Sea lettuce is the bright green algae that grow around the world in sheltered rocky coasts and in estuaries. Sea lettuce grows particularly well in 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.

The growth of sea lettuce in the Avon Heathcote Estuary is influenced by all of these factors.

The nuisance caused by the sea lettuce results from the large amounts of plant material that is growing over some winter seasons. Given the right conditions it can multiply in the spring and summer period breaking away and washing up on the shores and shallow areas of the estuary in smelly heaps.

The discharge of treated wastewater from the Christchurch Wastewater Treatment Plant is often said to be the cause of the sea lettuce. While the discharge will be one factor in the growth of sea lettuce it is not the major cause of excessive growth.

As part of the investigations into wastewater solutions the Council sought the best scientific advice we could obtain, using consultants from NIWA. Its answers indicated that complete removal of the wastewater from the estuary would reduce sea lettuce by up to 50% in some high-growth locations, but overall the reduction would be only about 20%.

NIWA also concluded that there would still be the potential for significant growth of sea lettuce with complete removal of the wastewater and a large reduction in nutrient loads from the two rivers.

The reason for this is that growth is limited as much by the transparency of the water, and by the shearing effect of wind and tidal flows, as it is by the supply of nutrients.

The Christchurch City Council responds to the nuisance caused by sea lettuce by picking up large accumulations in certain areas of the estuary. These areas are restricted to the high tide areas on the eastern side of the estuary. However, when the whole of the inter-tidal areas are covered in sea lettuce it is not possible to remove it all.

Other local authorities have similar issues with sea lettuce and find that in the years when large amounts become a nuisance the only solution is to pick up the sea lettuce from the high tide area.