It continually annoys when articles on sea lettuce blooms in Tauranga Harbour continue to blame the weather for a human problem. There is no evidence at all that the El Niño cycle has any direct relevance. The El Niño myth is convenient for EBOP as it means they do not have to take any responsibility for cleaning up the harbour from farm runoff, urban development or sewage spills.
I have been leading a sea lettuce research project together with top students and graduates, and our findings were recently presented at a Coastal Management Conference at Whitianga. The pattern of nitrogen assimilation in sea lettuce in the Tauranga Harbour compared with that found near Tuhua clearly indicates that sea lettuce is taking in nitrogen from the land, not the deep ocean. Oceanographers also agree that it is not possible for nutrients from deep water to enter the shallow harbour, as nutrient mixing occurs at depths below 20 metres.
The myth of an association with El Niño came about through an unpublished report by EBOP showing more unattached sea lettuce during the El Niño cycle. This can easily be explained through an indirect effect; more rain in the El Niño cycle, washing more human derived nutrients from the land and contributing to more sewage overflows.
Dr Michael C Morris
Lecturer in Environmental Management
Bay of Plenty Polytechnic