Marine Biology

What Goes on Beneath the Floes

The “underside” of sea ice in the Arctic and Antarctic is a unique habitat, where roughly 1,000 different species of algae, which are largely unaffected by cold or lack of light, flourish. Serving as food for small crustaceans, they represent the basis of food webs in the polar seas. AWI researchers Ilka Peeken and Hauke Flores are currently exploring the connections between the ice and different life forms in detail – which will in turn help them estimate the extent to which the warming climate will change these habitats.

Without the polar cod (Boreogadus saida) polar bears and other Arctic predators would die of hunger. The fish is believed to account for up to 75 per cent of the energy transfer between zooplankton and vertebrate predators.
An Arctic key species: Polar cod (Boreogadus saida) (Photo: Hauke Flores)
Caves, walls and grate made of ice: This image was made by a remotely operated vehicle (ROV) and shows the underside of heavily deformed Arctic sea ice. In the center one can see a orientation marker, colored back and white. It is one meter long.
An icy labyrinth (Photo: Alfred-Wegener-Institut/ROV Ronia/Meereisphysik)

Young polar cod are barely longer than a finger, making them easy prey for larger fish. The larvae and juvenile fish can only survive by hiding; and the best hiding place in their Arctic home waters is the sea ice. Millions of them seek shelter in the cracks and crevices under the ice floes, because the underside of the sea ice is hardly smooth as polished glass. On the contrary! It’s a virtual labyrinth of caves, cracks and sharp ridges. Sometimes these floes overlap, while at others they collide, as can be seen by the metre-wide sheets of ice jutting up vertically from the water. In some cases, terraces also form on the underside.

This craggy ice world not only offers the polar cod a safe haven. They also seek their own prey on the underside of the sea ice, where algae and countless amphipods and copepods can be found. In fact, the sea ice is practically an ocean floor densely populated by small life forms – the only difference being that everything is upside down.

Marine biologists still only partly understand how the food web under the ice works: given how difficult it is to access the underside of the pack ice, observations of polar cod or other fish there have been few and far between. It’s only been in the last few years that scientists have slowly begun to unlock the secrets of the food web, thanks to the advent of underwater robots and nets specially designed for use under the ice. Just when the polar cod eats, and which organisms are on the menu, are questions that no one can answer for sure. Our knowledge of another key polar species, the Antarctic krill, is also fragmentary. The small crustacean is similar to the North Sea brown shrimp and is considered a staple for the penguins, seals and whales of the Southern Ocean. As such, its role in the waters of the Antarctic is just as central as that of the polar cod in those of the Arctic.

The question of who eats whom

“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.”

AWI biologist Ilka Peeken (r) is focusing her research on ice algae. This image shows her and a colleague during investigations of a melt pond on Arctic sea ice.
AWI ice algae specialist Ilka Peeken (r.) during fieldwork on Arctic sea ice (Photo: Christiane Uhlig)
AWI biologist Hauke Flores, who leads the young investigators group Iceflux.
AWI biologist Hauke Flores (Photo: Steve Geelhoed)

Nothing doing without ice

The Arctic food web, depending on sea ice (Photo: ArcCoML)

The ice cover of the Arctic Ocean is a major pillar of marine life. Its freezing and melting determine as of when and for how long polar bears can prey on seals on the ice. The same time is available to amphipods and copepods to eat their fill of ice algae on the bottom of floes. The well-nourished crustaceans serve, in turn, as food for polar cod which are hunted by seals, sea birds and whales. When the ice area melts in summer, the larder of sub-ice species disappears as well. On the other hand, sunlight penetrates into the upper water layer. Algae start to grow, sink to the bottom and in this way supply the creatures of the deep sea with food. (Diagram: ArcCoML)

Surviving in the twilight

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.

Green instead of White: an Antarctic ice floe, containing countless ice algae
Green instead of White: an Antarctic ice floe, containing countless ice algae (Photo: Mario Hoppmann)
Ice algae attached to the underside of Arctic sea ice
Ice algae attached to the underside of Arctic sea ice (Photo: ROV Ronja/ AWI-Sektion Meereisphysik)

Turning fatty acids into fingerprints

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.”

Degraded pennate aggregates in cryoconite holes at the bottom of a partially open melt pond.
Degraded pennate aggregates in cryoconite holes at the bottom of a partially open melt pond (Photo: Mar Fernández-Méndez)

Algal blooms at the ice’s edge

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.

Food for the ocean depths

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.”

Melosira arctica grows on the bottom side of Arctic ice floes.
Melosira algae growing under Arctic sea ice (Photo: Mar Fernandez-Mendez)
Cucumber of the genuis Kolga feeding on ice algae.
Sea Cucumber Kolga feeding (Photo: Antje Boetius)

The SUIT net deployed in Antarctic waters

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

A net unlike any other

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.”

 

 

Night-time deployment of the SUIT net, which can dive under the sea ice to catch organisms living underneath the sea ice.
Night-time deployment of the SUIT net (Photo: Jan van Franeker – IMARES)