Low river flows lead to cyanobacteria warnings

Environment Canterbury reported today that river flows around the region remain low, resulting in cyanobacteria warnings at a number of swimming sites.

benthic-cyanobacteria.jpgSurface Water Science Manager Tim Davie said a small amount of rain in South Canterbury in the middle of the week helped some rivers and farmers a little, but was not enough to alleviate the problems from the prolonged dry spell.

“The long dry period has meant many rivers have irrigation restrictions in place, and has also led to problems with cyanobacteria at several popular swimming spots. Warnings are in place at 11 swimming sites, eight of which are in South Canterbury,” Dr Davie said.

“Cyanobacteria such as phormidium grow well during long dry periods where there are low flows and warm temperatures. Unfortunately this growth can proliferate into toxic mats that detach.”

Environment Canterbury works closely with the Canterbury District Health Board and territorial authorities to make sure warnings are in place at popular swimming sites. However, phormidium is also likely to be present elsewhere.

Canterbury Medical Officer of Health Dr Alistair Humphrey said the algae look like dark brown to black mats and can produce toxins harmful to people and animals.
"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips," Dr Humphrey said.

"If you experience any of these symptoms, visit your doctor immediately; also let your doctor know if you've had contact with dark brown/black algal mats or water in affected areas. Pets should be taken to a vet immediately if they show signs of illness after coming into contact with algal mats.”

Reticulated town water supplies are safe but people should not drink water from rivers with phormidium present. "Even boiling river water does not remove the toxin, so it should not be consumed," Dr Humphrey said.

Benthic cyanobacteria such as phormidium are naturally occurring throughout New Zealand. When they proliferate they are cleared out by flushing flows from high flows (from small to larger floods). A lack of flushing flows and sustained warm weather means ideal growing conditions for phormidium.

There is information on toxic cyanobacteria on the Environment Canterbury website, including a list of river sites where there warnings in place.
For information on cyanobacteria in rivers go to http://ecan.govt.nz/services/online-services/monitoring/swimming-water-quality/Pages/Potentially-Toxic-Cyanobacteria.aspx
For current river flow information go to www.ecan.govt.nz