New 'rock snot' theory rejected by NZ scientists

'Rock snot'. Photo / Stuart Munro

Controversial US didymo study not applicable to New Zealand, says expert.

Kiwi scientists have been left unconvinced by a new US study suggesting the pest didymo is not a recently-introduced foreign invader, but the result of native species responding to environmental change.

The microscopic algal pest, which can be spread by a single drop of water, has been found in more than 150 South Island rivers since it was first reported in the Lower Waiau River a decade ago. The so-called "rock snot" has not yet been found in North Island rivers and under strict biosecurity rules, people are legally obliged to prevent spreading it.

Scientists have maintained the pest was brought here but a new study, led by Dartmouth College and published in the journal BioScience, claimed the algae may have been native to much of the world for thousands of years. It suggested recent worldwide blooms of didymo were caused by a native species responding to changing environmental conditions rather than by accidental introductions, or the emergence of a new genetic strain.

The paper's lead author, Professor Brad Taylor, said didymo blooms were hastily attributed to human introductions or the emergence of new genetic strain because the absence of evidence was used as evidence of absence in many locations.

"Even in locations where rock snot had been recorded a century ago, this information was either ignored or the idea of a new genetic strain was adopted."

Algal blooms were often caused by excessive phosphorus and other nutrient inputs, but didymo blooms occur because phosphorus was low, the study found. When nutrients were rare, the algae produced long stalks that extended the cell into the water above to access nutrients, creating thick mats covering the river bottom.

"The paradox of didymo blooms in low-nutrient rivers is not really a paradox at all," Professor Taylor said. "However, the idea that low phosphorus can cause an algal bloom is hard for people to accept because we are all taught that more nutrients equal more algae."

The new research suggested rock snot blooms had become more common because of climate change and other human-caused environmental changes that were decreasing phosphorus to levels that promote the formation of didymo blooms.

But Kiwi scientists have disagreed with the study's assertion that this applied to New Zealand.

Niwa freshwater ecologist Dr Cathy Kilroy said while environmental changes might explain the expansion of blooms in the Northern Hemisphere, she believed didymo had been introduced here. While the study referred to processes that led to declining phosphorus in North American streams, there was no evidence those processes applied to New Zealand streams, she said. The South Island rivers affected by didymo had very low phosphorus long before the discovery of the first didymo blooms in 2004, and the subsequent rapid spread of didymo in the South Island indicated transport by humans.

"My view is that the expansion of blooms in the Northern Hemisphere starting in the late 1980s meant that didymo became much more common; ever-increasing international travel meant that it was inevitable that cells would eventually find their way to New Zealand," she said. "Once the cells were here, the many low-phosphorus waterways in South Island provided perfect habitat for blooms."

Dr Marc Shallenberg, a zoology research fellow at the University of Otago, agrees.

"Unfortunately, the all-or-nothing style of argument employed by the authors of this article provokes more than provides comprehensive answers to the complex problem of the widespread proliferation of didymo and other nuisance species."

What is didymo?
• A microscopic freshwater algal pest reported in New Zealand in the Lower Waiau River in 2004 and now found in more than 150 South Island rivers, but not the North Island.
• Can form a thick brown layer that smothers rocks, submerged plants and other materials.
• Can be spread by a single drop of water and people are legally obliged to prevent its spread.