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CNN
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Deep within Nelson Lakes National Park on New Zealand’s South Island lies a magical blue-purple lake. Surrounded by steep alpine forests and fed by glacier water from Lake Constance, this lake is small but no less important.
The lake was first discovered by the Maori iwi, Ngati Apa, who named it Rotomailewhenua, meaning ‘lake of the peaceful land’. This became a sacred place where they would cleanse the bones of their dead, believed to allow their souls to travel safely to Hawaiki, the ancestral homeland of Maori.
In more recent history, hikers passing through the national park have commented on the lake’s unusual colors and magical energy, but it was only in the last decade or so that scientists discovered that the water in this subalpine lake had “extraordinary optical purity” with visibility of 70 to 80 meters. This is consistent with values for pure water, making it “the most visually clear freshwater ever reported,” the researchers said.
Since then, the title “world’s clearest lake” and photos of its stunning scenery have been widely shared on social media, making the lake a popular tourist destination from December to March (New Zealand summer). But conservationists and the Ngāti Apa tribe are now concerned that this growing popularity could threaten the purity of the lake.
Their biggest concern is Lindavia, a microscopic algae known colloquially as “lake snow” or “lake snot” for the slime that hangs out just below the water’s surface. The algae is already present downstream from Lake Rotoiti, Lake Rotoroa and Lake Tennyson’s Rotomayrewhenua (also known as Blue Lake), and is at risk of being carried along trails in hikers’ boots and water bottles.
Lindavia, an invasive species in New Zealand, likely arrived from North America via fishing gear, speculates Phil Novis, a senior algae specialist at the government-owned environmental institute Landcare Research. The first record in this country was in the early 2000s, and it has spread quite widely since then. “The main vector is humans,” he says, explaining that in a previous study his team collected and tested sediment cores from 380 lakes in New Zealand, but the only one where Lindavia was present was one that was easily accessible to humans.
Even the smallest speck can permanently change a lake’s ecosystem, he added, and it can be carried very easily on water droplets. He recalls finding a sample of Lindavia in the chest hair of a man who swam several kilometers across Lake Wanaka in New Zealand’s Otago region.
Although not known to be toxic to humans, this algae excretes long viscous strands known as mucus, which can become a nuisance when concentrated and is known to clog fishing lines, boat filters, and hydroelectric systems. In the case of Rotomaile whenua, the mucus film it produces can obscure the lake’s exceptional clarity.
“We are deeply concerned,” says Jen Skilton, an ecologist and Taiao advisor (environmental manager) at the Ngāti Apa ki te Rā Tō Trust, a non-profit organization that supports iwi. “Once these invasive microorganisms enter the lake, they can have far-reaching effects, affecting water quality and the overall health of the lake.”
She added that this would be devastating for the Ngāti Apa people, for whom Rotomairewhenua has immense cultural and spiritual importance. Although they no longer use the lake for ancient rituals, it is part of their identity, she says. “We maintain a living link to our ancestral traditions and ensure that Rotomaire Whenua is preserved for future generations.”
Visitor numbers have more than doubled since 2013, when a study on lake clarity was published, according to the New Zealand Department of Conservation, which collects voluntary data provided by hikers at visitor huts near Rotomaire Whenua. Most people trek to the lake via two-day or seven-day circuits, or as part of the Te Araroa Long Distance Trail, which traverses all of New Zealand.
The title of “world’s clearest lake” and the social media buzz that comes with it has certainly contributed to its popularity, says Melissa Griffin, senior biodiversity ranger at Nelson Lake, Department of the Environment. “Before that, it was known as a beautiful place, but not many people went there. Then, with this name, the number of illegal travelers (hikers) who went to the lake really started to increase.”
As a result, the Department of Conservation’s Ngāti Apa ki te Rā Tō Trust and Te Araroa Trust have been working together to introduce biosecurity measures along the route. They set up a cleaning station next to the lake where Lindavia is located and installed educational signs requesting hikers to clean their shoes and equipment before heading to Rotomaire Whenua, potentially introducing new species.
Above all, through signage and videos shown on the Te Araroa Trail app, we remind visitors to avoid touching the water, whether it’s swimming, dampening a towel to cool down, or dipping into your GoPro to take underwater photos. This is not only due to biosecurity risks, but also out of respect for the location. “The waters of Rotomaire Whenua are ‘tapu’ (sacred) and entering the water is a violation of that,” Mr Skilton said.
During the summer, lifeguards (often Conservation Officers or Gertie Apa representatives) stay at the lake for extended periods of time to watch, talk to hikers, and explain the biosecurity risks, the lake’s cultural significance, and potential human impacts.
So far, there are signs that people are respecting this guidance, with fewer instances of visitors recklessly immersing themselves in the water or soaking towels after long, sweaty hikes. However, research shows that there remains a gap between understanding risks and taking proactive action to avoid them. Janet Newell, biodiversity ranger at the Department of Conservation, acknowledges that people read signs at cleaning stations and understand why they are there, but says that doesn’t necessarily lead to them using them. She says she believes that “it’s not me that’s the problem, it’s other people.”
“Opportunity and Responsibility”
Because Rotomaire Whenua is part of a national park, it would be difficult to limit the number of visitors, and this is not something the Conservation Authority would like to do. Instead, the group’s message is that while people go to enjoy these pristine environments, they should also consider the wider impact they are having.
While working as a caretaker at the cabin, Griffin’s responsibilities included picking up trash and cleaning toilets. “Increasing numbers means the toilets are having to be emptied more often,” she says. “Then you also have the cost of the helicopter and the carbon emissions of taking the helicopter to a remote area. Those are all things to consider when you think about the number of people going to a remote area.”
But at the same time, “I want people to go and have fun, watch it, and sit there with me,” she says, recalling her first visit. “When you stand next to the lake, it’s really special to get there. And it’s just peaceful out there. You hear a lot of birds, but the water is calm and looking into the water is incredible.”
“Increased visitor numbers bring both opportunities and responsibilities, and it is important that all visitors understand the importance of this site and take the necessary steps to minimize impact,” says Skilton.
“By adhering to regulations and guidelines, we can protect the lake’s unique ecological features and preserve its cultural significance for future generations.”
