You might just call it the return of the muck.
The green substance that every now and then covers the waterways of Lincoln is back – and it looks as weirdly amazing as ever.
The green mass has this time been spotted in the River Witham as it flows through Lincoln city centre.
It has certainly brightened up the water this week in what has otherwise been an overcast period.
But there are concerns that the weed can kill fish by using up the oxygen – and officials say it can double in size in a couple of days.
Dean Clark, who works at the M&S store in Lincoln, saw the greeny coating on Tuesday morning, August 18 – and said he’d never seen it look like that before.
He even stopped to take pictures of what he saw.
Mr Clark told Lincolnshire Live: “I work at M&S next to the river so I see the river daily. I’ve never seen it looking like this before.
“The vibrancy of the colour startled me at first, but it actually looks nice in a strange way.
“We did witness a man swimming in the pool yesterday.
“However, the green algae wasn’t about at that point so it literally appeared overnight.”
Some people on social media suggested it could be blue-green algae, which can be harmful to anyone who swims in it and potentially toxic to animals.
However, the Environment Agency has now confirmed this is not the case.
The agency say they sent an officer to inspect the water, and that the green plant seen in the river is duckweed – and is not toxic.
An Environment Agency spokesperson said: “We have not had any reports of blue green algae in the River Witham.
“Our officer has attended site and we have confirmed the green plant is duckweed and is not toxic.
“If members of the public see any fish in distress, please report it to our 24 hour incident hotline on 0800 80 70 60.”
Duckweed is often mistaken for algae, as it forms a similar thick, green substance in the water.
There are concerns that it can affect local wildlife – and in some places there have been reports that fish have been found floating on the surface as a result.
According to criticisms raised recently in Colchester, duckweed in a lake there has killed fish and had a serious impact on local wildlife.
Tony Newton, from Colchester, told the Daily Gazette Lexden Park is suffering from an invasion of duckweed.
He said: “The weed chokes the entire area of the water in a very short time.
“The weed dominates the lake and consumes the oxygen present in the water.
“Any fish in the lake risk the chance of suffocation.”
Colchester Council said that the problem there spread quickly because duckweed can double in size very quickly.
“Weed mass can double in size every couple of days given the right conditions and when our rangers visited the lake during the recent spell of very hot weather, the problem had spread to such an extent that several dead fish were seen floating on the surface,” a spokesman there said.
“We immediately contacted the Environment Agency to request specialist equipment to deal with the problem and are continuing to work with them to ensure oxygen levels in the lake return to normal as soon as possible.”
The Pondkeeper website says there can be some benefits to duckweed in ponds, as it can monitor the health of the water and stop algae growth.
But even it concedes that it is a ‘multiplying annoyance’.
“It becomes very tricky to remove, so if you have experienced it once then you are probably still experiencing it,” the site says.
“We regularly meet pond keepers desperate to rid their ponds of the tiny green plants.”
It can be hard to remove – so it could be around for a while.
According to the Wildlife Trusts, it can be very invasive – and cause problems for other plants.
Their website states: “Common duckweed can be highly invasive in a pond, container or even water butt, covering the surface of the water with ease. It can shade out other plants and use up the nutrients available, making it difficult for other wildlife to thrive. If you have a small pond in your garden that is covered in common duckweed, try raking it out and removing any dead vegetation which might add to its growth, or you could try using a pump to aerate the pond.