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BAY OF PLENTY REGIONAL COUNCIL TOI MOANA

Sea Lettuce FAQs

Frequently asked questions

What is sea lettuce?

Sea lettuce is a green seaweed.  There’s about five different species of it in Tauranga Harbour.

These are a mix between species that normally grow inside harbours and those that exist on offshore reefs.  The plant can grow prolifically and doesn’t need to be attached to the seabed to do so.

Sea lettuce grows from spores that attach to shells, rocks or other hard surfaces in the sea bed. It feeds on nutrients that come from both natural oceanic sources and land run-off.

Where does sea lettuce grow?

Sea lettuce is native to New Zealand. It is widespread throughout the Bay of Plenty and found throughout New Zealand including the sub Antarctic Islands.  It even regularly blooms in the most pristine locations such as the Auckland Islands.

Sea lettuce is also common in other parts of the world. There are many species and they can also bloom in very large concentrations. An example was seen globally during the 2008 Olympics in Qingdao, China where the sea lettuce bloom was so severe it stopped the sailing competition.

Why does sea lettuce grow here and end up on our beaches?

Tauranga Harbour has shallow flats and channels that are perfect for sea lettuce to grow in. It has plenty of nutrients here and good water clarity for photosynthesis.

Sea lettuce gets ripped off the sea-bed by strong on-shore winds or heavy seas. It keeps growing as it floats around and then gets washed up on the beaches with on-shore winds.

When washed up piles of sea lettuce decompose on the beaches or in shallow areas, they can release more nutrients into the harbour system, most likely promoting yet more sea lettuce growth.

What are the councils doing to manage excessive sea lettuce growth?

Bay of Plenty Regional, Tauranga City and Western Bay of Plenty District Councils are working together to tackle the sea lettuce problem in three ways:

  1. Reducing as much nutrient run-off from land as we can by entering Tauranga Harbour by: 
    • working with landowners to reduce agricultural runoff by fencing and planting waterways and adopting more sustainable land use practices.
    • stopping untreated point source discharges of nutrients such as from septic tank seepage and storm water drains through our consent compliance monitoring, sewerage and storm water management and pollution audit work. 
  2. Reducing the public nuisance factor by clearing sea lettuce build-ups from popular foreshore areas as needed.
  3. Regional Council has an ongoing environmental monitoring programme in place and is investing $210,000/year to support the University of Waikato Coastal Chair of Science and sea lettuce specific PhD projects to research and further understand the problem and identify viable management options.

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  

What are the rules about collecting sea lettuce from the water or foreshore?

Recreational use:

Recreational gatherers can collect sea lettuce from the water or shore as long as their methods are not in contravention of the Fisheries Act or associated regulations e.g. using a net to gather seaweed in a set net prohibited area.

Commercial use:

Sea lettuce collection that involves disposal of sea lettuce to commercial composting or orchard spreading is deemed to be commercial.

The commercial harvesting or collection of beach-cast sea lettuce is permitted in most parts of the Tauranga Harbour, but prohibited in all other estuaries of the Bay of Plenty. Where permitted, a fishing permit is required from Ministry for Primary Industries and all catches must be reported.

The commercial harvesting or collection of sea lettuce from in the water is currently prohibited in Tauranga Harbour and other Bay of Plenty estuaries.

What happens to the sea lettuce that councils collect from the foreshore?

Wherever possible, the sea lettuce is rinsed, dewatered and taken to a commercial composting facility where it’s washed and processed along with other green waste. In the rare instances that the collected sea lettuce is too badly decomposed, contaminated with sand or there’s too much of it for the compost facility to handle, it may be disposed of to landfill.

How much sea lettuce do the councils collect from Tauranga Harbour every year?

It depends on seasonal factors like wind, water temperature and rainfall that affect growth rates and the amount that gets washed onshore. Collection tonnages and costs since 2009 are as follows:

Year

2009/10

 2010/11

2011/12

 2012/13

2013/14

2014/15

2015/16

El Nino / La Nina status

Weak -moderate    El Nino

Very strong La Nina

Moderate La Nina

Neutral

Neutral

Neutral

Strong El Nino

Sea lettuce collected (tonnes)

2,815

1,187

280

1,213

280

350

496

Total cost (shared by Bay of Plenty Regional and   Tauranga City Councils)

$101,100

$41,400

$42,154

$69,742

$17,114

$35,659

$69,721

Is there more sea lettuce around now than there used to be?

Records from the Regional Council show that sea lettuce levels fluctuate from year to year. There have been blooms in the past and some of these have been substantial including peaks in previous El Nino years such as in 1991/92 and 1997/98.

The Regional Council’s work, now supported by new PhD research, shows a strong link to El Nino conditions where offshore winds move surface water out to sea and this is replaced by deep nutrient rich water from the ocean, therefore fuelling sea lettuce growth.

The graph below shows annual sea lettuce abundance as a percent of the long term average, since 1991. The graph also shows the correlation between sea lettuce abundance and the southern oscillation climate index (a measure of El Nino weather patterns).  

Can sea lettuce be used for anything?

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

It can be eaten as long as it’s not collected from anywhere near a stream or drain inflow immediately after heavy rain. To avoid illness from water-borne bacteria, it should only be gathered from a clean area of the harbour and thoroughly washed in clean fresh water or cooked before consumption.

Regional Council has previously funded trials on the use of sea lettuce for orchard mulching and biofuel conversion to help inform potential business ventures.

Sea lettuce is being used overseas and in some instances it is produced through aquaculture ventures.

The Bay of Plenty Chair in Coastal Science (funded by Bay of Plenty Regional Council) is leading collaborations with international researchers to see how their discoveries could be applied here such as for conversion to biofuel and use as high grade fertilizer.

What research is underway to help manage sea lettuce?

Locally, three PhD projects examining aspects of sea lettuce ecology are complete and in review. Another further PhD project has just been started.

  1. Julien Huteau researched nutrient uptake by sea lettuce and found that it can uptake nutrients and some pollutants from the harbour and release them again when the plant decomposes. The uptake patterns are different from those of sea grass and that there can be an antagonistic exchange between the two. He also found that sea lettuce transports nutrients and some metal pollutants throughout the harbour, making this available to organisms that eat the plant.
  2. Alex Port looked into drivers of sea lettuce in the southern Tauranga Harbour and found that nitrogen rather than phosphorus was the prime agent that triggered sea lettuce blooms and determined the extent of them. The nitrogen was sourced both from land runoff and ocean upwelling events. Mr Port also created a model that can be used to predict how much sea lettuce will turn up, and when based on measured nutrient loading. His model will be available next year as new information on nutrient sources is uploaded.
  3. Clarrise Nieman has been examining how sea lettuce interacts with or impacts on other marine life. She’s found that sea lettuce can impact mud flat communities by smothering them.
  4. Ben Stewart has just started research on nutrient sources and re-cycling in the harbour. This work is expected to quantify what’s in the sediments and what enters the harbour from land run-off and groundwater aquifers, as compared to ocean derived nutrient sources. Mr Stewart’s work will feed into the use of the model that Alex Port has developed. He’ll be estimating the age of groundwater that enters the harbour to help identify any legacy issues that might be contributing to sea lettuce growth. That work will be ongoing next year, initial results are expected to be available in early 2016.

The PhD findings are complemented by other coastal research work, including: 

  • Manaaki Taha Moana’s assessment of nutrient and species interactions in the intertidal zone throughout the harbour and has now started exploring the subtidal channel areas. 
  • MV Rena related mapping of near shore coastal habitats which have identified shallow emergent reefs with sea lettuce on them.
  • New work starting in 2016 to map sea grass health using drones. That will confirm the nutrient sources and sinks in the harbour as well as exploring the interactions between sea grass health and sea lettuce abundance.