Claims that the weather causes Tauranga sea lettuce blooms are ignoring evidence that pollution is a major factor, says Dr Michael Morris.

The freelance researcher and tutor is challenging a Bay of Plenty Regional Council report that says sea lettuce blooms are mainly caused by El Nino weather patterns.

A researcher says Tauranga's sea lettuce problem is linked to pollution from the land.

The El Nino westerly winds are said to drive coastal water offshore causing an upwelling of nutrient-rich deeper oceanic water which then enters the estuaries, feeding sea lettuce blooms.

Michael, a former staff member at the Bay of Plenty Polytechnic's marine and environmental management faculty, says El Nino is being favoured against strong evidence that pollution is the main cause.

He says the regional council report ignores recent research findings that there are more nutrients flowing into Tauranga harbour from the land than there are from the ocean.

'The report dismisses concerns that land based nutrients may play a part, by citing cases where point sources of nutrients (the technical term for nutrients that come from a single identifiable source, such as an outfall from a sewage treatment plant) have declined since the 1990s with no decrease in sea lettuce abundance,” says Michael.

'I am surprised that nobody writing or reviewing the report is aware that most pollution of our waterways is now caused by non point sources, especially from intensive farming, but also from storm water overflows, septic tanks, and urban development.

'These are harder to control than point sources because it is not possible to pinpoint one particular culprit.

'It is also considered politically unwise to antagonise farmers, which is why non-point courses are often ignored or explained away.”

In the Waikato, the regional council has discovered most Waikato River pollution is caused by dairy effluent, and not the sewage works at Hamilton and Ngaruawahia, says Michael.

The New Zealand coastal management conference in Whitianga in November 2010 was told the rivers draining into the Firth of Thames through dairy country are more polluted than even the Manawatu, which is one of the worst rivers in the world as far as pollution from excess nutrients is concerned.

Research by Julien Huteau and other BOP Polytechnic students presented at the Whitianga conference showed that nitrogen levels in sea lettuce declines the further the sea lettuce is from land, indicating land based nutrients.

'It is surprising that our findings were not even mentioned in the report,” says Michael.

'My students' studies from 2009 and 2010 confirmed that non-point sources are also responsible for pollution in the Waimapu stream, which flows into the town reach and would have a significant effect on the harbour ecology.”

E. coli and nutrient abundance in the Waimapu stream steadily increases with distance downstream, a clear indication of non-point pollution sources.

'I am not saying that the El Nino Southern Oscillation cycle is not important, however, we cannot control the weather, but we can control the nutrient loading into the harbour.”