Remote sensing with field validation can accurately predict biocrust abundance in the Dry Valleys of Antarctica.
by Abigail Borgmeier, graduate student at the McMurdo Dry Valleys LTER and LTER Graduate Writing Fellow
The “valley of the dead” is not as it may appear
The vast emptiness of the Antarctic Dry Valleys inspires awe. Robert Falcon Scott called this area “the valley of the dead” and reported that he had “seen no living thing, not even a moss or lichen.” Yet, the discovery of algae, nematodes, tardigrades, and collembola around glacier meltwater streams in the ecosystem proved his statement wrong.
It turns out that life in the Dry Valleys is even more widespread than initially thought. Biocrusts, chunks of living soil, can grow far from meltwater streams and lakes, and play an important role in carbon cycling in the ecosystem. A new paper by Dr. Sarah Power uses remote sensing and field surveys to show where biocrusts grow across the Lake Fryxell Basin within Taylor Valley, Antarctica (Image 1)—the most comprehensive biocrust model to date. As part of her doctoral research with the McMurdo Dry Valleys LTER group, Dr. Power harnessed the power of remote sensing to update the aboveground, terrestrial carbon budget of the Dry Valleys with her models. Her work is key to understanding life in the Dry Valleys and will help predict how the ecosystem will be affected by climate change.
Credit: Both maps were generated in Google Earth.
Why are biocrusts important in deserts?
Biocrusts are communities of living things that live on the surface of arid or semi-arid soils. Biocrusts might be more common than you think. They inhabit 12% of the Earth’s terrestrial surface and perform many crucial roles in dryland ecosystems, primarily being a major source of fixed carbon.
The McMurdo Dry Valleys is the driest, coldest desert on the planet. However, even in this extreme environment, biocrusts can thrive. In a place with so little life, these communities form an important backbone of the living ecosystem. Biocrusts form the habitat for the largest land animals in this ecosystem – tardigrades, rotifers, and nematodes. But even though biocrusts can persist in the Dry Valleys, they are far from dominant across what looks to be a largely homogenous landscape (Image 2)—and that makes figuring out where they grow a challenge.
Credit: Dr. Sarah Power, CC BY-SA 4.0.
Biocrusts can develop where there is a consistent source of water, near a meltwater stream bed or underneath a persistent snowdrift. However, according to Dr. Power, biocrusts are “not just going to be where it’s wet, there is more of a story here”. Outside of the biologically active meltwater streams, it appears that most of the Dry Valleys is bare soil.
What can we learn about biocrusts from remote sensing?
Dr. Power was able to combine remote sensing with a minimally invasive field survey within the Fryxell Basin to predict where biocrusts are able to grow across a large geographic area. Her approach mapped biocrusts without requiring a potentially invasive and time consuming field survey of the whole of Taylor Valley. “The biggest benefit of remote sensing is being able to detect things on the surface without actually being there,” says Dr. Power.
Credit: Dr. Sarah Power, CC BY-SA 4.0.
First, Dr. Power examined satellite images and soil moisture data to find sites where it was most likely that biocrusts would form in the Dry Valleys. After making predictions of where biocrusts might form, Dr. Power went to the Dry Valleys and ground-truthed the predictions she made from satellite imagery (Image 3 and 4). She collected field samples of biocrusts with the help of Dr. Jesse Jorna and Meredith Snyder.
Pairing remote sensing with field work gave a different set of insights than using either strategy alone. “Remote sensing helped with getting longer term dynamics, and on the flip side you can’t easily get measurements like electrical conductivity remotely, which ended up being really important,” Dr. Power says.
Biocrust habitat is complex
Turns out, predicting the presence of life isn’t so easy. Dr. Power’s surveys found that water in the form of nearby snow patches were important for biocrust presence, but water was not the only important predictor. Snowpack was crucial—snow offers cushy microhabitats for biocrusts to develop and persist (Image 5) by protecting them from wind, UV, and temperature extremes. Salt content, too, was key. There are a lot of salts packed into Dry Valley soils, so if melt water from snowpack accumulates salts in a depression in the landscape, biocrusts cannot survive. Electrical conductivity and topology were key to predicting the presence and abundance of biocrusts because electrical conductivity relates to how much salt is in soil and topography dictates where salt will accumulate.
Credit: Dr. Sarah Power, CC BY-SA 4.0.
Dr. Power used these findings to build habitat suitability models for biocrusts across the Fryxell Basin. The most robust models for predicting biocrust presence included a combination of factors from both field surveys (electrical conductivity) and remote sensing (topography and soil moisture).
The Dry Valleys are an extremely limited carbon system, so having a robust model of biocrust presence and abundance is key to an accurate carbon budget for this ecosystem. It was previously thought that most of the carbon lay in and around the streams. Dr. Power’s models show that there is a significant amount of carbon in the relatively dry, salty soils that appear to be devoid of life. She found that bare soil in the Fryxell Basin contains on average 85 g C m-2, while soil with biocrust contains on average 228 g C m-2. Dr. Power’s models are the first attempt to include biocrust in the basin’s carbon budget, and show that there is a significant amount of carbon that was overlooked when only considering the biota surrounding glacier meltwater streams and lakes.
Hotspots of life
The Dry Valleys of Antarctica have historically been a very stable environment. Harsh, but predictable. The organisms in this environment have evolved to survive cycles of freeze/thaw or wet/dry. Climate change is making these cycles increasingly unpredictable. For example, in March 2022, an extreme warming event caused the soil temperature to rise above freezing—far earlier than usual. “The type, frequency, availability of water will all impact [biocrusts] in compounding ways,” says Dr. Power. Her habitat suitability models will help researchers predict where biocrusts might exist as the hydrology of the Dry Valleys changes. The significant contribution of biocrust to the Dry Valley carbon budget highlights the importance of understanding and tracking how biocrusts are impacted by environmental change driven by climate change.
Credit: Dr. Sarah Power, CC BY-SA 4.0.
Climate extremes increasing in greater frequency threatens biocrust survival, and by extension, the carbon balance of the whole Dry Valleys ecosystem. Speaking about her experience doing field work in Antarctica, Dr. Power says that “One of my favorite parts of being in the field is walking around and it being silent. Nothing is happening, but you look on the ground and something is happening. You find a hotspot of life.” Even in the dry, salty soils of the “valley of death”, there is life if you know what you’re looking for.
