Illustration by Joana Ho
The first time I saw sea ice, it seemed like a mirage. As we approached it on the German research vessel Polarstern, other scientists who had previously done Arctic research started pointing out the sea ice in the distance. It just looked like a low hanging cloud over the ocean. But as we got closer, I finally realized the first bit of ice in the distance was a peninsula-type outcropping from an immense realm of frozen water, and we were heading straight into it. Our ship was a double-hulled ice breaker, and it easily pushed through the marginal ice zone with thin and broken chunks of ice. After a few hours of steaming, the ice became thicker, and you could feel the ship moving a little more slowly, yet 1-meter-thick ice was still no challenge for the ship. Our second day in the ice, we reached even thicker areas, but the ship persisted, often needing to back up and ram the thickest ice (3+ meters) multiple times to break through. Sometimes, the entire ship would tilt to one side and remain for several minutes, turning a normal corridor into a hill to be climbed. Although slightly annoying (possibly more so for those with motion sickness) and difficult to sleep through, ship life continued as normal through the backing and ramming operations. After all, labs needed to be set up and science needed to be done!
A) The ice sheet appeared as a shimmering white band on the horizon as we approached.
B) The marginal ice zone was easy for the ship to cut through, as it had loosely packed ice that was fairly thin. C) As the ship moved further north into the ice sheet, the ice was thicker and harder to move through. (Photo credit: Katyanne Shoemaker, License CC BY 4.0)
So why all this effort? Why were we trying to force ourselves into this thick ice sheet? Well, this was part of the yearlong MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) Expedition. Details of my experience leading up to this trip can be found here. The goal of the expedition was to study one stable piece of ice for an entire year, by allowing the ship to be frozen in the ice in the fall during the freeze-up, drift with the ice across the pole throughout the winter and spring, and then melt out in the summer, somewhere (hopefully) in the Fram Strait. Ocean circulation and wind currents are forces constantly acting on the ice, pushing huge sections across the Arctic Ocean. In September of 2019, when the Polarstern entered the ice, a team of physical oceanographers, sea ice physicists, and modelers carefully chose one special piece of ice to call home, our floe.
A map of the drift path during the MOSAiC expedition. The German icebreaker Polarstern drifted with the Arctic ice for nearly a full year, only leaving the chosen floe in June to exchange passengers and restock supplies in Svalbard.
(Image credit: Matthew Shuppe, License CC BY 4.0)
The ‘M’ of MOSAiC stands for Multidisciplinary, which this expedition most certainly was. Teams of scientists worked together to plan sampling locations, drill holes in ice a few meters thick from which equipment could be deployed, and carry out intensive sampling. Sea ice physicists did regular transect walks around the floe to measure ice depth, snow depth, and numerous other variables. Massive helium-filled balloons were tethered to the ice and carried equipment high into the clouds to measure atmospheric conditions. Seawater and zooplankton were collected from the side of the ship using wenches to send gear thousands of meters down. There was even a Remotely Operated Vehicle (ROV) that was able to perform measurements and collect samples (including zooplankton for my research) from directly under the sea ice.
The ROV, named Beast, being prepared for a dive under the sea ice (left). The ROV was tethered with an orange cable which carried information to the controllers in a small hut on the ice. On this deployment, Beast towed a zooplankton net behind it, which was released here by my lab mate (right). (Photo credit: Katyanne Shoemaker, License CC BY 4.0)
Throughout the cold (down to -35°C), dark winter, scientists worked on this ice floe collecting samples from the ice, ocean, and atmosphere. Luckily for me, my role in the expedition took place in the summer, when the Arctic sees 24 hours of sunlight and temperatures stay within a few degrees of freezing (generally -2 – +2°C). At these relatively warm temperatures, the ice can melt quickly from above and below. When the surface ice melts, you can see beautiful blue freshwater ponds (meltponds), which can eventually melt all the way through to mix with the seawater underneath. The ice also melts on the underside as light returns and temperatures increase in the Spring. This melting water brings with it microscopic organisms that were trapped in the ice when it formed, including photosynthetic ice algae. Once released back into the ocean, these ice algae can take advantage of the nutrients and sunlight to start a phytoplankton bloom.
This transition to a bloom period was exactly what I was interested in catching. Blooming phytoplankton in the ocean usually cascades into a burst of life higher in the food chain. My project in the MOSAiC Expedition was to look at what zooplankton in the Arctic are feeding on, and I was especially interested to see if they were feeding on ice algae directly or waiting until it was released into the ocean. Zooplankton in the Arctic come in many different sizes, from single-celled microzooplankton to jellies, pteropods, and krill that you could pick up with your hands. I was especially interested in copepods which are highly abundant throughout the oceans, but are pivotal in the Arctic food web.
A helicopter view of the ice sheet and the ship
shows varying shades of blue. The light blue
areas on the ice are ponds of melted water.
As the ponds get deeper, they can melt all the
way through to the seawater below.
(Photo credit: Katyanne Shoemaker, License CC BY 4.0)
Diversity abounds under the ice! Contents of a zooplankton net tow show various species and life stages of copepods and other zooplankton. The large orange copepod in the lower middle is a Paraeuchaeta, the fast swimming red ovals are known as Ostracods, and the animals with the long red antennae are Calanus copepods. Hundreds of hours were spent at sea sorting these animals for experiments and to photograph and preserve for lab analysis. (Video credit: Katyanne Shoemaker, License CC BY 4.0)
Different species of copepods have different life strategies. Some, like the blue glowing Metridia, are active all year and eat anything available to them, including phyto- and zooplankton and dead sinking particles. Others appear to be strict carnivores, like the large Paraeuchaeta. By far the most abundant group of copepods I saw in the Arctic were the Calanus copepods which have typically been considered herbivores but may also take advantage of smaller animal prey if available. The Calanus copepods feast in the phytoplankton-abundant summer surface water and build up fat stores in a lipid sac. This fat helps the copepods survive the long winter in a hibernation-like state known as diapause. The fat (Omega-3 fatty acids, like in your healthy fish oil supplements) also just happens to be super tasty to just about everything living around the copepods, and they are often on the menu for many whale species in the Arctic as well as polar cod.
The summer melt season in the Arctic brings with it light and food! These images show the developing green color in Calanus copepods’ guts, indicating a change in food type and abundance. The oil sac inside of these animals is also growing as the copepods eat, to store energy for the upcoming winter. (Photo credit: Katyanne Shoemaker, License CC BY 4.0)
Because being able to eat and store lipids is so important for copepods, their lives are delicately entwined with the seasonal rhythms of sunlight and ice melt. Some species of copepods only begin reproducing when there is an abundance of food, to ensure the survival of the largest number of their offspring. Other species lay eggs before the bloom period, in expectation that there will be sufficient food when the copepod babies (called nauplii) reach feeding age. My research focus is to understand what they are currently eating in relation to what is available in the surrounding seawater, and the timing of their feeding activity.
This work will become a baseline for current feeding cues in the central Arctic. We cannot currently predict if or how these seasonal patterns may be disrupted in a warming ecosystem. What we do know is that the polar regions are experiencing climate change more dramatically than anywhere else on Earth. How will changes in the rate and start of the ice melt season affect the seasonal patterns of the organisms that rely on sea ice? Only time will tell what the fate of the Arctic ecosystem will be.
Additional reading:
To find out more about how climate change is affecting the polar regions, see here: https://worldoceanreview.com/en/wor-6/climate-change-impacts-in-the-polar-regions/
For more information on the MOSAiC Expedition, including daily blog entries from a year at sea, visit: https://mosaic-expedition.org/
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