As previously reported, the ‘abnormal situation’ took place on Saturday, October 10.
At the time, it prompted a warning from the the Environment Agency to sea-users, who were urged to avoid swimming in waters around Eastney and Hayling Island.
Witnesses reported seeing large patches of stinky, discoloured water, which some feared was a further sewage leak.
However, at the time Southern Water said no water had been discharged, leaving residents perplexed.
Now, the Environment Agency’s acting area manager for Solent and the South Downs, Nick Gray, has revealed the incident was caused by algae.
Responding to a letter sent by Stephen Morgan, Portsmouth South MP, Mr Gray said: ‘Over the weekend we received more reports of discoloured water and sewage odours in our coastal areas extending as far east as Brighton. This led us to conclude this was a widespread, likely natural, event rather than focused around “point” sources such as sewage discharges in the Hayling and Southsea area.
‘On Monday our biologists analysed the photographs sent to us and social media posts and concluded that the light brown foam seen was typical of decaying algal material.
‘Algae can build up in nutrient rich waters particularly when conditions are relatively warm and calm.
‘Extensive growths of algae can discolour the water. As light levels decline with the on-set of autumn, the algae starts to decay producing unpleasant odours, foams or scums which can be mistaken for sewage.
‘We therefore believe the reports were due to this natural phenomenon on this occasion.’