Signs have gone up around some Wanganui lakes warning of algal bloom and others are being tested for toxic forms of algae, as the warm and settled weather continues.
Virginia Lake and Westmere Lake are experiencing an algal bloom, and signs have been placed on their shores to remind people to wash their hands if they come into contact with the water.
Barry Gilliland from Horizons Regional Council said tests had been done at Lake Wiritoa and Dudding Lake to check for toxic algae. The results will be available soon. He said counts of blue-green algae were climbing. At the moment the lakes were safe for recreational activities but that could change.
Mr Gilliland said the lakes had never returned a positive test for toxic algae but that did not mean they would not at some point in the future.
"There are different types of blue-green algae, some of which are toxic and some are not. Even if we do not get any toxic algae, we may have to close the lakes simply because of the very high levels of algae."
He said if that happened, the algae were likely to "bloom and die" quickly, within just a couple of weeks.
Mr Gilliland said blue-green algae thrived in warm temperatures and stable water conditions.
"That is certainly the case at both these lakes. In the shallow water the temperature is around 22C and the algae are responding to that."
Testing had been done in the Whanganui River at the City Bridge but it was clear, he said.
"The rivers and streams in the Wanganui district are not suited to blue-green algal growths," Mr Gilliland said.
Public health protection officer Bruce Butters said he was keeping a watching brief on algae levels.
"The levels are certainly creeping up, and we are watching cautiously," Mr Butters said.
Mr Butters said of most concern was Lake Wiritoa, because it was such a popular lake, used for a wide variety of activities.
Blue-green algal blooms are affecting waterways throughout the country. Lake Okaro (Bay of Plenty) and Rotokauri Reserve (Waikato) were closed last month, Tomahawk Lagoon (Otago) was closed earlier this month, and warnings have been issued for the Hutt River.
Mr Gilliland said the settled weather had meant very clear test results for Castlecliff and Kai Iwi beaches, although the beaches were tested for E coli, not algae.
He said Castlecliff was the most consistently clean beach in the entire Horizons district, although he warned that no beach was safe for swimming for several days after a flood.