Questions still swirl around ocean moss

Like candyfloss, but green and you can't eat it, the ocean moss gathered on Mahia Beach. Photo/Warren Buckland

More than 15 years after mounds of ocean moss began appearing on the beach at Mahia there's still mixed opinion about what causes it, whether it has a link to land use further south, and even how long it stays.

But at least it's useful, according to Mahia identity Bill Shortt.

"A lot of people use it in their gardens," he says.

"Great fertiliser," he observes. "Needs some entrepreneur to start up a fertiliser business."

The moss is believed to have developed from marine sources and has been identified as cladophora cf. montagae, and seems to have first been reported in quantity on the beach about 1999, although Mr Shortt says there were probably small amounts before that.

This week, it had formed in long strips on the beach, the fluffy-green stuff banked along the beach, settling in some places and developed by nature as if to resemble a scale-model coastline and cliff-face all of its own.

Washed up by recent swells common to the time of year, it was up to 30cm deep, but Mr Shortt says there have been times when it's been up to a metre deep.

"It's best described as similar to candy floss, except it's green, and you can't eat it," he says.

"I don't believe it is harmful to humans, but I would not be running an outboard motor through it."

Scientists doubt it is caused by anything land-based, but agree, as Niwa scientist Dr Wendy Nelson observed in 2010, that the algae could be exacerbated by human activities such as land use changes or sewage outfalls.

Hawke's Bay Regional Council senior scientist coastal quality Anna Madarasz-Smith said that wherever there is plant life, and it's fed by nutrients, the growth will be better.

Generally, it supports local belief that the algae is fed by run-off from rivers entering Hawke Bay, and they had a particular interest when they saw images of a similar floating algae causing delays to the 2012 Beijing Olympics yachting regatta at Qingdao Bay, reported to have suffered its worst algae blight in living memory, reports said.

"Same colour," Mr Shortt says, "but this stuff we have is on the ocean floor - theirs tended to float."

"The amazing story out here - it only appears in patches on the main beach, some 5km from Mahia Beach to Opoutama," he says. "None in Taylors Bay, Blacks Beach, or on the northern side of the peninsula."

Ms Madarasz-Smith says it is not an unusual algae along New Zealand coastlines.

Mr Shortt says: "As sea conditions subside it tends to die quickly with sunlight on it, change from green colour to a grey powder and dissipates into the sand it lies on."

As for the future of a local industry with fertiliser, it's unclear whether it's part of any quota management system, although people can take dead seaweed from the beaches as long as it's not for commercial purposes. It's possible that if it were wanted for commercial purposes, there may be some need to assess the environmental impacts.