No cyanotoxins despite blue-green Lake Omapere algal bloom

23 Feb 2018

Independent tests have not detected any cyanotoxins in water from algal bloom-affected Lake Omapere and the Utakura River. 

Colin Dall, the Northland Regional Council’s Group Manager – Regulatory Services, says an algal bloom in the lake had recently ‘tipped over’ in spectacular fashion, turning the lake and receiving river waters bright blue and green topped with a meringue-like white foam. 

Despite this, results received 4.15pm Friday 23 February on water samples collected by the regional council over the past week did not detect any cyanotoxins associated with the bacteria. 

However, while the toxins – which can pose health risks – were not present, the tests did show the cyanobacteria that can produce those toxins were present at very high levels. 

Earlier tests by the council itself this week had shown the water was also very low in oxygen, which Mr Dall says was probably to blame for a number of eel deaths in the area. 

Despite the fact no cyanotoxins had been found, Mr Dall reiterated earlier advice for people not to swim in the water or use it for household or stock supply. 

“The regional council will continue to sample and monitor the lake and Utakura River until the algal bloom recedes, which could still be some time away.” 

He says under the government’s ‘NZ [Interim] Guidelines for Cyanobacteria in Recreational Fresh Waters’, the regional council is responsible for sampling and testing water for cyanobacteria and associated toxins. 

It notifies the relevant district council and Northland District Health Board (NDHB) of the results, and when any action or alert levels are reached or surpassed.  Under the same guidelines, that district council and health officials are then expected to look after informing the public about the bloom, including issuing any associated health warnings and erecting any necessary signage. 

Mr Dall says the council typically monitors the lake monthly from November to April each year and initially became aware of increasing algal levels in November. 

While initial levels were not high enough to trigger any human health concerns, those results –  and others collected during January when monitoring had been stepped up by the NRC as a precaution – had been reported to FNDC and the DHB in line with the government guidelines. 

Mr Dall says despite being Northland’s largest lake, Omapere – at just 1.8 metres at its deepest point – is paradoxically also one of its shallowest and has naturally high fertility. 

This makes it more prone to algal blooms, and it has suffered a number since the early 1980s. 

“Typically, blooms like this are caused by a combination of conditions, including warm weather and increased nutrient levels, especially after run-off from heavy rain events, like that which occurred recently.”