Dogs are at risk and swimmers are being advised to keep out of the lower Maitai River after the deadly blue-green algae cyanobacteria has once again reached high levels.
Several dogs have died in previous years after eating clumps of the algae, which has also been a problem in the Waimea River.
In a release yesterday the Nelson City Council said the Maitai was affected from the motor camp downstream, and the problem was likely to persist over summer.
This means that most of the popular Maitai swimming holes will be out of bounds.
The council said temporary signs warning people to keep away from the water were being put up at the main entry points to the recreation areas. These would be replaced soon with permanent information and warning signs.
The algae was black or dark brown, typically with an oily sheen and musky smell, and could be easily seen along the river, it said.
Medical officer of Health Ed Kiddle said people had to assume that where there was algae, toxins might be present.
"An early indication of problems in the water is dogs getting sick after eating the algae, however there is a risk to humans if the levels of toxins in the water get high enough," Dr Kiddle said.
Symptoms from water contact may include allergic-type reactions like skin rashes, runny nose and eyes and sore throat.
If water with a high level of toxins is swallowed more serious symptoms can occur such as effects on the nervous system.
In dogs this usually shows as salivation, weakness and convulsions. People may have numbness and tingling and feel dizzy. Other toxicity symptoms include diarrhoea and vomiting and abdominal pain.
Cawthron Institute scientist Susie Wood is working in collaboration with council officers to learn more about why the algal blooms seem to have intensified this year.
The Cawthron monitoring programme is assessing cyanobacteria at six sites along the Maitai, including measurements of water temperature, flow, nutrients and sediment loads at each site to try to understand the triggers for cyanobacteria growth.
"The blooms usually occur during the summer months, when there has been a stable period of little to no rainfall and warm conditions. Our current research is exploring how nutrients and fine sediment in the river - which act as the algae's food source - are related to these bloom events," Dr Wood said.
"These toxic river algae have killed over 100 dogs around New Zealand in the last five years and I advise people to take these warnings seriously."
She has previously told the Nelson Mail that cyanobacteria is "one of the most toxic natural compounds you'll ever come in contact with".
Just a couple of grams of the algal mat formed in a bloom were sufficient to paralyse and kill a small dog within five to 15 minutes, she said, with the danger to humans particularly related to small children, who might ingest the algae while playing at the water's edge or swimming.
Council communications manager Angela Ricker said the most important thing people could do was keep themselves and their dogs out of the river where there's algae.
"If anyone comes into contact with the water and experiences a reaction they should seek urgent medical attention."
Cyanobacteria was present in all of Nelson's waterways, but there had been no reported problems in other rivers around the region this summer.
Meanwhile, the council is still waiting for the results of water and tissue sample tests taken after thousands of pilchards died in the lower Maitai last week. A chemical spill has yet to be ruled out as the cause, though the saltwater fish might have died after being driven into the river by predators such as kahawai in the harbour.
Dog owners were advised to swim their pets from the dog walking area at Tahunanui Beach, she said.
Further information on blue-green algae along the Maitai is available on Nelson City Council website at: nelsoncitycouncil.co.nz.
- The Nelson Mail
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