KEY POINTS:
It's green, ankle-deep and threatening to cause a stink at Mt Maunganui.
Sea lettuce has washed up in thick patches, some up to a kilometre long, on the Mount beaches in the past few days.
Locals say it is the largest quantity of the grass-coloured algae in years,
and, unless the tide takes it away, it will begin to rot and smell.
The regional council, Environment Bay of Plenty, has monitored sea lettuce in the area since the early 1990s and says its appearance is linked to an El Nino weather event.
"When we get a big abundance of sea lettuce, it coincides with the El Nino events and we're in an El Nino at the moment," council scientist and manager of environmental investigations John McIntosh said.
"When we get predominantly offshore winds, the coastal waters get blown offshore and the deeper waters up-well, so you get the abundance of nutrients from that source coming in and that's when the sea lettuce really takes off."
He said it was likely that the sea lettuce had been swept around Mauao, the mountain at Mt Maunganui, from Tauranga Harbour.
The worst recorded outbreak was in the early 1990s, but locals at the Mount's Omanu Beach yesterday said they had not seen it in such quantities before.
"In all my days I've never, ever seen it here like this," said Ian Dale, 54, who has lived in the area all his life.
Large quantities of sea lettuce in summer create an oversupply in winter, causing it to flourish in spring, Mr McIntosh said.
"When it starts to grow again, instead of just growing from little rosettes the size of a new 20c piece it's starting as a big sheet, so the next year you get a worse infestation."
He said the problem was likely to get worse this year but would mostly affect Tauranga Harbour. The algae were not a threat to humans, but could cause problems at the city's busy port.
"It does accumulate in that deeper water at the port and has been sucked into ship engines."
The sea lettuce at the Mount is a common type, ulva lactusa, found worldwide. Large quantities can sometimes be an indication of pollutants in the water, but Mr McIntosh said that was not the case here.
Sea lettuce is eaten in some countries but ulva lactusa is not a recognised edible seaweed, although it has been used as compost.
Environment BoP is not planning a clean-up, saying the responsibility lies with the local council.
Tauranga City Council said it was aware of the "unusual occurrences" of sea lettuce on the beaches and staff were monitoring the situation.
"The logistics of removing the sea lettuce with machinery from the open coast locations would be difficult," a spokeswoman said. "Efforts are made to protect the dune system."