Is the writing really on the wall for the Waikato River?

Thursday, 16 April 2026

Boaties plough through a massive algal bloom at Mangakino’s Lake Maraetai earlier this year. A Hamilton-based scientist says the bloom’s links to geothermal activity and the presence of gold clams, needs to be investigated.

River researcher Adam Hartland says multiple pressures on the Waikato River system are pushing it closer to tipping points “where recovery becomes slow, costly and uncertain'.

Hartland is an Adjunct Associate Professor in freshwater biogeochemistry at Lincoln University, and a senior scientist at Lincoln Agritech’s Hamilton office, and says the river is “becoming a case study in what happens when very different environmental pressures hit the same system faster than authorities can respond”.

“The science is telling us, in real-time sensor data, that the [river] system is moving toward thresholds we do not want to cross.”

Adam Hartland says links between gold clams, geothermal activity and algal blooms needs to be further investigated.

He says the current monitoring and governance of the river “was not designed for the compound pressures now acting on the river”, and is putting at risk drinking water supplies for nearly half the country’s population.

Recent Waikato Times reports, and a subsequent RNZ investigation, have documented worsening toxic algal blooms in hydro lakes in the upper Waikato River with locals describing water “so green it resembles the ‘Incredible Hulk’, dogs becoming violently ill, and mats of toxic slime covering the surface”.

He said multiple pressures on the river, including the ingress of mineral-rich fluids from nearby geothermal fields, increasing carbon dioxide, and the presence of invasive gold clams, are upsetting the water’s chemistry and putting at risk drinking water supplies for almost two million people in Hamilton and Auckland.

Toxic algal blooms in Lake Ohakuri have been getting progressively worse. The water is eventually treated and served up to almost two million people in Hamilton and Auckland.

“The reporting captured genuine community frustration and institutional fragmentation.

“But to turn concern into effective action, we need to understand why blooms keep forming where they do.

“Otherwise, interventions risk missing the mark. The Waikato cannot afford misdirected effort.”

Toxic algal blooms in the upper Waikato River’s hydro lakes turn the water into a glowing pea-soup making them unsafe to swim in.

He said a recent “near miss” at Lake Karapiro, where in January 2026 a monitoring buoy recorded oxygen near the lake bed dropping rapidly toward levels that would suffocate aquatic life, was averted only because storms overturned the water column and mixed oxygen back in.

“What prevented a crisis was not management action but weather. This near miss, averted by luck, is a warning, not a reassurance.”

Using Lake Ohakuri’s algal blooms as an example, Hartland said its location next to the Ohaaki-Broadlands geothermal field saw geothermal activity releasing heat, carbon dioxide (CO₂) and mineral-rich fluids into the water, that promote the growth of cyanobacteria.

“This includes iron, a nutrient toxic algae need to thrive.

Gold clams have been found to consume large amounts of calcium carbonate ‘disrupting the water chemistry treatment plants rely on’.

“Whether decades of fluid extraction have altered the rate of influx of CO₂ and iron remains untested, but the proximity to geothermal fields is striking.”

He said a recent field trip where a mobile sensor was placed in the upper Waikato River produced “stark” results.

“Carbon dioxide concentrations in the geothermal zone reach ten times the background level and the isotopic signature confirms the source as volcanic, not biological.

“The water does not return to background levels even by the time it reaches Lake Karāpiro, more than a hundred kilometres away.

“That lingering excess CO₂ could be feeding algal growth well beyond the volcanic zone.”

He said invasive gold clams that have rapidly colonised the river from Port Waikato to Lake Maraetai were another emerging issue as they strip calcium carbonate from the river, and are “releasing arsenic in forms that could slip through conventional [drinking water] treatment processes”.

He said initial research, now under review, found gold clams were stripping around 14 tonnes of calcium carbonate from the river every day, “disrupting the water chemistry treatment plants rely on”.

As the clams breathe, they pump carbon dioxide into the water and consume oxygen, tipping the river’s balance away from a system driven by plant-like photosynthesis (which produces oxygen) and toward one dominated by respiration (which releases CO₂).

Hartland said those two very different stressors, and how their combined effects drive algal blooms were “an open and urgent question”.

“Geothermal CO₂ enriches the water from below, sustaining conditions that help toxic algae grow far downstream.

“The clams, spreading upstream into the geothermal reaches, add a second source of CO₂ through their breathing, while depleting oxygen and stripping calcium.”

He said current monitoring programmes cannot answer that question.

“This is not a criticism of any single agency - national monitoring protocols now predate the compound pressures the river faces.

“The question is whether we can build the governance and data-led operational protocols to match the pace of change, before the next bloom or near miss becomes the event we failed to prevent.”