Environment Canterbury says that despite cooler temperatures and higher river flows in most of the region, many of Canterbury's hill-fed rivers remain prone to developing potentially toxic algae. Dog owners are also advised to be vigilant around small ponds and slow-flowing waterways in both urban and rural areas as potentially toxic algal mats and scums can be harmful to dogs.
Rivers where large mats of the cyanobacterium (blue-green algae) Phormidium have been observed in recent weeks include:
Hurunui District: Waipara River at Stringers Bridge and Teviotdale Bridge – Phormidium is present at both of these sites in areas where water is shallow.
Waimakariri District: Ashley River/Rakahuri – Extensive growth of Phormidium has been observed in the lower areas of the Ashley River below the Ashley Gorge, particularly near the Rangiora-Loburn bridge and upstream of the SH1 bridge. There are small amounts of Phormidium at the Ashley Gorge.
Selwyn District: Selwyn River – In the upper Selwyn River near Whitecliffs Phormidium is growing in patches in shallow water.
Timaru District: Opihi River – Large amounts of Phormidium were observed at Saleyards Bridge, the SH1 Bridge and Waipopo.
Timaru / Waimate districts: Pareora River – There is extensive growth of Phormidium in the Pareora River at Brassells Bridge, the Pareora Huts and upstream of Evans Crossing to the Dam. Mats are thick and detaching from the rocks in some places.
Waimate District: Otaio River – Large amounts of Phormidium have been observed upstream of the bridge at the Otaio Gorge.
Where river flows have removed algal mats from the river bed these may accumulate further downstream at the river’s edge and river users, particularly dog owners, are asked to be vigilant.
In addition to the rivers named above, Environment Canterbury is issuing a general warning to river users throughout the region that Phormidium is likely to be present in some river areas. Dog owners should keep their animals away from any areas of rivers where dark brown/black algal mats are present. Dogs are attracted to the musty odour of the mats and may become ill if they eat them. People should not swim in areas where there are large amounts of dark brown/black algae.
The potentially toxic, naturally-occurring cyanobacterium (blue-green algae), Phormidium, is found in rivers throughout the country, but only forms problematic thick blooms at some sites under the right flow conditions. In thick blooms, the dark mats can peel off the substrate and show a lighter coloured underside. Although district or city councils may place signs warning of Phormidium, these may not be seen at the numerous river access points, hence the need for people to treat every low-flowing river cautiously.
Please report any sightings and locations to the regional council in Timaru or Christchurch on 0800 324 636 (0800 EC INFO).
For further information or in the case of illness or suspected illness, seek advice from your doctor. If animals become sick, contact your veterinarian.
Further information: Dr Cathy Kilroy, NIWA (on behalf of Environment Canterbury) on 03 343 7883, or speak to your environmental health officer at your local district or city council, or for further information on health risks contact Community & Public Health on 03 364 1777.
Or see:
http://ecan.govt.nz/services/online-services/monitoring/swimming-water-quality/Pages/river-warnings.aspx
Phormidium in shallow water
Phormidium mats amongst green algae at water's edge