02/07/2026
Below is the English translation of an article written by Akseli Innanen for the Finnish Newspaper - Helsingin Sanomat,
At the World's Clearest Lake
Nelson Lakes National Park, New Zealand
Algae found in neighboring lakes threaten a unique landscape in New Zealand.
Blue Lake, or Rotomairewhenua, is the clearest lake in the world.
The captain carefully inspects our equipment as my partner and I step into the boat on Lake Rotoroa.
We're heading to another lake—the clearest freshwater lake in the world. It's called Blue Lake, or in Māori, Rotomairewhenua, meaning the lake of peaceful lands.
Visitors can only approach it with impeccably clean equipment. If there were even the slightest sign of moisture or mud, our captain, Dudley White, would intervene.
Rotomairewhenua's serene clarity is under threat. An invasive species of algae, lindavia, has already spread to other lakes within the national park. A single drop of water or a microscopic fragment of algae clinging to equipment can carry it from one body of water to another. Once the algae reaches a lake, it is impossible to remove.
"Check, Clean, Dry" is the guiding principle for preventing the spread of lindavia.
For White, who operates a water taxi service, protecting the lakes has become an increasingly important responsibility.
"We explain to our customers how we want to care for the environment and these lakes. We work together with the Department of Conservation and the local Māori iwi", White says.
The fastest way to reach Rotomairewhenua is to cross Lake Rotoroa by boat and then hike into the mountains. Our boat reaches the jetty at the southern end of Lake Rotoroa, and we review our schedule. In a few days, we'll radio for pickup from the nearby Sabine Hut.
At the hut stands a cleaning station. The first signs warn visitors about "lake snow," the sticky slime produced by blooms of lindavia algae. Although this gelatinous material floating in the water is not known to pose an immediate risk to humans, its effects on wildlife, biodiversity, and especially the world's clearest lake remain unknown.
Visitor numbers at Rotomairewhenua have risen since 2011, when the Te Araroa Trail, a long-distance hiking route spanning the length of New Zealand, was opened past the lake.
Only weeks after the trail opened, researchers declared the lake the clearest in the world, measuring an astonishing 80 meters of horizontal underwater visibility—almost identical to that of distilled water.
John Wotherspoon, the operational manager of Nelson Lakes National Park, worried about the attention the announcement would bring. At first, people wanted to drive directly to the lake.
"The lack of a road discouraged many from trying. But the popularity of the Te Araroa Trail has continued to grow," Wotherspoon says.
We hike through steep valleys, shaded beech forests, and avalanche paths, following the vivid blue waters of the Sabine River.
Tiny droplets of honeydew glisten on the trunks of the beech trees. They are produced by scale insects, which feed on tree sap and excrete the excess through extraordinarily long a**l tubes, providing a sweet food source for other fauna.
The honeydew tastes similar like honey.
Late autumn is quiet on the trails. Things were very different around Christmas and New Year's, when Wotherspoon worked as the hut warden at Blue Lake Hut during the busiest hiking season. The hut, located near Rotomairewhenua, is designed for just 16 people.
"There were nights when nearly thirty people stayed there. It was pretty intense."
At that time, visitors were content to simply admire the lake, but over the years, everything from frisbees to inflatable pool toys has ended up in the water. Drone footage of the lake has appeared on social media despite drones being prohibited in national parks. Last season, someone was even reported to have gone swimming in the lake.
"In the past, the information provided by the Department of Conservation was all people knew about the lake. Nowadays, social media can encourage people to visit and imitate things they've seen others doing," Wotherspoon reflects.
After a day and a half of hiking through the wilderness, we arrive at Blue Lake Hut.
Near Rotomairewhenua, warning and prohibition signs are present. If information alone could protect the lake, its future would be secure. The message is simple:
Look—but don't touch.
So we simply look.
The wind dies away in the valley cradled by the mountains. Rotomairewhenua becomes perfectly still, its surface like glass. I stare in fascination at colors shifting from deep blue-violet to brilliant neon green.
It's as though the surface of the water disappears, revealing a vast natural aquarium. Although the lake is only a little over eight meters deep, for a moment it seems to stretch all the way toward the Earth's core.
By midday, a gentle breeze ripples the water. I walk across a meadow of tussock grass toward the far end of the lake, where a stream tumbles down from higher ground.
Scientists believe the lake's extraordinary clarity is the result of a natural filtration system. Water flowing from the higher Rotopōhueroa Lake and nearby glacial streams travels underground through deposits left by an ancient Ice Age landslide. This natural filter removes suspended particles and dissolved organic matter with remarkable efficiency. The lake's water is also completely renewed every three days, helping keep it exceptionally pure despite sediment and nutrients washed in by rainfall.
A visit to Nelson Lakes National Park shows how a close-knit community is working, each in its own way, to protect something beyond measure.
During the summer, Tara MacDonald, events coordinator for the Ngāti Apa ki te Rā Tō iwi, also served as the Blue Lake Hut warden. She helped visitors understand the Māori perspective on the lakes.
"The lakes are part of a living spiritual landscape connected through whakapapa. When people understand that, their relationship with these places becomes more caring and more responsible," MacDonald writes.
Whakapapa is a Māori concept describing the interconnected lineage that links people with their ancestors, their community, and the natural world across generations.
Through the local iwi, the significance of protecting the national park takes on a deeper meaning.
"Our perspective reminds people that these places are living cultural landscapes—not untouched wilderness separated from people.”
I find myself reflecting on the kōiwi ritual. Traditionally, the bones of deceased Māori men were washed in Rotomairewhenua, while those of women were washed in the upper lake, Rotopōhueroa. Through this ceremony, the spirits of the dead were prepared for their journey to Hawaiki, the ancestral homeland and final resting place in Māori tradition.
Respect for these sacred customs is another important reason why visitors must never enter the lakes.
Lengthening shadows urge us to begin the journey back through the national park.
Farther down the valley, a blue duck—one of New Zealand's most endangered birds—dives into the rapids. Its presence is a sign of exceptionally clean water and a healthy ecosystem. Yet the bird also faces a threat from an invasive predator familiar to Finns: the stoat. Fortunately, intensive trapping programs are underway throughout the park.
Back at the jetty on Lake Rotoroa, we wait for Captain White to collect us. As we stand by the shore, I watch New Zealand longfin eels gliding beneath the water's surface.
At the end of their long lives they undertake an extraordinary migration to deep waters near Tonga to spawn. The precise location of their breeding grounds, like so much else about their lives, remains a mystery to us.
Everything is the same seamless whole.
