Nasties in the water

Science and hydro field staff have detected significant growths of the potentially toxic river cyanobacteria Phormidium sp. in the upper Manawatu River and tributaries. These growths have been responsible for the death of dogs in many parts of the country in the past few years. It is highly likely that these growths are also occurring in the lower Manawatu, Mangatainoka and other rivers throughout the region at the moment. Horizons are currently monitoring the growth of cyanobacteria at State of the Environment sites and working with the Cawthron Institute to address the risks caused by cyanobacterial toxicity in rivers.

The following information should be helpful for people working or recreating in rivers over the summer:

• Growths of potentially toxic cyanobacteria are relatively common on the bed of stony/gravel rivers in summer, during periods of warm water temperature and low flow and have been observed from time to time throughout the Region's rivers.
• Cyanobacterial growths are black and 'slimy' and tend to occur on the bed of shallow areas in the river.
• Affected rivers will have a strong 'musty' smell.
• The growths/mats can become exposed on gravel beaches or drift downstream as flows drop over summer.
• Toxicity to dogs (and stock) generally occurs through scavenging and ingestion of the algal material within the river, or on gravel beaches.
• Human contact with toxic cyanobacterial growths may cause skin irritation, but the main risk is to animals who may eat the material.
• Growths are likely to persist throughout summer until removed by elevated flows after rain.
• Any suspect black growths seen by staff should be reported, and if possible a GPS reference and photo taken.
• Members of the public should be advised when walking dogs to avoid allowing their dogs to scavenge in or along the gravel beaches of rivers that contain obvious black growths on the bed.

Here is a link to some useful information from DHBs in the Acukland area.