic matter can consume dissolved oxygen, especially at night or as vegetation dies, leaving fish unable to breathe. Sewer overflows and occasional diversion of Senne water can cause additional, abrupt damage to aquatic life.

That makes the dead fish a symptom of a structurally fragile system rather than proof of one recent act of pollution. The region’s water-management plan says none of the monitored Brussels water bodies achieved “good ecological potential” in the 2019 assessment. At the same time, the canal supports numerous fish species, making each mortality episode a setback to ecological improvements recorded since Brussels’ wastewater-treatment plants entered service.

City to Ocean wants part of the response to begin above the waterline. Its “Disconnect Your Roof” campaign, promoted by the City of Brussels

Organisation

City of Brussels

Municipal authority providing guidance on rainwater reuse and managing local public space.

Why it matters

Municipal authority providing guidance on rainwater reuse and managing local public space.

About

The City of Brussels is the largest municipality and historical centre of the Brussels-Capital Region, as well as the capital of the French Community of Belgium, the Flemish Region, and Belgium. The City of Brussels is also the administrative centre of the European Union, as it hosts a number of principal EU institutions in its European Quarter.

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The European connection is regulatory rather than geographical: this is Brussels the city, not an incident involving the EU institutions headquartered there. Regional monitoring forms part of Belgium’s obligations under the EU Water Framework Directive, which requires authorities to assess and improve ecological water quality. The immediate next step is more basic, however. Brussels Environment and the Port of Brussels need to clarify what sampling found, how many fish died, whether carcasses were removed and whether oxygen-restoring measures were used. Until those answers emerge, the best-supported conclusion is that algae and oxygen depletion killed fish in an already vulnerable canal, while the event’s precise trigger remains unconfirmed.