Toxic blooms and invasive clams are forcing a rethink on the Waikato River

The Waikato is the longest river in New Zealand, central to the identity and culture of the Waikato River iwi and is the source of drinking water for almost half of the country’s population.

It also becomes a case study in what happens when very different environmental pressures hit the same system faster than the authorities can respond.

Recent RNZ research reports worsening toxic blooms in upper Waikato lakes. Residents around Lake Ohakuri describe the water as green as the “Incredible Hulk”, dogs are very sick and a carpet of toxic sludge covers the surface.

These conditions are far from Te Ture Whaimana o te Awa o Waikato, the official view for a safe river for swimming and food gathering.

The report captured the real social confusion and organizational divide. But to turn concern into effective action, we need to understand why flowers continue to grow where they do.

Otherwise, the steps can be dangerous to lose the mark. Waikato can’t afford a wrong effort.

The place of the worst blooms is the light. Lake Ohakuri sits near the Ohaaki-Broadlands geothermal area, where decades of hot water extraction for power generation has caused the earth to sink nearly seven meters.

That thermal activity releases heat, carbon dioxide (CO₂) and mineral-rich fluids into the water, all of which encourage the growth of cyanobacteria. This includes iron, a nutrient algae needs to thrive.

Whether decades of extraction have changed the flow rate of CO₂ and iron remains untested, but being so close to geothermal fields is intriguing.

According to the bottom line

Until now, no one has measured how much geothermal CO₂ actually dissolves in the river or how far it travels downstream.

During our latest field campaign, we installed a mobile sensor near the upper Waikato and a method known as stable isotope analysis to record carbon fingerprints and begin to fill this gap.

The results are clear.

Carbon dioxide levels in the geothermal area reach ten times the background and the isotopic signature confirms the source as volcanic, not biological.

A large amount of dissolved CO₂ escapes into the atmosphere as the river passes through the hydro lake chain. The water does not return to its original state even if it reaches Lake Karāpiro more than a hundred kilometers away.

That CO₂ that’s going on can even feed the algal over the volcano.

The golden clam thing

The geothermal area is not the only place of pressure. The invasive gold clam (Corbicula was bright) has added to the Waikato rapidly since its discovery in 2023.

The clams have been sealed all the way to Lake Maraetai, directly below Ohakuri.

Our research, which is under review, shows the clams are pulling up to 14 tons of calcium carbonate into the river every day, disrupting the water chemistry treatment plants that depend on them and releasing arsenic in forms that can slip through conventional treatment processes.

As clams respire, they pump carbon dioxide out of the water and consume oxygen, shifting the river’s water from a plant-driven system such as photosynthesis (which produces oxygen) to one controlled by respiration (which releases CO₂).

The more pressures, the greater the risk

In January 2026, our monitoring signal in Lake Karāpiro recorded oxygen near the lake bed dropping to levels that would suffocate aquatic life.

What prevented disaster was not administrative action but the weather. Powerful storms churned up the water and re-mixed the oxygen.

This near miss, avoided by luck, is a warning, not a guarantee.

Two very different pressures are coming together in one river. Geothermal CO₂ enriches the water from below, maintaining conditions that help toxic algae grow far downriver.

Clams, which live up to geothermal areas, add a second source of CO₂ through their respiration, while consuming oxygen and removing calcium.

What this double pressure will mean for algal blooms – when they form, how long they last and how strong they become – as the clam population increases, is an open and urgent question.

Current care can’t answer it. Toxic algae are sampled monthly in 4 hydroponics, and results take days to return. This is not to criticize any one organization; national monitoring protocols have been in place before the combined pressures the river faces.

The difference between knowing and doing

The local community called for buoys that kill algae, network cameras and wash the lakes. This shows a reasonable desire to take visible action, but without understanding the main reasons for the blooms in these special places, we are in danger of treating the symptoms instead of the causes.

Two million people drink Waikato water. Thousands swim in it, fish in it and collect mahinga kai (food collection) along its length. Iwi has ties to it that extend across generations.

Science is telling us, through real-time sensor data, that the system is moving towards thresholds we don’t want to cross. The monitoring and governance architecture that we inherited was not designed for the current pressures in the river.

The question is whether we can build governance and data processing protocols to keep up with the pace of change, before the next bloom or near miss becomes an event we failed to prevent.