By Gerald Ford
Toxic algae in Masterton’s Waipoua River is likely caused by farming and may be impacting stream life, according to an agricultural scientist.
Mark Heath, an environmental scientist from Wellington Regional Council, was commenting on an outbreak of cyanobacteria, which is commonly known as toxic algae or algal bloom.
The nutrient build-up that causes it can be from rural or urban sources, Dr Heath said.
In the case of the Waipoua, the river runs by Queen Elizabeth Park and a few industrial sites, but “our assumption is most of it is coming from agricultural land use upstream”, Dr Heath said.
“The other key components are rainfall, high sunshine and warm temperatures.”
Cyanobacteria feeds off nitrogen in the waterways. In attaching itself to rocks the organism is able to draw freefloating nitrogen from the water by trapping material underneath. It appears as black or brown “algal mats” on the riverbed.
The material can be dangerous and even fatal to dogs and cause skin irritation for swimmers. Warning notices have been placed along the Waipoua River bank jointly by Greater Wellington Regional Council, Masterton District Council and Regional Public Health.
Besides the health risks to people and dogs, there has been limited research on the effects of cyanobacteria on other stream life, Dr Heath said.
“We don’t know enough.”
However there is evidence that cyanobacteria is toxic to invertebrates including some species of mayfly, which are a food source for fish.
Trout exposed to sub-lethal levels of cyanobacteria showed an increased metabolism, which is a sign of stress.
Greater Wellington note on its website this summer that “the amount of black or brown toxic algal mats growing on the river bed has reached dangerous levels and detached mats are starting to wash up on the river’s edge”.
The Greater Wellington website also lists the Waipoua site and a swimming spot nearby at the Ruamahanga River along Te Ore Ore Road as providing a “low risk of illness”.
The Ruamahanga River at the Cliffs has a general rating of “caution”, and swimming spots at Kokotau, Morison’s Bush and Waihenga River have a general rating of “moderate risk of illness”, although the most recent tests do not breach bacteria alert levels.
Monitoring of Wellington waterways and coastal areas is carried out by Greater Wellington Regional Council and district councils in partnership with Regional Public Health on a weekly basis over the summer months. Results of the monitoring and any warnings are posted to http://www.gw.govt.nz/is-it-safe-to-swim/.
Other information about toxic algae, including a guide of what to look for, can be found at www.gw.govt.nz/toxic-algae-faqs.
