Sea-lettuce plague cleared from popular beach

Boat trailers on the foreshore of Tauranga Harbour draped in sea lettuce. Photo / Alan Gibson

One thousand tonnes of rank, unsightly sea lettuce has been cleared this summer from one of New Zealand's most popular beaches and its surrounding coastline.

The native algae, knee-deep in some places, plagues Tauranga's inner harbour and is an unsightly blot on Mount Maunganui's shoreline.

Environment Bay of Plenty spokesman Rob Donald said the proliferation of the weed this summer was the worst since the mid-1990s.

Twice as much as usual had been collected by a grooming machine imported to deal with the problem.

Complaints by residents - many living in million-dollar properties on the harbour - and from owners of popular cafes at the Mount prompted a $60,000 clean-up, organised by the Tauranga City Council and Environment Bay of Plenty.

Until recently the council relied on the tide to clean coastal areas such as the Mount's main beach, but last night the mess was being cleared away by machinery for the third time this summer.

The mats of weed can tangle fishing boats' nets, and if its growth is particularly heavy it kills everything underneath it, such as shellfish.

Council parks ranger Craig Fea said it was also a health risk if left to decompose too long.

"In extreme cases the hydrogen sulphide gets to toxic levels, which can be a threat to children and dogs on windless days. It becomes black and slimy and is harder to clean up."

Some studies suggest the increased amount of sea lettuce is a result of polluted run-off in the region, but Environment Bay of Plenty scientists say it is the result of an El Nino weather pattern.

They say the westerly winds of El Nino force nutrient-heavy ocean swells into the harbour feeding the sea lettuce blooms.

The council hoped the 1000-tonne infestation was a one-off, as the sea lettuce bothered residents and and holiday-makers.

"Some of the beaches are iconic resorts - we really have to keep them as clean as possible. When it is hot there is a real odour," said Mr Fea.

Mr Donald said more research was needed to understand the effect pollution had on encouraging the bloom.

When the weed grew in manageable amounts, it was a good food source for sea life and could be collected for use as a fertiliser.

Local orchardists and commercial composters had enthusiastically picked up the weed after it had been raked up by the machinery.