Algae and weed left a pond totally covered after a bloom described as the “worst in recent years”.
After weeks of sweltering weather the pond in Queens Park, Brighton, was left completely choked by duckweed and blanket weed – a form of algae.
At the bloom’s peak the surface of the water could not even be seen beneath the thick covering.
Volunteers have been battling to try to clear the pond but say there are simply not enough of them.
Chris Lowe, of the Friends of Queens Park, said the build-up has been the worst in recent years but while it may be “unsightly” it is not dangerous to wildlife.
He said: “The growth rate of duckweed is speeded up by high nutrient levels and sunlight, the pond returning to more or less clear water in the winter.
“The lack of a hard frost this year meant that the residual population of duckweed was not killed off.
“Duckweed is an example of geometric progression as in the right conditions, it can double every one or two days.
“So if it takes 100 days for the pond to get half covered it then takes, in theory, only one day to cover completely.
“Manual clearance is one solution but as there are tons to remove this is only likely to be a partial solution with the number of volunteers we have.”
Jess Price, conservation officer at the Sussex Wildlife Trust, said the Queen’s Park growth is the “largest” she has seen.
She said: “This is a very large bloom and they can grow really quickly and are tolerant of low levels of nutrients.
“You would have to physically clear it up but on a pond this big it is going to take a lot of work and it grows back so fast.”
Other parks in the county, including Brooklands Park in Worthing, have also been hit by blooms of the algae and weed.
A spokesman for Brighton and Hove City Council said they are aware of the problem and are looking into finding a solution.
Anyone interested in volunteering with the Friends of Queen’s Park can call Park Ranger Lindsay Cattanach on 07818 092719, Chris Lowe on 07761 674827 or visit www.friendsofqueens parkbrighton.net