Why have rivers, ponds and canals in Greater Manchester turned green?
Scores of people have asked that very question after spotting unusually-coloured stretches of water across the region.
Well, there are a couple of reasons.
Some bodies of water, such as the Rochdale Canal through Oldham, appear to be covered in a thick green film. That’s duckweed.
At other spots, including Heaton Park, the water itself looks to have turned a murky olive colour. That’s thought to be down to algae.
Visitors to Heaton Park, Philips Park and Bramhall Park have also spotted yucky-looking water.
The recent warm weather has allowed plants to thrive, with rivers, ponds and canals ending up looking like pea soup.
Algae and duckweed grow rapidly when it’s hot – and in bodies of water in urban areas, because of higher levels of nutrients caused by pollution.
The Freshwater Habitats Trust says plants feed off farming waste, animal faeces, food thrown to ducks, and runoff from roads and pavements.
Algae and duckweed are not a cause for concern, experts say. Both occur naturally, with algae providing food for scores of species.
Duckweed even provides homes for tiny aquatic beetles.
Pictures from the Rochdale Canal show a blanket of thick green duckweed on the water.
Empty bottles and other junk can be seen floating on top.
Strangely, there is a distinct line where the green film ends and the cleaner-looking water begins.
From afar, it’s not the prettiest of sights, but up close, it’s strangely beautiful.
Other stretches of water, such as at Heaton Park, look a bit more grim.
Louie Ramsden, an environmental scientist at the Canal and River Trust, said: “We want people to continue to enjoy the lovely waterside setting but to be aware that there’s a current outbreak of duckweed in the water.
“Duckweed is a real challenge on the canals at the moment and with more hot weather on the way, it’s going to continue growing.
“It grows so rapidly and is tricky to collect because the individual pieces are so tiny.
“The weed itself isn’t harmful – in fact lots of ducks, geese and some fish like to eat it – but it looks bad, traps rubbish, and it can cause problems for boaters too.
“We’re monitoring the situation and we want to remind people to take care when out walking with dogs and with their families, and asking people not to throw litter onto the weed.”