Natural but a nuisance
Sea lettuce is a naturally occurring green algae that is native to New Zealand. Sea lettuce can cause problems for the community when it grows during summer months and is deposited on beach and foreshore areas in large quantities.
Sea lettuce 'blooms' can affect the way the harbour looks, and how people can use the harbour for business and pleasure.
When it decays, sea lettuce produces an offensive sulphur odour. It can drift around the harbour and nearshore coastal waters and interfere with fishing nets and lines. It can also affect commercial shipping at the Port of Tauranga, wrapping around ship propellers and blocking cooling water intakes.
Sea lettuce varies in abundance from year to year and place to place. Its presence is most strongly influenced by uncontrollable factors such as wind, tides and coastal currents which affect water temperature and natural nutrient levels.
The graph below shows fluctuations in sea lettuce abundance in Tauranga Harbour as a percentage of the long term average, since monitoring began in 1991.
Research by Regional Council has shown that large sea lettuce blooms in Tauranga Harbour are a natural event that appear linked to the El Nino weather pattern. El Nino creates enhances growing conditions for sea lettuce with improved water clarity (due to lower rainfall and less land run-off), and oceanic upwelling which brings cooler, nutrient-rich waters into the harbour from off-shore.
Regional Council is responding to community concerns about sea lettuce in three ways:
1. Reducing the amount of nutrients entering Tauranga Harbour by working with land owners, business owners, local councils and the wider community to:
- reduce diffuse nutrient sources such as from agricultural runoff and nutrients that drain through the soil and into groundwater which eventually enters the harbour
- prevent point sources of nutrients such as from septic tank seepage and storm water drains
2. Investing in research to further understand the problem and identify viable management options
3. Reducing the public nuisance factor and hydrogen sulphide risk by working with Tauranga City and Western Bay of Plenty District Councils to remove sea lettuce from high use public areas around the Tauranga Harbour. Watch a New Zealand Herald video clip from one of our 2013 sea lettuce clean ups.
Read the report on Sea Lettuce and Nutrient Monitoring in Tauranga Harbour 1991-2010. See our research and monitoring page for other research reports about sea lettuce and other harbour issues.
You can help manage sea lettuce by collecting it from the foreshore and using it as a fertiliser or compost supplement inyour garden. Wash it and use it sparingly to avoid salt build-up in your soil. Find out more in this Sea Lettuce and the Garden booklet, produced by Tauranga HarbourWatch Inc.
First Tauranga Harbour sea lettuce clean up for 2016/17 summer. 28 November 2016 at Beach Road, Otumoetai.
Coastal Science Chair, Chris Battershill talks about sea lettuce and current research to manage it. December 2015
Tauranga Harbour Projects Manager, Bruce Gardner talks about how Regional Council is managing sea lettuce. December 2015
Why sea lettuce washes up at Matua - January 2017