Scientists say the former forest plants appear to have been washed into the alpine lakes eons ago and created a sunless niche.
New Zealand is one of the few places worldwide to support unusual and rare bryophytes that thrive in light levels of less than 1% of the sunlight falling at lake surface.
The plants were first discovered in lakes Wānaka, Hāwea and Wakatipu in the 1980s. Scientists from the Otago Regional Council and Niwa this year embarked on a new survey of the submerged plants, hoping to build on knowledge gained over the decades and establish a regular, three-year monitoring protocol.
Niwa freshwater ecologist and diver Mary de Winton first studied the plants in the 1990s and returned to the lakes this year for another look.
"Bryophytes are usually found in moist environments like damp forest floors and shady rock faces, but we saw them in Lakes Wakatipu and Wānaka up to 50m down.
"It’s a really unusual place for them to grow, but we think they were washed into the lakes eons ago and have found a niche in the quiet twilight depths.
"If it wasn’t for the extreme transparency of our lake water, we don’t think this ‘freak of nature’ would have survived," Ms de Winton said.
ORC lakes scientist Hugo Borges said this year’s survey was successful and more work has been planned later this year.
"We haven’t done a full analysis yet, but we successfully revisited all sites and found deep-water bryophyte in some of them.
"We used a remotely operated vehicle to explore the lake bed, so have hours of footage to study. This will give us a better understanding of the extent of bryophytes coverage, which we can compare to the previous surveys," Mr Borges said.
The two organisations will schedule a dive programme every three years to monitor the submerged plant communities, using this year’s survey as a baseline.
The monitoring programme would be crucial for tracking environmental change, while the depths the plants grow down to would provide an indicator of the long-term clarity of the lakes, Mr Borges said.
"Despite being simple organisms, deep-water bryophytes play a vital role in ecosystem functioning by providing habitats, improving water quality and aiding biodiversity.
"They are highly sensitive gauges of water transparency, light transmission, sedimentation rates, temperature fluctuations and changes in dissolved gases, all of which give us clues into the health of our freshwater environments," he said.
