Scum, foam and sludge on Bay beaches is being described as a natural occurrence of surf diatoms.

Bay of Plenty Regional Council has confirmed the sludge is not associated with the discharge of dairy farm effluent, as some may have suggested.

Scum that is all a part of nature.

Surf diatoms are a type of phytoplankton, which is a main source of food for productive shellfish beds in the surf.

An abundance of these indicates a healthy habitat.

The council's pollution prevention team leader, Steve Pickles, says the recent dominance of onshore weather has contributed to the build-up of surf diatoms, particularly on Tirohanga and Hikuwai beaches.

'It's definitely become worse with the recent stormy weather,” says Steve.

'Surf is stirred up more and the scum is washed up.

'We get the same occurrence most years with changing weather patterns.”