The Maitai is likely to be out of bounds for swimmers over the summer after a potentially lethal algae was found in parts of the river.
Toxic blue-green algae (also known as cyanobacteria) is currently present at very high levels.
Blue-green algae produce toxins that can be potentially lethal to dogs and harmful to humans.
The lower Maitai from Maitai Camp downstream is seriously contaminated and city council expects the problem is likely to persist over summer.
Temporary signs warning people to stay out of the water are being erected at the main entry points to the lower Maitai recreation areas.
These will be replaced soon with permanent information and warning signs. The algae is a black or dark brown colour, in appearance, typically with an oily sheen and musky smell and can be easily seen along the river.
Medical Officer of Health Dr Ed Kiddle says, "People have to assume that where algae is present, toxins may 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.
"Symptoms from contact with water 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.
"Given that the problem has shown up early in the Maitai River the research being undertaken is very important to try to identify why this has occurred."
Dr Susie Wood from the Cawthron Institute 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 and 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. 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" says Dr Wood.
Council river and stream health monitoring across 28 sites indicated that algal cover was relatively low in September, with less than 5 per cent cover at sites on the Maitai, Wakapuaka and Lud rivers. In November a noticeable increase in cyanobacteria was observed. As a result, the city council and the Cawthron developed a more detailed monitoring programme.
Nelson City Council Communications Manager Angela Ricker says "We're working closely with Cawthron and NMDHB's Public Health team to minimise the risk to public health. We want people to be fully aware of the risks and to take the warnings seriously. The most important thing people can do is 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."
Further information on blue-green algae along the Maitai is available on Nelson City Council website at: http://www.nelsoncitycouncil.co.nz/environment/water-3/freshwater-2/cyanobacteria-toxic-blue-green-algae/
Cyanobacteria is present in all of Nelson's waterways but has particularly high coverage in areas of the Maitai.
There have been no reports of problems in other rivers around the region and dog owners are advised to swim their pets in the permitted area at Tahunanui Beach.
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