Jane Nees BOP Regional Councillor www.janenees.co.nz

Tauranga Harbour periodically experiences severe blooms of sea lettuce, which reach nuisance proportions at times – especially where it is washed onto beaches.
Sea lettuce grows both in the inter-tidal and sub-tidal reaches of the harbour, but is most abundant in the shallow sub-tidal habitat.

It can grow at very fast rates and it is estimated to be able to grow up to 28 per cent per day in the Tauranga Harbour given the right nutrients, light and temperature levels.

The Bay of Plenty Regional Council recently received a report which provides an updated picture of sea lettuce abundance in the harbour and the environmental factors which influence sea lettuce blooms. It is obvious from the report that the abundance of sea lettuce is the result of a complex interaction of factors.

Key points from the report are:

The growth rate of sea lettuce increases up to a maximum water temperature of between 16-20C and falls to zero growth at 25C. So autumn and spring in sub-tidal zones has near optimum growth conditions and reduced sea lettuce in summers with sustained higher water temperatures.

Our shallow harbour and high sunshine hours mean there are few light-related limitations to growth of sea lettuce.

Nutrients are the other main factors contributing to growth rates. Key nutrients affecting growth are nitrogen and phosphorous – the higher the concentration, the faster the growth, although there is an ideal ratio of both.

Wave energy, current speeds, wind direction and storm action all have an impact on where – and how much – sea lettuce ends up in our harbour.

There is a clear relationship between sea lettuce blooms and climatic patterns. Sea lettuce is more abundant during El Nino periods and less likely to bloom during La Nina periods.

The old theory of deep sea up-wellings of nutrients during El Nino periods causing additional nutrients in the harbour, resulting in sea lettuce blooms, is not substantiated by recent analysis of nutrients in tissue and water. This latest research doesn't show any evidence of a climate associated deep ocean nutrient influx to the harbour.

Other controlling factors like cooler water temperatures in summer in El Nino events are likely to be influencing growth rates.

It is recognised that there are land-based contributions to sea lettuce blooms. There is clear seasonal and geographic trends in the extent to which freshwater nutrient-inflows affect nutrient content in sea lettuce tissue. It is difficult, however, to correlate variations in terrestrial input with overall sea lettuce abundance in the harbour.

The bottom line is that a lot more work needs to be done to fill gaps in our knowledge revealed by the recent report because of the underlying complexity of the interacting variables influencing sea lettuce growth. Costs and options for completing the recommended monitoring and research work on sea lettuce will be considered as part of the development of the council's Tauranga Harbour Programme.

If you have any views on this, or any other issue, please email neesj@xtra.co.nz or phone me on 07 579 5150. Or visit www.janenees.co.nz