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.
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.