With temperatures set to hit the mid-30s by Saturday, Hawke's Bay swimmers are urged to use their eyes and nose before dipping their toes.
A spokesperson for Hawke's Bay Regional Council said hot, dry weather created ideal conditions for potentially toxic algae (mat-forming cyanobacteria) to grow, particularly when river flows were low and stable.
"Do a quick visual check when you arrive at a river or stream," the spokesperson said.
"If the water looks murky, discoloured, or has an unusual smell, it's best to choose another spot.
"Also, if dogs are showing an interest in eating or sniffing any black, brown, or dark‑green slimy mats on rocks in shallow areas or at the river's edge, keep them out."
Mat-forming cyanobacteria are naturally occurring bacteria that grow on the beds of rivers and streams.
They form thick, dark brown or black mats that cling to stones in shallow, slow-moving areas. They are a normal part of river ecosystems and tend to increase during warm, stable, low-flow periods.
While they are natural, some species (the most common is Microcoleus) can produce toxins that are harmful to people and animals when the mats become dense or detach and accumulate along the river's edge.
Their growth follows seasonal and climatic cycles, which is why there is a higher risk during hot, dry summers.
The council spokesperson said conditions changed continually.
"We monitor key sites closely and issue warnings when levels exceed health guidelines," they said.
"The recent rain provided some short-term relief by increasing river flows and creating a 'flushing flow' that can detach mats from the riverbed and improve oxygenation.
"However, mats can re-establish quickly once the flow drops again. With temperatures forecast to rise, we may see warm weather and relatively low flows similar to what we experienced in late December."
The council also recommends waiting two to three days after heavy or prolonged rain before swimming.
"Heavy rain flushes contaminants from urban and rural land into waterways that may cause illness."
HBRC monitors water quality at more than 35 swimming spots in Hawke's Bay, including beaches, rivers, ponds and lagoons.
Before you head off for a swim, check its website for safe spots at hbrc.govt.nz/hawkes-bay/swimming
LDR is local body journalism co-funded by RNZ and NZ On Air.
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