After the Ice

by Alun Anderson

  The profusion of life inside the ice and on its undersurface is one of the Arctic’s best kept secrets. A lump of sea ice may look much the same as ice that you take out of your refrigerator, but it is not. Sea ice began as salty water. As it freezes, salt is squeezed out of the growing ice crystals, and left surrounding them as a briny liquid which is just too salty to turn into ice. Instead, the brine flows into a network of tiny channels which gradually interconnect and broaden as they flow down to the bottom of the ice and reach into the sea beneath.

  A few years ago I heard about a remarkable experiment carried out at the Alfred Wegener Institute, Germany’s main polar research institute. Scientists took a lump of sea ice and spun it in a refrigerated centrifuge (the ice scientist’s equivalent of a spin drier) until all the brine drained from it. Then they filled the ice with a special resin which they could harden by shining ultraviolet light on it. After the ice was allowed to melt away, they were left with a beautiful resin cast of the branching channels, big and small, within the ice.2 Powerful microscopes showed that the insides of the ice were as complex as the interior of a sponge: there were broad routes, several millimeters across, leading down to the bottom of the ice, and vast numbers of narrow, interlinked byways no wider than the finest fibers of a spider’s web.

  Rolf Gradinger, one of the team who made that cast back in 1991, is now at the University of Alaska in Fairbanks. When I called him up to ask him about his work, I was thrilled to find that he is still as madly excited about sea-ice tunnels as if he had discovered them yesterday. “That three-dimensional cast was really new. It was so amazing to see how connected everything was,” he says. “It was just an eye-opener to see that for the first time. Whenever I work on sea ice, I have that picture in my mind.”

  Many of the tunnels in the ice are lined with algae, often along with dense mats of bacteria. The algae themselves are remarkable. They live in a dim, twilight world. “They are unique microbes on the earth,” says C.J. Mundy, an ice algae researcher at the University of Quebec at Rimouski. “The algae are so well acclimated to the faint amount of light that they are able to increase the amount of chlorophyll, their photopigment, to collect more of the light.” Down in these tunnels where the algae grow, strange-looking grazers with exotic names come to feast.3 There are rotifers, with a bloblike body driven by a crown of lashing cilia and flagellates which, as their name suggests, are powered by whiplike flagella. These in turn end up as prey for tiny carnivores, including the wormlike turbellarians and nematodes.

  The “tunnel world” sets automatic constraints.4 “It is unique and quite mind-boggling,” explains Gradinger. “Some of the brine channels are just so small that the bigger grazers don’t make it in. The physical properties of the brine channel and the network diameters create size restrictions that control the biology.” Inside the ice tunnels, it pays to have a very flexible body. Evolution has shaped some of the predators, such as the turbellarians, so that they can squeeze themselves into tunnels less than half their usual body diameter in the hunt for prey.

  Watching the inmates of the tunnel world is not easy. The best you can do is to cut a slice of ice about half a centimeter thick, put it under a microscope, and “sit in a cold room at-2°C and see what happens,” says Gradinger. “Turbellarians; I like them a lot I have to admit,” says Gradinger. “They may look like little red blobs, but under the microscope they are very active. You can actually see them moving around through the brine channels, and see how they squeeze themselves through very, very narrow parts.”

  Gradinger has found that young polychaete worms are thin enough to squeeze inside some of the channels and graze on the rich pastures within. In the ice off the coast of Alaska, he has seen astonishing numbers of these worms—more than 100,000 of them per square meter—feeding in the ice for a few months in spring. That is a graphic reminder of the abundance of life that ice algae can support.

  I worry about what will happen to these unique worlds if the sea ice begins to melt away completely in the middle of the summer. Gradinger explains that the creatures that live in the seasonal ice found close to the shore have always experienced ice that melts in summer and refreezes each winter. In summer, they live in niches among sand grains in the bottom of the shallow seas, and when winter comes they return to the newly forming ice. The richer varieties of creatures that live in old ice far out on the deep Arctic Ocean are different. “These organisms don’t survive in seawater or on the seafloor, so if you remove the sea-ice cover you remove these species,” says Gradinger bluntly. That means that with the loss of the old sea ice a unique, almost unknown world may vanish. “Yes,” says Mundy, “you can think of these animals as like the polar bears, but the polar bears are visible.”

  The ice algae are not the biggest source of food in the Arctic—plankton blooming in open water are more productive (when and where there is open water)—but they do have a special importance. Ice algae begin growing as soon as the sun returns to the frozen Arctic and light shines through the ice. “The sea ice provides a very early pulse of food, from March to the end of April, that is available for herbivores long before anything is happening in the water,” explains Gradinger. “At that time of the year, concentrations of algae in the bottom layer of ice are four orders of magnitude higher than what you see in the water column. It is an enormous, high-density food patch.” The ice algae kick-start the ecosystem in spring. The tiny grazers—the amphipods and copepods—that feed under the ice are also the key food for enormous numbers of fish, especially the Arctic cod that is a vital link in the web that feeds birds, bigger fish, seals, and on to bears.

  Later in the year, in the parts of the Arctic where the ice melts away in summer, phytoplankton will bloom in the open water where there is far more sunlight than down under the ice. As a result they will provide an even bigger pulse of growth than the ice algae (provided there are enough nutrients in the water). That leads immediately to an interesting question: in the future, when the Arctic loses more and more summer ice, and more open water is exposed for longer each year, will the Arctic become a more productive, if very different, sea than it is now?

  The answer seems to be that it will. Already the first signs of change are arriving: after the sudden fall in the sea-ice area in 2007, plankton started to grow in the newly exposed open water over a longer season. In a single year the total productivity of the seas leapt by 40 percent. That change was recorded by Kevin Arrigo, a biological oceanographer from Stanford University in California, who took measurements from a satellite-borne sensor that measures the “greenness” of the surface ocean.5 I ran into Arrigo while we were both waiting to board a plane in Chicago en route to the American Geophysical Union’s giant conference in San Francisco. I asked him what he thought of that huge change. “I was very surprised,” he said. “Forty percent is a big increase. I thought there would not be enough nutrients in the water for such an increase. There must be some process bringing nutrients up from the deep water below.” He explained that 30 percent of the increase he saw came from the greater area of open water available, but 70 percent came because there was simply a longer time when the seas were free of ice.

  Whether the whole Arctic would always be a more productive place if the ice shrank away to nothing every summer is more debatable. The Arctic seas might end up capped by a layer of freshwater from melting ice and faster flowing rivers that prevents nutrients from getting up to the surface. Or the open waters might be stormier and mix nutrients more thoroughly. In the future, says Mundy, “you could have nutrient limitation or you could have open water plus the ability to mix the water better by coupling it to the atmosphere. Which one will take over we don’t know yet.” The Arctic is a big place and the answer will not be the same everywhere. “You really have to think of the Arctic Ocean as a patchwork of different systems, and each of the systems has its own rules and regulations,” says Gradinger.6

  A leap in the ocean’s total productivity might sound like good news for Arct
ic residents, and it could be if they were all very adaptable to new conditions. But such a radical change could just as easily be a disaster that simply encourages different animals to move into the Arctic. The special creatures that live in the Arctic now are adapted to its particular rhythms and to the timing of the ice melt. If that rhythm changes, then the whole ecosystem may be thrown out of balance, destroying the web of relations within the Arctic from bottom to top, leaving the Arctic a very unfamiliar place.

  We have some clues as to what could go wrong from research that is already underway across the Arctic at the University Centre in Svalbard. Here, marine biologists Jørgen Berge and his postdoctoral researcher, Janne Søreide, study blooms of plankton beneath the ice and in the nearby seas. I found out about their project CLEOPATRA (Climate Effects on Planktonic Food Quality and Trophic Transfer) by chance when I saw some pictures from the project in the little museum in Longyearbyen.

  One picture showed a group of researchers in fur and woolly hats having a picnic in the snow outside a small, tent-shaped wooden hut with a bent stovepipe sticking out of its roof and a much-patched old wooden door as its entrance. Another showed their laboratory bench, which seemed to be made of planks perched on a row of red, orange, and blue-and-yellow-striped oil drums, parked inside a tent. This looked like fun and I called them up to find that they have a delightful tale to tell, but one that could only be put together by researchers who are happy to live with cold, ice, and bears.

  Berge and Søreide work out at Rijpfjorden, a remote spot on the most distant northeasterly island of the archipelago. They have to fly out there by helicopter and live for weeks crammed into that wooden hut I saw. A large tent does indeed serve as a laboratory. “It works very well. It’s not well insulated, but we don’t need so much comfort,” says Søreide stoically. “We could sleep in tents, but as this place is known for being crowded with polar bears, the hut is better.” So far the bears have been more interested in chasing seals than researchers. “We have curious bears but not problem bears,” says Søreide. Still, everyone obeys the strict laboratory regulations and carries a rifle.

  Everywhere in the Arctic, the zone alongside the edge of the sea ice where phytoplankton bloom in spring as the ice melts is immensely important. The fresher, lighter meltwater helps trap the algae near the water surface where there is lots of sunlight and the bloom may stretch thirty miles out across the water. Satellites can easily pick out the band of pale green from space.

  The ice-edge zone is always buzzing with life. Seals and larger seabirds are busy catching fish, walrus climb out on the ice in between diving to the shallow bottom to feast on clams, smaller birds like the dovekie are dipping into the water to pick out copepods, piratical jaeger rob less agile birds, whales swim along the ice edge trawling up plankton or catching fish, and polar bears wait patiently at the ice edge in hope of snatching a seal. In a day’s cruise along the ice edge to the north of Svalbard, all these scenes will pass by, just as they will elsewhere in the Arctic where the summer ice edge lies in shallow waters. Deeper inside the Arctic, the seas are frozen over and there is a great silence.

  Among the abundant life at the ice edge, the cold-water copepod Calanus glacialis is a critical link in the food chain right across the shallow Arctic seas. Although just a tiny shrimplike creature, six millimeters from head to tail (excluding its two long antennae), Calanus glacialis is the most important grazer of the Arctic seas. It is found in the billions near and under the ice where it fattens up at great speed. For the copepods, “fatten” is the right word. As they gobble up plankton during the short summer, they build a store of high-energy lipids that will see them through the cold, dark winter when there will be nothing to eat.

  To huge numbers of fish and birds, this copepod is a perfect little package of high-energy food. Copepods provide a royal feast for Arctic cod, Boreogadus saida (quite different from the larger Atlantic cod that we eat and called “polar cod” in Europe), which live under and close to the ice.

  Out at Rijpfjorden, Calanus glacialis live in special abundance, and Berge and Søreide have found that it has a clever trick to make the most of the ice and its melt. When the sun returns to the Arctic in spring and rises high enough for its rays to penetrate through the ice, the algae that live beneath immediately begin to grow. The huge ice-edge bloom comes later, when the ice starts to melt. The copepod takes advantage of both blooms. In mid-April, they feed on the spring ice algae and reproduce. When the ice-edge bloom comes in July, their offspring feed, develop, and build up lipid reserves for the winter. “By taking advantage of the earlier ice algae bloom and then the phytoplankton growth season, glacialis can fulfill their life cycle in one year, despite living in a harsh Arctic environment,” explains Søreide.

  The copepods must have their timing right. Up here in the really high north the summer is very short. “The time in between these two blooms may also be critical,” says Søreide, “because it takes time for the copepods to develop so they can feed on the second bloom.” In a warmer world, when the ice melts earlier and faster and the seas are warmer, the timing could all go horribly wrong

  Calanus glacialis has a smaller, skinnier relative called Calanus finmarchicus that prefers warmer waters. A little farther south it lives in staggering numbers and feeds the herring and Atlantic cod of Iceland’s seas. But it’s not big enough to provide for the creatures of the High Arctic. “The energy content in these two Calanus species is so massively different,” explains Berge. “There may be as much as five times more energy in glacialis than in finmarchicus.”

  If the summer ice vanishes quickly in a changing Arctic, so too will those ice blooms. Because Calanus glacialis is geared to cold water and the timing of the ice blooms, it will be at a disadvantage to its relative finmarchicus, which is adapted to warmer, open seas. “A change in ice conditions would mean a change in the composition of Calanus species, which would have an immense impact on top predators including whales and seabirds,” says Berge. Already more southern areas of Svalbard that have had several ice-free summers have been taken over by the southern species, while the more northerly areas are still dominated by the High Arctic glacialis.

  The story of Rijpfjorden and its copepods may be one tiny tale from one far corner of the Arctic, but it reinforces a bigger point: it is not just the total productivity of the Arctic seas that is important to the web of life it supports but also the “when” and the “where,” the timing and the place-leading to what ecologists call “match-mismatch” problems. A tiny copepod that is well adapted to the old Arctic gives way to one from the south that does better in the new Arctic. But the whole shape of the pyramid may then change too. How big a change might come, we don’t yet know.

  In the more southerly Bering Sea, we already know of a major change to the ecosystem that is driven by changes in the “when” of the sea-ice melt. The phytoplankton don’t just feed the zooplankton, fish, and birds in the waters around them. A rain of them also sinks slowly to the bottom of the sea, giving life to the shellfish, crabs, and bottom-dwelling fish that are particularly abundant in the Bering Sea. They in turn feed larger animals that dive to the sea bottom in search of prey. Both pelagic (water-dwelling) and benthic (bottom-dwelling) creatures are ultimately dependent on the phytoplankton which capture the sun’s rays at the surface. But which set of creatures will get the biggest share of the food partly depends on the timing of the sea-ice bloom.

  If the ice edge bloom starts early in the year, the water is still too cold for many of the zooplankton and fish that would usually eat them to appear in large numbers. The result is that fewer plankton are eaten and more fall through the water to provide a bigger feast for life at the sea bottom. If the plankton blooms later, then the water is warmer and the zooplankton and fish are there to mop everything up. Life in the water gains and the creatures of the sea bottom lose. So the timing of the bloom has a big impact on the overall structure of the ecosystem.

  Already in the more southerly
Bering Sea, the impact of this change is being felt. The whole ecosystem is shifting in favor of sea dwellers and away from bottom dwellers.7 The number and distribution of fish, clams, crabs, and even jellyfish are all changing as the pelagic zone beats the benthic zone. As the timing of one event changes, the whole ecosystem shifts direction.

  Related changes may come to the High Arctic. “Shifting the sea ice causes different water temperatures, which in turn cause different zooplankton to grow. These might consume everything in the water column and then nothing is left to sink to the seafloor,” warns Gradinger. “The implications are huge because we have these benthic feeders living in the Arctic such as the gray whales or the walrus or the bearded seals, and for them a shift to a stronger pelagic component will be disastrous because they can’t feed off small copepods. You might have a complete shift in the ecosystem structure.”

  Can we predict what will happen, I asked Gradinger. “There are so many factors that are relevant and everything is always affected,” he sighed. “These interactions are not linear, so we can come up with some ideas about linkages, but we can’t make a predictive model right now.”

  The “where” of the ocean’s productivity is going to be just as important, and once again its impact will be just as hard to predict. There is one place, however, where we can expect to see a big, unwelcome change. In the new Arctic, if the edge of the sea ice moves too far out to sea, disaster will strike many colonies of breeding birds.