What is May Rot in the sea? Locals 'revolted' as 'smelly brown sludge' infests sea at UK beaches that 'looks like rancid chicken soup'

“Smelly brown sludge” has been spotted at Brighton beach and along the south coast of England leaving locals and holidaymakers alike “revolted”. Users were quick to post pictures of the sea on the south coast over the weekend on X, formerly Twitter, showing the murky brown water.

One user wrote: “I’ve been swimming in Brighton from April or May through to September/October every year since 2001. I’ve been saying to all who talk about sewage concerns that it didn’t bother me. But this year it’s too much- there’s a smelly brown sludge across the surface of the water.”

Another user responded to the post that they were unable to swim at Cooden beach in Bexhill due to the sea being “filthy” and smelling “very bad”. The person added that environmental health “were there two hours after we reported it and said it wasn’t sewerage” but instead “May Rot”. The user wrote: “A lot of it about this time of year. I’ve never seen it before.”

Another person posted on X stating that the brown sludge is caused by algae blooms and not sewage. The person said: “This is the time of the year when you get algae blooms which have a very ozone smell as they die off, also it is fish spawning season which can form as a scum too, it seems as this is in several parts of the coast.”

A third person also wrote that the water was “so dirty” in Saltdean and “looked like rancid chicken soup”. Southern Water confirmed to NationalWorld: “This is algal bloom and was investigated over the weekend to make sure that it was not related to any of our assets and operations. Our teams also took samples to confirm this along with an investigation into these reports.”

So what is May Rot? And why is it causing the sea to smell?

May Rot occurs when the tiny plant-like organisms living in the ocean called phytoplankton increase in numbers due to sudden warmer weather and brightness levels. Brighton & Hove City Council said the algal blooms on its beaches were “harmless” and were “nothing to worry about”.

However, the Environment Agency and Southern Water said some blooms can be toxic and advised people to avoid them.