Life, Death and Predation on the Greenland Ice Sheet

The Greenland Ice Sheet is the world’s second-largest body of ice (Source: Krzysztof Zawierucha).
Mention the Greenland Ice Sheet, and chances are that you conjure up the image of a barren, white wilderness, dominated by ice and devoid of life. In fact, the ice sheet and its coastal outlet glaciers support thousands of small pools that teem with bacteria and animals. “A world of microbes exists in these tiny, frozen, cold pools on glaciers. There’s life, death, and predation happening,” marveled glaciologist Aurora Roth.

These little pools are called cryoconite holes, pockets in the surface of glaciers that are usually ovular or circular. Cryoconite holes can be quite small and shallow, or as wide as a meter and up to half as deep. “People first notice cryoconites because they look so odd, like honeycomb. The textures are visually striking,” says Roth. She added that they constitute such an extreme environment that scientists look to them to understand the evolution of simple life forms on Mars and other planetary bodies. A recent paper in Limnology by Krzysztof Zawierucha et al. analyzed cryoconite communities on the margin of the Greenland Ice Sheet and found that the distribution of microfauna at the edge of the sheet is random, without clear ecological determinants like water chemistry or nutrient availability.

Sediment lies at the bottom of cryoconite holes of various sizes (Source: Krzysztof Zawierucha).
Cryoconite holes (and their larger versions, puddles and lakes) are full of water, and contain debris deposited by wind, rockfall or water flow. Small debris particles can be bound together by cyanobacteria into granules, which eventually erode into mud. Both granules and mud foster communities of bacteria and animals that comprise the biotic hotspots of the ice sheet. Microorganisms are the top consumers in cryoconite food chains, a position impossible for them to occupy in most other ecosystems, where they are eaten by larger organisms. Such unusual dynamics make “this icy world more and more fascinating,” Zawierucha told GlacierHub. “Despite the fact that they are in microscopic size, they are apex consumers on the glacier surface, so they are like polar bears in the Arctic or wolves in forests,” he said.

Zawierucha conducted his fieldwork at the beginning of polar autumn and was struck by the changing colors of the tundra, the musk oxen and the impressiveness of the ice sheet, which together created a landscape that felt right out of a fairy tale. As he trekked through wind and rain to collect samples from cryoconite holes, puddles and lakes, he often felt as if he was in a science fiction movie about “icy worlds in other galaxies.”

Zawierucha poses against the dramatic landscape of the ice sheet during fieldwork (Source: Krzysztof Zawierucha).
Back in the lab, Zawierucha found rotifers and tardigrades swimming around in his samples, two hardy invertebrate groups that also live in freshwater, mosses, and for the tardigrade, extreme environments–tardigrades are the only animals that can survive outer space. The invertebrates were far more common in granules than mud. The paper suggests two reasons for this disparity: the mechanical flushing action of water that forms the mud and the food source the granules provide for the invertebrates. The samples boomed with other types of life, as well: they contained thirteen types of algae and cyanobacteria, plus different groups of heterotrophic bacteria.

The flushing process, and the way it affects the animals which it displaces, raises many questions for Zawierucha. How much wind or rain is required to remove the animals from the cryoconites? “How many of them are flushed to downstream ecosystems, and how many stay in the weathering crust on glaciers?” he wonders. And what happens once the animals are out of their holes? Zawierucha harbors what he calls a “small dream,” to find active animals in the subglacial zone (the hydraulic systems under a glacier). “If they are flushed to the icy wells, are they able to survive under ice?” he asks.

Tardigrades are found on all seven continents, in a range of extreme environments (Source: E. Schokraie et al./Wikimedia).
In the future, Zawierucha would like to continue to close what he calls the “huge knowledge gap” between the vast amount of research devoted to microbial ecology on glaciers and the dearth of information about animals. “Even if their distribution is random, they still may play an important ecological role in grazing on other organisms,” he believes.

Tardigrades, some species of which are black in color, may have an even bigger effect on glacial dynamics and global climate. Tiny though they are, populations of black tardigrades in cryoconite holes, which Zawierucha has found on alpine glaciers, can reduce albedo and increase melting of the glacier surface. This may constitute a positive feedback loop that hastens glacial melting, but more studies are required to prove this, Zawierucha says.

One positive feedback loop is clear. Higher temperatures increase the melting of glacier surfaces and spur microbial activity, which in turn speeds up the process of melting, according to Zawierucha. As the Greenland Ice Sheet continues to melt, the animals that call it home will be disturbed, though it is difficult to anticipate the end result. How tardigrades, especially species unique to glacial habitats, will respond to higher flushing rates and removal from glaciers is of particular interest. Perhaps the tardigrades will adapt, or perhaps they will go extinct, says Zawierucha.

Zawierucha prepares samples for transport off the ice sheet (Source: Krzysztof Zawierucha).
Faced with an uncertain future, glaciology projects that cross disciplines make Roth hopeful. “It’s a good example of what happens when you look at a system through an interdisciplinary lens,” she told GlacierHub. “When you bring in a biologist, you see the difference in the questions they ask and things they unearth.”

Now is the time for such interdisciplinary research: more studies of animals living on the Greenland Ice Sheet will help scientists understand how this important freshwater reservoir, and Earth’s climate, will respond to global warming.

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