Manawatu¯ swim spots may be toxic

A potentially toxic bacteria in several Horowhenua and Manawatu¯ swim spots has been lingering for weeks.

High levels of cyanobacteria was found in the Ohau, Mangatainoka and Pohangina rivers on December 20, prompting the Midcentral District Health Board to release a health warning.

Cyanobacteria can produce toxins harmful to people that swallow it or come into skin-contact with it. This can cause vomiting, diarrhoea and skin irritations.

Yesterday, Midcentral couldn’t confirm whether the algae still posed a health risk but an online update from Horizons Regional Council’s swim spot monitoring team showed it was still unsafe to swim at spots in Ohau, Mangatainoka, Pohangina, Manawatu¯ River, Makakahi, Mangahao and Lake Horowhenua.

Horizons science and innovation manager Abby Matthews said bacteria levels could change during the week so swim spots were monitored weekly and updated on the council’s website on Thursdays or Fridays.

Cyanobacteria warnings were lifted two weeks after no monitored sightings, she said. Commonly, there was a higher occurrence of cyanobacteria in warm weather but not all cyanobacteria was toxic, Matthews said. ‘‘Some types of cyanobacteria can produce toxins, although the factors that trigger toxin production are not completely understood.’’

It occurred naturally, so it could not be entirely removed, she said. Interventions such as riparian planting and landuse management could help reduce the occurrence of cyanobacteria in some catchments, but drivers were complex and site-specific, Matthews said.

Warnings can also be seen on the Land Air Water Aotearoa website and at the swim spots.

Horizons measures about 80 swim spots in the wider Manawatu¯ from November to April each summer to determine their swimmability. Horizons uses a traffic-light system to highlight how safe a swim spot is. Green is good to go, amber means there are potential health risks and red is unsafe.

Often, beaches were deemed safer for swimming as low levels of E coli and cyanobacteria were found. Yesterday, nine beach spots were classed as safe for swimming, one posed a potential health risk and the Tasman Sea at Foxton Beach was deemed unsafe.