Many walkers and dog-walkers have been puzzled by the light brown and white foam that has appeared in large clumps around both the Portsmouth and Hayling sides.
And, as well as the foam, there are large mats of green algae floating on the surface of the water.
News reader Stuart Phillips said: ‘There is a white foamy substance that has entered the water somewhere. It is along the beaches at the entrance to Langstone Harbour and out at sea.’
An Eastney dogwalker, who did not want to be named, added: ‘You normally see green algae and foam during the summer. This year it seems particularly bad.’
But expert Louise MacCallum, environment officer for Langstone Harbour, said both were natural phenomena.
About the foam, she said: ‘Some people call it spume.
‘It’s caused by diatoms which are a kind of plankton.
‘The cells in their bodies break down and then get churned up by the waves. It produces a foam.’
She said the phenomena is dependent on the weather.
The algal blooms are caused by nutrients in the water from agricultural run-off and sewage discharges.
Ms MacCallum said: ‘It’s not great, to be honest. It’s here in much higher quantities than it would be naturally if there were no human influences.
‘It can stop birds from getting to the mud to get their food.’
She added: ‘A lot of work has been done to try to reduce nutrient input.
‘I would like to see if you took pictures now and in 10 years time, we would definitely see an improvement.’