Key Points:

A naturally occurring native green alga that grows in sheets and in sheltered harbours in temperate conditions can be a nuisance.

Early Maori used the edible sea lettuce in cooking. Fish, such as parore, feed on it.

Water movements cause sea lettuce to break off from its growth beds and drift ashore where it rots. Its high sulphur content means it smells as it rots.

Sea lettuce has started to spread along Tauranga's beaches and harbour again but scientists say it shouldn't be as big a problem this summer as it has in the past.

Environment Bay of Plenty coastal scientist Stephen Park said the amount of nutrients in the harbours and estuaries was usually low and the growth of sea lettuce slow, but large blooms happened regularly in spring.

The blooms tended to match the El Nino weather pattern, when persistent westerly winds drive coastal water offshore. In these conditions extra nutrients are available in the ocean, causing the blooms.

This summer, however, predictions are for La Nina to continue through summer.

"This is good news and means growth this summer should be lower than average," Mr Park said.

Sea lettuce was not an indication that water quality in the harbour had dropped.

"It's a naturally occurring green alga that is native to New Zealand and the clear, shallow waters of Tauranga Harbour provide an ideal habitat for it," Mr Park said.

Tauranga City Council will carry out some limited clean-ups over summer. Priority is given to larger blooms in areas of high public use, said Emily McNie, council park ranger for the coast and harbours.

The council also helps residents collect sea lettuce from beaches and funds clean-ups next to harbour reserves after large blooms.

- Bay of Plenty Times