What Goes on Beneath the Floes

Who and what is actually living or hunting underneath or at the underside of the Antarctic sea ice? Penguins of course, whales, and everybody knows leopard seals. But what about these colorful organisms? They were all caught with our under-ice net SUIT. Afterwards our Dutch colleague Jan von Franeker put the animals in an aquarium onboard Polarstern, identified their species and took these amazing photos. 

“We’re just getting started,” say AWI sea ice ecologists Hauke Flores and Ilka Peeken, who are working together to better grasp the food webs – or who eats whom – under the ice. “For example, we want to determine which types of algae are important for the food webs and how those webs could be affected by climate change,” adds Ilka Peeken. To do so, the researchers not only have to identify which species live under the ice, but also grasp each one’s respective biology.

With regard to the polar cod, the AWI scientists believe the juvenile fish spend the first one or two years of their lives under the sea ice of the central Arctic, where they most likely feed on amphipods and copepods. It’s not until their third year that the fish swim toward the coast, where they join with schools of adult cod. A behavioural model for the future? According to Hauke Flores, “We still can’t predict what will happen if the Arctic ice continues to melt.” Will the habitat of the polar cod shrink as a result? And if so, will this result in a critical mass regarding the difference in populations between the juvenile fish and the adults on the coast?

Some theories claim the polar cod isn’t so closely bound to the ice; instead, those animals that can’t find food in open water in the autumn simply turn to the ice. “If that theory’s right, the polar cod should be able to cope with the melting ice,” says Flores. “But even if that proves true, there’s the risk that competing species from the south – like the Atlantic cod – would increasingly encroach on the polar cod’s habitat, forcing it to fight for its food.”

But regardless of whether or not the disappearing ice will mean new challenges for the polar cod, the most central role for survival in the Arctic and Antarctic is played by the “ice algae” – types of algae that live in sea ice or on its underside. What makes them so special? According to AWI biologist Ilka Peeken: “Like everywhere else in the world, in the waters of the Arctic and Antarctic there are algae that float freely in the water as phytoplankton, but as long as the water is covered by a layer of ice, they lack the light they need to grow. In contrast, ice algae can thrive in the twilight under the ice.”

Like forest ferns, which can flourish in the shadows of trees, the single-celled ice algae can get by with only minimal light. And each one can divide every two days, which means a mass of algae weighing 200 grams can grow to a kilogram in just four days. In large numbers they can form veritable carpets on the underside of the ice, offering ideal feeding grounds for amphipods, copepods, krill and other animals.

Living in the half-light under the ice also has its advantages for the ice algae: though there may be less light under the ice floes of the Arctic and Antarctic, they enjoy relatively constant access to what light there is, because, unlike the phytoplankton in open water, there are no currents or waves to pull the algae into deeper (and darker) waters.

Further, ice algae aren’t exclusively found on the underside of the ice. AWI researchers have also discovered them in the sea ice itself, in the minute, porous passages and channels that form when seawater freezes. Once the first ice crystals form, they no longer absorb any salt from the water. Instead, the salt gathers between the crystals and forms the passages and channels mentioned above, before ultimately trickling out the underside and back to the sea.

But how do the ice algae get in these brine channels to begin with? With the first freeze, they cling to the channels, braving both the extreme cold and the high salinity. For comparison: sea-ice brine can be up to six times saltier than North Sea water. But that doesn’t seem to bother the algae: AWI biologist Ilka Peeken estimates that more than a thousand different species of single-cell algae and related single-cell organisms inhabit this extreme environment.

And the algae do so despite the fact that the waters of the Arctic and Antarctic are characterised by frequent phases in which there is practically nothing to eat – particularly in winter, when the water under the ice is hardly mixed and washes in very little fresh food. “But the plants and animals have adapted to this, too,” claims Peeken. Many of the ice algae, like the group known as pennate diatoms, can store large quantities of fats, especially the Omega-III fatty acids. This in turn makes the pennate diatoms an attractive source of nutrition for animals.

Many marine life forms can’t synthesise Omega-III fatty acids themselves, but instead absorb it from vegetable sources – which is fortunate for the AWI researchers, as the fatty acids offer the ideal substance for using painstaking detective work to reconstruct food webs: “Specific algae groups synthesise specific fatty acids,” explains Flores. “When an animal tends to primarily feed on a certain group of algae, they accumulate the specific fatty acids from that group in their tissues. We analyse these fatty acids, and the results allow us to recognise what the animal group’s preferred food source is.”

Researchers used to assume that things were largely dormant under the sea ice in winter and spring; we now know that the animals remain active throughout the winter. Though juvenile fish and krill larvae slow their metabolism, they still feed from time to time. Yet we still know very little about what the algae do in the winter.

When the long night of the polar winter comes to an end, so does the relative quiet under the ice: as soon as the daylight returns, the ice algae start to reproduce at breath-taking speed. “After the long winter, these algae are the first food source for animals in the under-ice community,” says Peeken.

The awakening of spring also heralds the summertime melting of the sea ice, and with it the gradual disappearance of the ice algae’s haven. In their stead, those algae that float freely in seawater, the phytoplankton, begin to reproduce. Especially at the edge of the ice that covered the Arctic through the winter, huge algal blooms often form.

There are two causes for this phenomenon: firstly, there’s far more light here than in the twilight under the ice. Secondly, freshwater from the melting ice now flows into the sea. Since freshwater is lighter than saltwater, it remains on top of it – a bit like a cocktail where the barkeeper stacks up layers of different fluids. Thanks to these stable layers, there is very little water circulation. As a result, the phytoplankton remain in the sun-filled upper layer, which offers them ideal conditions for growth. Further, the massive algal blooms at the ice’s edge promote the growth of small crustaceans – which the AWI researchers surmise is also the reason the polar cod feeds there.

Due to global warming, the Arctic sea ice has displayed intensive melting for several years now, and not just at the margins. At the same time it has grown thinner, and meltwater pools on the ice’s surface that reach down to the seawater below are an increasingly common occurrence. These produce patches where light can pass through the ice – and ideal growth conditions for the phytoplankton. The result: isolated algal blooms.

Yet the intensive melting of ice in the Arctic is also producing other phenomena in forms researchers have never seen before. In the summer of 2012, an expedition team on board the research icebreaker Polarstern made a rare observation. That summer was characterised by the most intensive Arctic sea ice melting the researchers had ever witnessed. The ice was thin and riddled with meltwater pools, making it far more transparent and offering ideal conditions for the diatom Melosira arctica to flourish. In many spots it formed long strands and fist-size clumps under the ice. Once the ice had all but disappeared, these clumps rapidly sank to the depths of the sea.

Since the researchers aboard RV Polarstern were onsite, they seized the opportunity to send underwater cameras to the ocean floor. The images they captured were astounding: on the normally practically lifeless seafloor, sea cucumbers and brittle stars could be seen devouring the clumps of Melosira, clearly demonstrating that any food that drifts down from above is quickly consumed, even in Arctic waters.

The work of Hauke Flores and Ilka Peeken shows that a number of open questions remain concerning food webs in the Arctic and Antarctic. Yet today the researchers can gain deeper insights into the symbiotic communities under the ice than ever before, not only with cameras, but also using a specially designed net that allows RV Polarstern to sweep under the sea ice for kilometres at a time.

The “Surface and Under Ice Trawl” (SUIT) net, developed by researchers at the Dutch marine ecology research institute IMARES, can be used to capture e.g. fish and small crustaceans. “We have also equipped it with light sensors that allow us to measure how bright it is under the ice,” says Hauke Flores. Flores, Peeken and their teams plan to use these light measurements to more closely explore the relation between available light and algae development. The AWI researchers and their Dutch peers are currently the only scientists in the world to employ such an advanced net system. Their chief goal is clear; as Peeken summarises: “In the future, we want to be able to translate the ice thickness and snow cover into precise information on the brightness and living conditions below. We could then accurately estimate the biomass, which would allow us to determine how productive and important specific regions of the Arctic and Antarctic are for marine life.”

This video, filmed and produced by our Dutch partners from the research centre IMARES shows, how the SUIT net was deployed during a Polarstern expedition into the Weddell Sea in late summer 2013. The footage at the beginning of the video as well as all underwater footage was recorded by a small GoPro camera attached to the net's metal frame, looking strait ahead into the direction the net is pulled to. Source: Jan van Franeker - IMARES

The work of Hauke Flores and Ilka Peeken shows that a number of open questions remain concerning food webs in the Arctic and Antarctic. Yet today the researchers can gain deeper insights into the symbiotic communities under the ice than ever before, not only with cameras, but also using a specially designed net that allows the Polarstern to sweep under the sea ice for kilometres at a time.

The “Surface and Under Ice Trawl” (SUIT) net, developed by researchers at the Dutch marine ecology research institute IMARES, can be used to capture e.g. fish and small crustaceans. “We have also equipped it with light sensors that allow us to measure how bright it is under the ice,” says Hauke Flores. Flores, Peeken and their teams plan to use these light measurements to more closely explore the relation between available light and algae development. The AWI researchers and their Dutch peers are currently the only scientists in the world to employ such an advanced net system. Their chief goal is clear; as Peeken summarises: “In the future, we want to be able to translate the ice thickness and snow cover into precise information on the brightness and living conditions below. We could then accurately estimate the biomass, which would allow us to determine how productive and important specific regions of the Arctic and Antarctic are for marine life.”