A “sick stunt” likened to something you would “see Mr Burns order on The Simpsons” saw chemicals pumped into the River Lea to turn it fluorescent green, just four days after thousands of fish died there.

Hackney Gazette: The River Lea before the fluorescein dye was poured inThe River Lea before the fluorescein dye was poured in (Image: Archant)

Although artists Sam Bompas and Harry Parr – who were commissioned to come up with the “grand finale” of the Open East Festival to celebrate the reopening of the Queen Elizabeth Olympic Park on Saturday – said they were “genuinely sorry,” the river was dyed the same lurid colour with Fluoroscein the following night.

The intention behind the sound and light project dubbed “Chromatopsia” was to celebrate “The River Lea story” – which, according to arts organisation Create, London “encapsulates how the Olympics has brought new life and vibrancy to the local environment for everyone to enjoy”.

People who had been horrified to find thousands of dead fish in the very same river on Tuesday after heavy rain washed pollution into it from roads, complained that it was inappropriate to add another chemical to the “toxic cocktail”.

The Environment Agency (EA) claims the chemical Fluoroscein is safe and gave approval along with the Canal and River Trust for the stunt, which was developed in conjunction with the Barbican arts centre, Create London and the London Legacy Development Corporation.

But a 2011 study conducted by a university in the United States raised concerns on toxicity and Green Party campaigner and vet Caroline Allen said it beggared belief that organisers should try and highlight the River Lea as “some sort of legacy success while it is a dangerous and toxic mess.”

“There is a nasty toxic soup going on in there and you can’t help but worry that adding something else could have unexpected consequences,” she said.

“For me the real issue is that they went ahead with this event celebrating something that is a disaster area. “I think it raises wider questions about the truth behind the legacy claims.

“If they are willing to try and spin thousands of dead fish what else can we believe?”

In a statement released before the event had even happened, EA director Howard Davidson predicted: “The River Lea has dazzled tens of thousands of spectators tonight and its image will be unforgettable.”

However following the stunt, dozens of “gobsmacked” Hackney residents took to social networking site Twitter to express their dismay, like @Hackneyhaz who asked Create whether they were planning to dye the river green again.

“Cos nothing shouts “eco” like a load of dye in a natural resource,” she said, continuing: “It’s like some sick stunt you’d see Mr Burns order on The Simpsons.”

Brian Devlin, chair of the Docklands Canal Boat Trust, which runs community boat project MV Challenge on the River Lea, is concerned the Prescott Lock - installed before the 2012 Games - has slowed the flow of the river and makes it more likely for contaminants to build up.

The cosmetic alterations create permanently deep non-tidal water within the Queen Elizabeth Olympic Park as part of the landscaping to widen the river. Mr Devlin said: “The fluoroscein will wash downstream eventually, but the effects might be felt for a long time.”

Theo Thomas who runs campaign group Thames21’s Save the Lea campaign added: “We use more than 3,500 chemicals which find their way into rivers.

“I’m against pouring more in when we don’t need to.”

A spokesman for the EA said the chemical, which it normally uses in practical scenarios to detect leaks in watercourses, “breaks down when exposed to sunlight and will have no lasting effect on the river and wildlife”.

Sam Bompas told the Gazette they had done an “enormous amount of research” to eliminate risk.

“If you drink an enormous amount of water it’s unsafe and you will die, but if you have one glass of water you won’t die, anything in a high concentration can be challenging but the way we were using the dye was safe,” he said.

Mr Bompas was aware that hundreds of fish had died during the preceding week, but added that the spectacle had been six months in the making and a decision was made to continue with it.

“I’m really sorry people are upset. I genuinely am because the point of us doing this is to give people a lovely time. That’s what I truly care about,” he said.