Northland's red carpet of algae - where does it come from?

A blanket of red algae has created a mess on the beach and forced swimmers and surfers further north. PHOTO/ John Stone

Waipu locals and holidaymakers have an extra reason to hope it doesn't rain in the next week or so.

A red carpet of seaweed that Mother Nature has again rolled out at the southern end of Waipu Cove is pungent, but smells nowhere near as bad as last year, say locals.

That's because this year's weed is still alive. It is only when it dies that it begins to rot, and smell.

Its primary cause of death is fresh water, meaning it is susceptible to heavy rain, or rough seas that could wash the weed into a nearby fresh water stream.

Waipu resident and coastal engineer Andre Labonte sent off a handful of the algae last summer for testing at Niwa in Wellington and results showed there were about 25 species in the mix but the dominant algae present were Plocamium and Acrosorium ciliolatum.

While this year's mix had not been tested, he said if it was similar to last summer the species both grew on the ocean floor.

If there was a storm or ocean swell, as there was just before Christmas, the algae would dislodge and float to the sea surface.

In the increased sunlight the algae would continue to grow and undergo photosynthesis and "bloom", creating giant, red blobs which eventually washed ashore.

"There have been no waves washing up the beach since to take it all away so it just accumulates," Mr Labonte said.

A crust had formed on top and sealed in the moisture allowing the algae to survive for some time.

However, when it mixed with fresh water it would die and begin to rot.

"There is no indication the algae mix on the beach is toxic. But it might be a different story in the stream when it dies there. I wouldn't be swimming in there when that happens," Mr Labonte said.

The weed appeared on Christmas Eve and covers a section of beach about 60 metres long and in the water it has spread about 500m in the surf.

Yesterday swimmers were not deterred by the algae and took to the gentle surf to cool off.

Ruakaka Lifeguard patrol captain Maysha Ahrens said an overnight change in wind had caused the red algae to spread toward the front of the clubhouse after initially being contained to the beach's southern end.

"We have tried each day to move the flags a little bit further north to keep it outside the algae. But today there's no way we can avoid it."

Standing in the southern carpark near the reserve where a fresh water stream flows to the sea, a pungent odour filled the air.

Mangawhai friends Carol Collett and Kay Acraman had been enjoying a coffee at the Cove Cafe when they got a waft of the seaweed.

As they walked over the sand dunes to the beach the source of the smell became evident.

"It stinks. But last year it was worse," Mrs Collett said.

Auckland visitor Sophie Gower at first glance said she thought: "It kind of looks like that insulation stuff you use for the walls. I was going to go for a swim here but I might just migrate further down the beach."

Anton Trist of Camp Waipu Cove said the layer of algae was not yet thick enough to warrant its removal.

He had discussed it with the Northland Regional Council (NRC) this week and it was now a case of "wait and see".

The NRC held the consent for removing the algae, but the camp would likely pick up the cost if removal became necessary, Mr Trist said.

"We're hoping it just goes away. But if it gets bigger we will trigger the consent process and get into action."

He said last year the weed volume and odour was far worse and required immediate action.