The balmy summer should have made surface temperatures in Rotorua's lakes perfect for swimming - if not for the blooms that came with the warmer water.
In a summer remembered for its above-average readings, the popular lakes recorded their warmest temperatures in nearly a decade over the peak summer months of January and February.
The ramifications this had for the lakes' hydrology and other factors were observed by scientists from the National Institute of Water and Atmosphere, who have published their findings in Scientific Data, published by the leading environmental journal Nature.
"This summer has been unusually hot, resulting in very shallow mixed layers in lakes," Niwa lake scientist Piet Verburg said.
"As a result of the balmy conditions cyanobacterial blooms have occurred, even in nutrient-poor lakes where this is normally not seen such as Lake Tarawera, in the Bay of Plenty."
In January, blue-green algae levels found in Lake Tarawera caused the Bay of Plenty Regional Council to elevate it to an "amber alert" level, meaning the situation would be monitored but did not come with a formal health warning.
Dr Verburg's work, and the research of other NIWA scientists, is part of the Global Lake Temperature Collaboration (GLTC), an international group assembled to provide better access to global lake temperature records.
The GLTC project was established in 2010 to build a global database of lake surface temperatures, including both satellite data and "on the ground" measurements from collection programmes such as NIWA's.
It was now widely recognised that global and regional climate change had important implications for aquatic ecosystems, with recent studies revealing significant warming of lakes and reservoirs throughout the world.
The observed rate of lake warming was sometimes greater than that of ambient air temperature, and rapid changes in lake temperature had profound implications for hydrology, productivity, and biotic communities.
New Zealand has about 3820 lakes with a surface area greater than one hectare, and NIWA and regional councils collect lake temperature data for about 120 of them, from Lake Taupo, the North Island's largest lake to Lake Okareka, one of the smallest.
The data was valued globally as there are fewer lakes in the Southern Hemisphere than the Northern, and these datasets broadened the global reference.