What Have We Done?

Sumner Beach - Christchurch

Sumner 2008

The Press | Monday, 07 January 2008

The curse of the summer sea- lettuce bloom is back to irritate Christchurch beach-goers. Sea lettuce from the Avon- Heathcote Estuary has broken free from its usual habitat and is landing in clumps on Sumner Beach. 

Avon-Heathcote Estuary Ihutai Trust chairman Alex Drysdale said sea lettuce always flourished where humans spread waste – the estuary being the end point for Christchurch's treated sewage. 

"It is a little unusual but not unknown," Drysdale said of the recent Sumner invasion. "Given the hot weather and the conditions, sea lettuce would not normally make its way to Sumner Beach unless there had been a storm and it broke off, but there have been a few high winds here lately." Drysdale said the quantity of lettuce was just a symptom of the long-term pollution in the estuary which would be eased after the city's ocean sewage outfall was completed. "Everything that can be done has been done and it will be several years before nutrients in the estuary fall enough to inhibit sea-lettuce growth.

"It really is just a natural event until the pipeline is built."

Last year, the Christchurch City Council started work on the $87 million ocean outfall pipe which must be completed by September 2009 but is expected to be finished by the end of this year.Timing of the Matua sea lettuce bloom mirrors the timing of the Maketu Estuary sea lettuce bloom, which has been observed to annually coincide with seasonal pasture growth rates. Pasture growth rates are largely dependent upon seasonal weather patterns and upon soil nutrient levels. As seasonal weather patterns have remained unchanged the significant increase in sea lettuce spring growth rates must be attributable to an increase in nutrient levels. Sea lettuce is but one example of the disregard that has prevailed both within this country, and upon this planet for the natural health of the waterways. Water right applications to discharge effluent are currently being heard within this region and one might well ask what right they have to pollute our water. The rivers and streams of our world are mirrored by the arteries and veins of our bodies. 

http://www.stuff.co.nz/4346664a10.html

What is Sea Lettuce?

Sea lettuce is a bright green marine macro algae found around the world on rocks along sheltered rocky coasts and in shallow estuaries. The sea lettuces are collectively grouped under the category of Ulva species. Ulva is a macrophyte algae (as opposed to a planktonic algae), which grows in sheets two cells thick usually less than 30cm in diameter. Ulva is rich in nutrients such as vitamin A and B. Early Maori used the completely edible sea lettuce in cooking. Sea lettuce is still eaten today in salads and stews around the world. It is a very vigorous grower which adapts readily to changing conditions.

Rotting Sea Lettuce - Why such a problem?

Sea lettuce can grow very fast when nutrients, temperature and light conditions are favourable. Biomass can increase at a rate of up to 20% per day. Wave action causes the organism to break off from its growth beds and be washed ashore where it rots. The smell can be very offensive because of the high sulphur content of sea lettuce. The offensive smell can occur intermittently between late November and early April.

What Factors Influence Sea Lettuce Growth?

Increased levels of nitrogen and phosphorous in coastal waters in particular agricultural runoff is often carried down to the sea in the rivers. Sea lettuce grows more in spring. Growth rates are linked to temperature with growth decreasing below 15oC and above 20oC. Growth is light sensitive so the lack of light with increasing depth means that the algae prefers shallow water.

More information on the problem is available from the http://www.envbop.govt.nz website. use the 'search website' feature to look for 'sea lettuce'.

What Have We Done?