Severe sea lettuce season ahead

Sea lettuce on the beach at Tay Street in January. Photo/George Novak

Local authorities are bracing themselves for a severe sea lettuce season, enhanced by the strong El Nino weather pattern expected this summer.

Sea lettuce is a naturally occurring algae native to New Zealand. Its growth is mainly influenced by coastal currents, water temperature and nutrient levels. Tauranga City Council and Bay of Plenty Regional Council are preparing for a big season.

Regional Council Tauranga Harbour projects manager Bruce Gardner said trends in monitoring information, gathered since 1991, showed the largest blooms of sea lettuce occured 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. 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," Mr Gardner said.

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

Mr Gardner said the Te Maunga compost facility was on board again this year, but he was also on the lookout for alternative disposal options, especially in 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. 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".

Mr Gardner said with careful washing and dilution, beach-cast sea lettuce could 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," Mr Gardner said.

"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 the larger discharge points around the harbour have been removed or had treatment systems in place since the mid 1990's."

Regional Council coastal scientist Rob Win said sea lettuce was responsive to any nutrient source, but the particularly large blooms happened when there was less land run-off, cooler water temperatures, and a higher proportion of natural, ocean derived nutrients in the harbour system.

"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," Mr Win said. The PhD reports are due to be complete 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. Further information about sea lettuce in Tauranga Harbour is available at www.boprc.govt.nz/sealettuce.