Introduction

Arctic sea ice is the layer of frozen water covering the northernmost ocean on our planet. Sea ice moves constantly in response to ocean currents and winds, and is also sensitive to changes in temperature. It used to be several meters thick and would only partly melt in the summer. With the current increase in temperatures due to climate change, more and more ice melts over the summer. This means that now a larger part of the ice cover is young and thin, because it only had one winter to grow [1]. These changes have consequences for the climate system and for organisms living in the Arctic Ocean (reviewed in [1]). For example, changes in the ice thickness affect the amount of sunlight and heat that reaches the ocean. More heat accumulated in surface waters leads to less ice growth and further ice melt. More sunlight below the ice enables more algal growth in the ocean. Since algae are responsible for food production the same way as plants are on land, it is important to know how many and which type of algae will grow in the Arctic Ocean. This will help us to predict how other organisms, such as fish and sea birds will respond. In 2015, we took part in a research expedition in the Arctic to collect data that will help us address this challenge.

During the research expedition, a Norwegian research ship named Lance was anchored to sea ice exiting the Arctic Ocean (Figures 1, 2A; [2]). The ship drifted with the sea ice for half-a-year, without using motor power. From January to June 2015, we used the ship as a base to carry out measurements of the atmosphere, ice, and ocean, such as temperature, snow depth, ice thickness, and the amount and type of algae present (Figure 2). Our measurements from the expedition taught us about new features emerging in the Arctic Ocean as a consequence of ongoing climate change. To learn more about the project go to our webpage www.npolar.no/nice2015 or look for the hashtag #nice2015arctic on social media. Here we will explain our findings on how changes in sea ice affect the “home” of algae, the sea ice, and underlying ocean habitats.

What Makes the Algae Grow Where They Grow?

We, the sea ice biologists, were especially interested to study the algae. Almost all food in the ocean is produced by algae, which are capable of producing organic sugars using sunlight, just like land plants do. This process is called photosynthesis. In the water column and inside the sea ice, the algae are microscopically small and are called phytoplankton and ice algae, respectively.

In spring, we were surprised to find high amounts of phytoplankton growing in the water under the sea ice [3]. Typically, very little sunlight reaches the water below the sea ice, because it is reflected away by the white ice and snow. Most of the phytoplankton present were of the species Phaeocystis pouchetii (Figure 2D). This algae species is unicellular but the cells can connect together to form colonies, which are up to 0.2 mm in diameter and which are possible to see with your bare eye.

Later in spring, toward the end of our expedition, we found high amounts of algae on top of the ice, in a layer between the thick snow and the relatively thin ice cover (Figure 2E; [4]). Typically, the main habitat for ice algae is the bottom of the ice which interacts with sea water. When we analyzed these algal communities under a microscope, we found that they consisted of species that are typically found in the water column, such as chain-forming diatoms (Figure 2F). These species are not typical in the sea ice.

To find out how these algae ended up between the snow and ice and what made algal growth possible in these habitats, that is, the different types of “homes,” we turned to our oceanography and sea ice physics colleagues to hear what had happened with the ice cover in the previous months (Figure 3).

Storms Break Up Ice Floes Into Smaller Pieces

Some of the most exciting processes we observed during our research expedition happened during several powerful winter storms. These storms came from the south and brought strong winds, rapid changes in air temperature, and heavy snowfall [5].

The strong winds during the storms pushed the ice around and broke it into pieces, making it more fragile and deforming it, more like a boulder field [5]. This opened up areas of open water, known as leads. Eventually these leads were covered by thin ice, but they still let high amounts of sunlight pass through, compared with thicker ice covered in snow [6].

Connecting all our findings, we discovered that the leads enabled the algae to grow below the sea ice, by allowing the necessary sunlight to pass through for algal photosynthesis [3]. The algae we found, Phaeocystis pouchetii, can cope with changes between low light under the thick ice and high light intensity under the leads better than some other algae, which may explain its success under this type of ice [3].

Heavy Snow Pushes the Ice Surface Below Water and Lets It Flood

Throughout our research expedition, our colleagues, the sea-ice physicists, found an unexpectedly deep layer of snow on top of the sea ice. This layer of snow was approximately half a meter thick. The snow accumulated on top of the sea ice during several storms earlier in the winter [5].

We found that the sea ice was very thin in relation to the deep snow layer. The fluffy texture of snow makes it a highly effective insulating blanket over the ice. This keeps the ice warm, compared with the cold atmosphere, and therefore prevents the ice from growing thicker during the cold winter.

On several occasions, we saw that the weight of the heavy snow pushed the surface of the ice below the water level. Imagine sitting on a swimming float in a pool. This caused the snow and ice to be flooded with seawater, which created a layer of slush. Together with the water, small amounts of algae were brought to the top of the ice. These algae acted in a similar way as seeds would and started an algal community in the slush layer, which explained our finding of the water column species. The slush creates a good place for algae to grow and reproduce, because there is more sunlight available than under the ice and there are less animals (zooplankton) eating them [4].

Sea Ice Can Also Melt From Below

Melting of the ice can also make it thinner and lighter in relation to the snow cover and lead to surface flooding. In addition to being melted by sunlight in spring and summer, sea ice can melt from below even if the water is just a few degrees warm. During storms, the strong winds stir the ice and the ocean below the ice, mixing up warmer water from deeper layers to the surface where it melts the ice [7]. This is a special process that happens in certain parts of the Arctic, where warm ocean currents from the North Atlantic flow into the Arctic under the ice cover (Figure 1). This warm Atlantic Water is usually about 2°C, which is a few degrees warmer than freezing Arctic waters at -2°C. With global ocean warming, we can expect that at some point in the future, the ocean will be able to melt the sea ice from below even without the help of storms: warm Atlantic water under the ocean surface only needs to be as warm as 5°C to melt sea ice very effectively, without the extra stirring caused by storms [8].

Conclusions—What Will the Future Arctic Look Like for Algae?

We think that during our expedition we saw what lies ahead for new algal life in the Arctic. We know ice is getting thinner and thus snow cover may get thicker in relation to the ice. This could make the occurrences of slush layers more common and make it an important habitat for algae in the future. Deeper snow covers resulting from more frequent storms may also make it more common.

Winter storms are increasing in frequency and this is likely due to climate change. As the Arctic sea ice continues to thin, it is becoming more sensitive to these winter storms. We expect that the ice will break more easily and allow more sunlight to pass through in the future, which could make algae growth below the ice happen more often.

Our research helps to predict how the different parts of the Arctic sea ice cover respond to climate change and how the Arctic ecosystems may change as a consequence. Yet there are still several questions we have no answer to: for example, is this happening all around the Arctic already and how will it progress toward the North Pole? What will the ocean be like when all the ice has melted in the summer? What type of algae will live in the future Arctic? There is so much more to find out!

Glossary

Arctic: ↑ The Arctic is a polar region located at the northernmost part of the Earth, often defined as the region north of the Arctic Circle (66° 33′ N). Around 60% of the Arctic is sea, much of which is covered by year-round sea ice.

Sea Ice: ↑ Sea ice is frozen ocean water. It forms, grows, and melts in the ocean. In contrast, icebergs, glaciers, and ice shelves form on land and are made of fresh water from compacted snow.

Ocean Currents: ↑ Oceanic currents describe the movement of water from one location to another, similar to wind in the atmosphere. Ocean currents are driven by wind, water density differences, and tides. Ocean currents affect the Earth’s climate by driving warm water from the Equator and cold water from the poles around the Earth.

Algae: ↑ Organisms that grow in the water and can fix carbon from the atmosphere like plants do on land. In the high Arctic, the algae present are tiny, single-celled organisms.

Habitat: ↑ A place or area with certain environmental conditions, in which certain type of organisms live.

Photosynthesis: ↑ The process in which algae and plants convert, with the help of sunlight, inorganic carbon from the atmosphere or the water into organic compounds, such as sugars, that they use for growth. Other organisms rely on these sugars and other compounds for their food. In addition, oxygen is released in the process.

Diatom: ↑ Type of single-celled algae that has a wall made out of a mineral called silicate, which is like glass. They are very abundant in the sea and important in the Arctic food webs. There are many different species of diatoms and they can be identified under the microscope.

Ecosystem: ↑ An ecosystem is a group or community of interconnected living organisms and their environment. Examples of marine ecosystems include coral reefs, deep sea, and of course Arctic ecosystems.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The N-ICE2015 campaign was led and supported by the former Centre of Ice, Climate and Ecosystems at the Norwegian Polar Institute. The Research Council of Norway project Boom or Bust (no. 244646) was central for the biological part of the campaign, including supporting HK during the N-ICE2015 campaign. MF-M was supported by Program Arktis 2030 funded by the Ministry of Foreign Affairs and the Ministry of Climate and Environment, Norway (project ID Arctic). We would also like to thank the project leaders and fellow scientists on N-ICE2015, and the captains and crew of RV Lance for their assistance. AM acknowledges support from the ARC Centre of Excellence for Climate Extremes (CE170100023). PI was supported by the Norwegian Research Council project 287871.