Under thinning ice at Lake Fryxell

In January, divers were able to return to the bottom of Lake Fryxell in the McMurdo Dry Valleys. Waikato University professor Ian Hawes donned his dive suit as part of a NZ-US team carrying out long-term ecological monitoring. They found that the ice on the lake is thinning, and this is changing the microbial life that grows in this extreme environment. Videographer Anthony Powell worked with the divers to bring back pictures from the bottom of the lake, and TVNZ's Laura Frykberg helped tell the story.

The McMurdo Dry Valleys are a rare place in Antarctica, as they are not covered in ice. It’s basically a big cold desert, and microbes are by far the dominant life form. This finely-balanced, extreme environment is a natural lab for studying climate change. New Zealand scientists have had a special relationship with the Valleys for decades, building up long-term data sets that help us understand what is changing.

The valley lakes are fed by melting glaciers. Water flows into the lakes during Antarctica's 'warmest' months, but there is no outflow stream, though some water does evaporate. Generally, there has been gradual rise in lake levels in recent years, and there has been a thinning of the lake ice cover and an increase in lake water temperature.

In the 22/23 field season, there was an intensive effort at Lake Fryxell to study the consequences of these recent changes in environmental conditions. This involved collaborative research between the Antarctic Science Platform, the US Long Term Ecological Research Programme and the University of California. The main goal is to develop an understanding of the resilience of inland aquatic ecosystems to the types of stresses that are anticipated to accompany ongoing climate change.

The team dives and deploys Remote Operated Vehicles (as well as robots like the one pictured above) to take environmental measurements. They take samples for further analysis and place sensors to gather data, some of which will monitor lake conditions throughout the year.

The bottom of the lakes is a unique world, where the microbial mats (lake slime) grow into Gaudi-like structures. The types of microbes and their structures vary at different depths. Microbes in this extreme environment are of great interest, and not only as indicators of climate change. They also give us an understanding of how life on earth may have evolved, and of what life might look like on other planets.

As you can see, not all the tools the scientists use are high-tech.

The team found that lake ice has been thinning, from 4.5 m in 2014, 3.6 m in 2018 to 2.4 m in 2023. This allows more light to filter into the lake, which increases productivity, and changes types of microbes and where they live within the lake waters.

Preliminary observations suggest that a consequence of greater light feeding the microbes has been an increase in the phenomenon of mat 'lift off'. Lift-off occurs when oxygen production from photosynthesis forms bubbles within mats. If there's enough bubbles, mats tear off of the lake bottom and float. This has always been seen in shallow parts of the Dry Valley lakes, including Lake Fryxell, and there is a specific class of microbial mat known as lift-off mat.

This season, for the first time, the divers observed that a class of mats known as pinnacle mats, which typically grow are greater depths than lift-off mat, were also producing bubbles and lifting from the lake floor.

The significance of this is that these floating mats can become trapped under the ice, freeze in over winter and migrate to the surface of the lake ice. The 'freeze-dried' organic matter then blows into valley soils. Recent estimates are that about 80% of soil carbon is derived from lakes. These polar desert lakes are different from typical lakes in tropical and temperate regions, which tend to be sinks for catchment carbon, rather than sources.

This is the ‘greening of Antarctica’ in action as the world warms - this is the carbon that feeds life, like garden mulch, rather than the carbon that heats up the planet as emissions. As you will hear in Ian’s interview, future generations may see open water instead of ice-covered lakes, and there could be life in the McMurdo Dry Valleys beyond the lake shores.

- Read TVNZ's full story, Unique Antarctic ecosystem under threat from climate change