Bay of Plenty residents are being warned to expect beaches to turn green over the coming months, with a severe lettuce season predicted.

With a strong El Nino weather pattern predicted this summer, Tauranga City and Bay of Plenty Regional Councils are bracing themselves for a busy sea lettuce season around Tauranga Harbour.

Bay of Plenty Regional Council Coastal Scientist Rob Win taking a close look at sea lettuce at Pahoia, Tauranga Harbour last month. Photo: Supplied.

Sea lettuce is a naturally occurring algae that's native to New Zealand. Its growth is mainly influenced by coastal currents, water temperature and nutrient levels.

Regional Council Tauranga Harbour Projects Manager, Bruce Gardner, says trends show that the largest blooms of sea lettuce occur during periods of drier weather and persistent offshore winds associated with El Nino weather patterns.

'It becomes a nuisance for people when it smells or gets in the way of boating and swimming,” explains Bruce.

'So the councils work together, to make sure public calls are responded to and popular foreshore areas are regularly checked.

'Our aim is to locate and collect any large piles of beach-cast sea lettuce before they get too anaerobic and smelly.

'We're putting pick-up and disposal contracts in place for the season now, so we'll be ready to go as and when problem piles are identified.”

A total of 350 tonnes of sea lettuce was collected from city beaches and Ongare Point last year – all of which was used for commercial compost production at Te Maunga.

Bruce says the Te Maunga facility is on board once again this year, but he's also on the lookout for alternative disposal options, especially at the northern end of the Tauranga Harbour.

'We'd love to hear from anyone that thinks they can make good use of collected sea lettuce,” he adds.

'If it's a bumper season in the northern harbour, we need to minimise the high cost of cartage back to Te Maunga.

'Transporting the collected sea lettuce adds to clean-up costs, so having some alternative disposal options closer to accumulation sites would be a big help.”

With careful washing and dilution, Bruce says beach-cast sea lettuce can be an effective compost or orchard mulch.

'It can be used to make biofuel too, but that's not currently a viable business proposition because of issues with supply, quality and consistency.

'It's a common misconception that sea lettuce growth is caused mainly by sewage and industrial pollution into the harbour.

'We know that's not currently the case for Tauranga Harbour because all of the larger discharge points around the harbour have been removed or had treatment systems in place since the mid 1990s.”

Regional Council Coastal Scientist, Rob Win, says sea lettuce is responsive to any nutrient source, but particularly large blooms happen when there's less land run-off, cooler water temperatures, and a higher proportion of natural, ocean derived nutrients.

'In addition to our regular monitoring, we've been supporting two University of Waikato PhD projects that will add to our information base about sea lettuce dynamics,” says Rob.

The PhD reports are due to be completed later this year.

Sea lettuce concerns should be reported to Bay of Plenty Regional Council's Pollution Hotline on: 0800 884 883 or Tauranga City Council on: 07 577 7000.