Toxic algae bloom season may be coming to an end in New Zealand, but major lakes across the Waikato are still rife with health warnings.
By Jordan Smith of Local Democracy Reporting
Data provided by Te Whatu Ora - Health New Zealand outlined lengthy health warnings for four lakes when assessed against national guidelines for cyanobacteria levels.
Lake Rotoroa (Hamilton Lake) has had three health warning periods since September 2023, with one currently in place. Forest Lake (Lake Rotokaeo) holds an identical record, with all three warnings being from March 2025 onwards.
Additionally, both Lake Waahi and Lake Waikare have had one health warning in place continuously for the past five years.
Waikato Regional Council's Environmental Science Manager Dr Mike Scarsbrook said most lakes in the Waikato are "highly degraded, with shallow lakes being the worst affected".
Loss of native plant life is most rampant there, he added, becoming dominated by toxic algae.
Three key factors for the rampant toxic growth; the supply of nutrients as well as changes in water flow and climate.
Specific nutrients such nitrogen and phosphorous brought in by extensive increases in agriculture urban and horticulture land use have provided a hotspot for algae to grow, Scarsbrook said.
"The change in land use and the intensification of that has brought with it more nutrients, more sediment and that's fuelled the algae that grow in the lake."
He added the lakes now "don't flush nearly as much as they used to," changing the physical habitat of the water.
Climate change however has been major culprit over recent years, Scarsbrook pointed out, referencing the change in temperature and rainfall patterns wrecking havoc on lake health.
"The climate is warming, we're seeing changes in rainfall patterns, droughts and more intense rainstorms that bring in more sediment from eroded catchment."
Waikato Regional Council have been doing "quite a lot of work" in Lake Waikare looking at how to reduce the frequency of toxic algae blooms, Scarsbrook says, with the yardstick for lakes across the Waikato is to make it healthy enough for community use.
"It's not taking the lake back to how it was in the 1960's or earlier, it's simply the community is looking to have a lake that they can swim in and boat on again. That's the primary community value people worry about."
In order to begin clawing back the damage, Scarsbrook said "we have to think about the things that we can control" in the short to medium term, with the major one being the nutrients and sediment being fed into lakes.
"We know that when we do restrict the amount of nutrients, the lake water quality will improve and we've seen that in other places around New Zealand, like Rotorua for example."
He said the problem isn't new in New Zealand and added most farmers are putting in efforts to reduce nutrient and sediment input.
"There's an increasing trend of intensification but at the same time we're trying to reduce the overall levels of nutrients [so] a bit of chasing our tail really."
Scarsbrook emphasised the revitalisation is "gonna take a lot of hard work, a lot of resource and there's no one single solution".
"We have to have holistic thinking rather than just picking one thing and focusing on that."
Local Democracy Reporting is local-body journalism co-funded by RNZ and NZ On Air.
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