A large algal bloom has been blamed for discoloured water and foam sighted off parts of the south coast.
Beachgoers in Hampshire and Dorset were among those reporting a “frothy scum” in the water.
Naturalist and diver Steve Trewhella said the bloom was on a scale unprecedented in recent years.
Mr Trewhella, who took samples from the sea off Kimmeridge on Saturday, said a “huge” slick of algae was currently floating off the south and south west coast.
He said: “I haven’t seen a bloom of this particular species on this scale before.”
Mr Trewhella said the slick may have been expanded by sediment from the recharging of beaches.
Bournemouth and New Forest councils said they had received reports from beachgoers who were worried about “pollution”.
Jo Lally, who visited Milford on Sea in Hampshire on Monday, said: “The sea is very brown and slimy, roiling with brown, gloopy particles, and… the stench of raw sewage is almost overpowering.”
New Forest District Council said early indications were that it was algae and that a deposit of oil was not suspected.
The advice to coastal users while the extent and cause was unknown was “to exercise caution.”
The Environment Agency said the “brown frothy scum” seen in Dorset had been identified as Phaeocystis, a non-toxic algae which could be whipped by the wind into a brown foam.
The Agency said: “Once onshore it breaks down into a brown slime which can smell similar to sewage.”
Source: BBC News 31st May 2016