Nine cattle have died after drinking water from algae-contaminated Lake Rotongaro.
The seven cattle and two calves had access to the lakeshore and had been drinking the water. Environment Waikato staff sampled the lake on Thursday after the cattle were found and measured very high levels of the blue-green alga Microcystis and the algal toxin microcystin.
Levels of algae at the edge of the lake were 40 times higher than stock drinking water standards and toxin levels were 760 times higher. Environment Waikato has advised both MAF and the Medical Officer of Health.
The farmer had farmed the area for 34 years and never had cattle deaths from lake water before. The lake was fairly inaccessible but was used by families in the area.
Environment Waikato water scientist Bill Vant said algae occurred naturally in rivers, lakes and streams and flourished during hot, fine conditions. Algal blooms were worsened by other factors, such as the amount of available plant nutrients, phosphorus and nitrogen within a lake. Most algae were harmless but high levels of blue-green algae were toxic to both people and animals.
Parents should ensure that they and their children were not exposed to the toxins as they risked skin infections and serious intestinal illness, he said.
The cattle deaths were also a reminder that stock should be prevented from accessing waterways, both to ensure they did not contaminate water and to prevent them drinking contaminated water.
Several lakes in the Waikato are currently contaminated with algal blooms, with levels well above the guidelines for swimming, boating or jetskiing. High algal counts mean that health warnings are in place for lakes Hakanoa, Kainui, Ngaroto, Waahi and Whangape. Environment Waikato routinely monitors lakes and rivers in the Region, and reports algal levels on its website www.ew.govt.nz