A green brown scum sighted along beaches mainly in the Eastern Bay of Plenty is not ‘cow poo' but surf diatoms – a naturally occurring single celled organism.

The foam and sludge has been tested following complaints from members of the public.

Beach scum is actually surf diatoms – a plant. Picture: Supplied.

'This sludge, whilst unsightly, is nothing to be concerned about as it is a common non-toxic sea plant even though it is a simple single-celled one,” says Bay of Plenty Regional Council senior environmental scientist Stephen Park.

The sludge is actually made up entirely of phytoplankton, the base of the marine food chain, upon which other marine creatures feed, says Stephen. The blooms occur naturally even in pristine habitats and it is usually a sign of a healthy ecosystem.

'The species of plankton found are usually surf diatoms which live near the shore zone on the open coast. Plankton numbers tend to increase through winter and spring so when weather conditions are favourable their numbers build up.

'That combined with tide and wave conditions mean the diatoms can form foamy slicks that wash up on our beaches. The scum can get pretty thick (20-30cm) and cover 2m wide bands on the beaches but they are not usually that bad.

'Unfortunately, they can smell unpleasant as they break down in the sunlight but they do not produce toxins and are not associated with bacteria or virus which might cause sickness. So there is no need for concern for contact from humans or animals.

'So whilst the sludge may be unsightly for people enjoying our beach, there is no need for concern as this is a normal occurrence for this time of year.”