“We’ll be looking at how we can best control the growth of the sea-lettuce beds and finding the most effective, practical means of removing the lettuce stranded on shore. Options to be considered for controlling growth include mowing and suction dredging operations by boat,” he says.

The Council currently removes large accumulations of lettuce from certain areas of the estuary three times during the summer months, when rotting sea lettuce emits a strong, sulphur-like odour. These areas are restricted to the high tide areas on the eastern side of the estuary, where the Council has resource consent to use tractor-mounted rakes and front-end loaders.

“We’ll be looking at whether other mechanical methods will work on the softer sediments in Humphreys Drive. Trials of alternative removal methods such as shore based suction may also be undertaken.”

The Council is funding research into the impact of sea lettuce on creatures that live in the estuary sediment through a $5000 research grant to Canterbury University. It is also seeking further research proposals on estuary ecology including sea lettuce.

Sea lettuce is a bright green algae found naturally throughout New Zealand and the world. It grows in sheltered rocky coasts and in estuaries; particularly shallow estuaries where light can penetrate the clear water and water temperatures are warmer. Nitrogen and phosphorus in coastal rivers or other discharges aid this growth. Weather patterns that influence the nutrient levels in the oceans also impact on the growth of sea lettuce in tidal waters.

Mr Couling says all these factors influence the growth of sea lettuce in the estuary and similarly, in other parts of the country.

“In Tauranga, for example, sea lettuce blooms have caused extensive problems in Tauranga Harbour.

“Weather patterns, of course, are beyond the Council’s control. Given the right conditions, the lettuce can multiply in spring and summer periods, breaking away and washing up on the shores and shallows of the estuary in smelly piles from November to March.”

He says the discharge of treated wastewater from the Christchurch WasteWater Treatment Plant is often said to be the cause of the sea lettuce.

“The discharge is a significant factor, but it is not the only cause of significant growth.”

Investigations by the National Institute of Water and Atmospheric Research (NIWA) concluded that there would still be the potential for significant growth of sea lettuce, even with the complete removal of the wastewater from the estuary and a large reduction in nutrient loads from the Avon and Heathcote Rivers.