One of the popular Wairarapa swimming spots along the Ruamahanga River, between Double Bridges and the Waingawa River. Photo: Supplied/LDR
An expert is warning Wairarapa swimmers about risks caused by contamination of the region's rivers.
Greater Wellington Regional Council senior environment adviser Penny Fairbrother says, while toxic algae in the rivers is currently at low levels, other risks remain.
"We monitor numerous spots on Wairarapa rivers," Fairbrother said. "At present, the vast majority of sites are showing it's not suitable for swimming."
While algal bloom was unusually low for this time of year, there was no room for complacency, she warned.
"Rivers can be contaminated with high levels of bacteria, mostly as a result of heavy rainfall.
"That's as a result of high rainfall washing contaminants into the river making it unsuitable for swimming."
Bacteria in rivers could cause infections like gastroenteritis, which make swimming dangerous.
She said four of Wairarapa's five rivers were at either red or orange levels for bacteria - the Ruamahanga, Waiohine, Waingawa and Waipoua. Only the Tauherenikau River was safe for swimming.
The 'golden rule'
"The rivers should clear back to green level within 48 hours, depending on rainfall levels," said Fairbrother. "The golden rule is not to swim for two days after heavy rain."
A green level indicated it was safe for swimmers and pets.
Fairbrother said rain could be a double-edged sword - too much rain risked elevating bacteria levels, while too little meant conditions became ripe for toxic algae to bloom.
There was a 'Goldilocks' period between heavy rainfall and very low rainfall levels.
"Ideal conditions for toxic algae are warm weather, stable river flows, and no or little rain. Once we have a decent rainfall, it has a scouring effect, which washes the algae off the rocks.
"It's almost like a washing machine effect."
She said, once the algae washed down the river and reached the sea, it couldn't survive.
"Beautiful warm dry periods allow the algae to flourish."
She said, until recently, Wairarapa rivers had toxic algal bloom at low levels, but that had changed at two sites.
Last week, the Waipoua River at Colombo Rd and the Ruamahanga River at Te Ore Ore were both at amber levels for algae.
"That indicates the presence of moderate levels of algae."
An amber alert indicated relatively low risk for dogs and swimmers.
"As a rule of thumb, dogs should never be allowed to scavenge from the river bank," Fairbrother said. "While the risk remains low at this level, people should exercise caution."
Danger for dogs and swimmers
There had been no algae red level in Wairarapa this year so far.
That alert level would mean more than 50 percent of the river was covered in black mats or there were detached mats.
Detached mats were big patches of bloom, which could float to the river's edge.
"That's when it becomes really dangerous for dogs," she said. "Dogs really love the smell and will eat the mats.
"It's highly poisonous for dogs. It releases a neurotoxin, and dogs and other mammals are highly susceptible to that."
Fairbrother said an algal mat the size of a 50-cent coin could kill a dog, if ingested.
"We are also really worried about young children, who could put it in their mouths."
She said there had been signs of algae early this season, but regular bouts of rain had made the difference and kept the risk low.
Do not relax
It was still too early to predict what could happen for the rest of the season.
"Do not relax," she warned. "If we get 2-3 weeks of amazing weather in February or March, there is every possibility of a big algal bloom.
"We monitor right through until April."
LDR is local body journalism co-funded by RNZ and NZ On Air
