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

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

It is also becoming a case study of what happens when very different environmental pressures affect the same system faster than authorities can respond.

A recent RNZ survey recorded worsening toxic algae blooms in Upper Waikato hydro lakes. Communities around Lake Ohakuri have described the water as so green that it resembles the “Incredible Hulk,” dogs becoming severely ill, and toxic mucus coating the surface.

This situation is a far cry from the statutory vision of Te Ture Waimana o Te Oura o Waikato, a river safe for swimming and foraging.

The report captured the real frustrations of the community and the divisions in the organization. But to turn concern into effective action, we need to understand why blooms continue to occur where they occur.

Otherwise, there is a risk that the intervention will miss the mark. Waikato people cannot afford to strive in the wrong direction.

The location of the worst blooms will give you a clue. Lake Ohakuri is right next to the Ohakuri Broadlands geothermal field, which has sunk nearly 20 feet after decades of extraction of hot fluids for power generation.

This geothermal activity releases heat, carbon dioxide (CO₂), and mineral-rich liquids into the water, all of which promote the growth of cyanobacteria. This includes iron, a nutrient necessary for the growth of toxic algae.

It remains to be determined whether decades of fluid extraction have altered the rates of CO₂ and iron input, but the proximity to geothermal areas is surprising.

Tracking downstream effects

Until now, no one had measured how much geothermal carbon dioxide is actually dissolving into rivers, or how far downstream geothermal carbon dioxide travels.

During a recent field trip, we deployed mobile sensors along the Upper Waikato and introduced a technique known as stable isotope analysis to capture carbon fingerprints and begin to close this gap.

The results were grim.

Carbon dioxide concentrations in the geothermal field are 10 times higher than background levels, and isotopic signatures confirm that the source is volcanic rather than biological.

As rivers pass through hydrolake chains, large amounts of dissolved carbon dioxide escape into the atmosphere. Even after reaching Lake Karapiro, more than 100 kilometers away, the water does not return to background levels.

The excess carbon dioxide that remains may be promoting algae growth far beyond volcanic areas.

gold shell element

Geothermal areas are not the only places under pressure. Alien species goldfish (The clams were shining) was discovered in 2023 and has rapidly established itself in the Waikato.

Currently, clams can be found just downstream of Ohakuri and upstream to Lake Malaetai.

Our research, which is currently under review, shows that clams are stripping approximately 14 tonnes of calcium carbonate from rivers every day, disrupting the water chemical treatment plants they rely on and releasing arsenic in a form that can bypass traditional treatment processes.

When the clams breathe, they pump carbon dioxide into the water and consume oxygen, tipping the balance of the river from a system driven by plant-like photosynthesis (which produces oxygen) to one dominated by respiration (which releases carbon dioxide).

Multiple pressures and complex risks

In January 2026, a monitoring buoy in Lake Karapiro recorded that oxygen near the lake bed was rapidly dropping to levels that suffocated aquatic life.

It was the weather, not the actions of management, that prevented the crisis. A severe storm physically upended the water column and brought the oxygen back into the mix.

This near miss was avoided by luck, but it’s not a relief, it’s a warning.

Two very different stressors are now converging in the same river. Geothermal carbon dioxide enriches water from downstream, maintaining conditions conducive for toxic algae to grow far downstream.

Spreading in upstream geothermal areas, clams deplete oxygen and remove calcium while adding a second source of carbon dioxide through respiration.

As clam populations continue to expand, an open and urgent question is how this dual pressure will affect algal blooms, including when they occur, how long they occur, and how severe the blooms are.

Current monitoring cannot answer that. Toxic algae are sampled monthly in four hydro lakes, and results take several days. This is not a criticism of any particular institution. Current national monitoring protocols predate the combined pressures facing the river.

Gap between knowing and doing

Local communities called for ultrasonic algae removal buoys, webcams to be installed and the lake to be cleaned. This reflects an understandable desire for visible action, but without understanding the underlying causes of blooms in these specific locations, we risk treating the symptoms rather than the causes.

Two million people drink Waikato water. Thousands of people swim in it, fish, and gather mahinga kai (traditional food collections) along its length. Iwi have intergenerational obligations.

Science shows real-time sensor data that the system is moving towards a threshold it doesn’t want to cross. The oversight and governance architecture we inherited was not designed with the complex pressures currently acting on the river.

The question is whether you can build governance and data-driven operational protocols to keep up with the pace of change before the next bloom or near-miss becomes an unpreventable event.