Media release

Hamilton Lake warning due to potentially toxic algae

Posted On: Thursday, 18 February 2010

The public are being advised to keep their dogs out of Hamilton Lake (Lake Rotoroa) and avoid all contact with the water due to a potentially toxic bloom of blue-green algae.

Hamilton City Council staff raised the alarm about the algae yesterday and an initial sample examined by the National Institute of Water and Atmospheric Research (NIWA) confirmed the presence of two species of cyanobacteria, otherwise known as blue green algae. This organism may form toxins – leading to a warning to avoid all contact with the water.

Concentrations will occur on the down-wind side of the lake and these areas particularly should be avoided.

NIWA will conduct further analysis to confirm the concentration of the cyanobacteria and detect whether it is forming toxins. If the presence of toxins is confirmed further information will be issued.

Blue green algae blooms can occur when hot, still weather leads to low oxygen conditions in the lake sediments and the release of nutrients, particularly phosphorus, into the lake. The blue green algae fix nitrogen so a surplus of phosphorus gives them an opportunity to expand very quickly.

The bloom will die down as it follows its natural course and as weather conditions change but there may be concentrations of green matter and scum around the edge of the lake for some time and this may contain toxins. Care is advised.