Toxic blue-green river algae 'a warning sign'
The appearance of blue-green toxic algae in the region's rivers is considered a warning sign of their decline.
Scientist Dr Susie Wood of Cawthron Institute said new research showed the proliferation of the naturally occurring algae (cynobacteria) was brought about by low summer water flows and nutrient concentrations in the rivers, specifically elevated levels of nitrogen associated with surrounding land use.
"It's an early warning that these rivers are starting to decline," Dr Wood said.
The toxic strain of the algae can be fatal to dogs and considered harmful to humans.
Monitoring of the district's northern rivers showed low levels were present in the Motueka and Lee rivers and high levels in the lower reaches of the Waimea River between the Wairoa confluence and Appleby Bridge, where a dog died two weeks ago after swimming.
Dr Wood said the algae's pre-Christmas arrival in the region might be considered early, but the toxic algae was present in some parts of New Zealand from October. It was not just associated with pastoral land use and also appeared in rivers such as Wellington's Hutt, where it passed through urban reaches. The algae grew on the river's bottom from where it sloughed off earthy-smelling mats on the riverbanks.
"Some rivers have it present for many kilometres," she said.
Dr Wood said national research of the algae's occurrence and increase started in 2005 after a number of dogs died after swimming in the Hutt River.
The research work was carried out by a range of scientists including researchers and students from Cawthron and Canterbury and Victoria universities.
The research was supported by most of the country's regional councils with some government funding. Tasman District Council was not party to the programme.
"Until this year I would have said its incidence was patchy in their region," Dr Wood said.
"But the Waimea is a bit of a concern and now there are some spots in the Motueka."
Tasman District Council environmental information manager Rob Smith said while nitrate levels were a factor, the algae's bloom was mainly driven by low summer river flows.
Signs had been erected at the Waimea River warning swimmers, dog owners and parents of young children of the risk. Warning signs would be erected elsewhere when needed, he said.
Environmental resource scientist Trevor James said the district's northern rivers would be monitored weekly over summer. The Buller catchment was only monitored near Murchison for water quality, not algae.
Tasman did not yet buy into national research on the algae, said Mr Smith.
"We haven't had an issue like this before," Mr James said.
- The Nelson Mail
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