As summer begins, Whanganui's Wiritoa and Dudding lakes are both suitable for swimming.

During the swimming season Horizons Regional Council tests the water weekly at swim spots across the region.

At the coastal lakes the council looks for bluegreen algae, also known as cyanobacteria. Some species of the algae can be toxic, and even the non-toxic ones can irritate swimmers when they are at high densities.

The algae are at very low levels so far. Their density tends to increase as water temperature warms.

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Swimming is not advised when densities are over 10 cubic millimetres per litre of water. Last January they reached five times that in Dudding Lake in January, and the water had a greenish look.

Health warnings were put up at the lake for a few weeks, but some swimmers took no notice.

The weekly water samples are taken at the boat ramps of both lakes. The algae tend to be denser at the lake surface on calm days, and at the leeward lake edge on windy days.

Horizons spokesman Barry Gilliland says it's always good to do a few circuits of the lake in a boat before any swimming. Doing so mixes surface algae deeper into the water.