The Bottom of the Arctic’s Food Web Is of Top Importance

As our planet’s climate warms, its most rapidly changing region is the Arctic Ocean and surrounding seas. Warming causes many changes, including the melting of sea ice and a decline in the amount of water that is covered by ice. These changes impact organisms at every level of the food web. In this article, we explain how changes in temperature affect the quality of food available for animals that live in the Arctic. We focus on changes near the bottom of the food web, involving tiny plants that dwell inside and below sea ice, and tiny animals that drift in the Arctic seas. Shifts in the abundance and quality of the smallest organisms in the Arctic Ocean affect larger organisms, such as polar bears and whales. Changes at the base of the food web must be considered if we want to protect the creatures that call the Arctic home.

As our planet's climate warms, its most rapidly changing region is the Arctic Ocean and surrounding seas. Warming causes many changes, including the melting of sea ice and a decline in the amount of water that is covered by ice. These changes impact organisms at every level of the food web. In this article, we explain how changes in temperature a ect the quality of food available for animals that live in the Arctic. We focus on changes near the bottom of the food web, involving tiny plants that dwell inside and below sea ice, and tiny animals that drift in the Arctic seas. Shifts in the abundance and quality of the smallest organisms in the Arctic Ocean a ect larger organisms, such as polar bears and whales. Changes at the base of the food web must be considered if we want to protect the creatures that call the Arctic home.

HOW A CHANGING CLIMATE AFFECTS THE ARCTIC
Since the start of the Industrial Revolution, over years ago, our planet's climate has changed drastically. Temperatures have soared at a faster rate than any other time in the past million years! At the most northern and southern tips of our planet are the polar regions, the Arctic and the Antarctic. These are the coldest regions of Earth, where temperatures average well below • C. In winter, the top layer of the ocean freezes, creating what is called sea ice. Sea ice can range from paper thin ice, which melts very quickly, to incredibly thick ice that reaches heights of m and can survive for many years. Sea ice has a cooling e ect on the climate, acting as a refrigerator and keeping the rest of the planet at habitable temperatures.
As the temperature of our planet has increased, the environment has reacted in unique and alarming ways. In polar regions, large areas of sea ice are melting. The once snowy, white polar regions are being transformed into large areas of blue, open ocean. The warming of the polar regions has created a lot of questions that need answering. So, let us shed some light on how rising temperatures could a ect the Arctic's marine ecosystem.

WHY ARE PHYTOPLANKTON SO IMPORTANT?
At the base of the marine ecosystem, we find very small, but very important, plant-like creatures that drift in all seas. These creatures are called phytoplankton. Due to the microscopic size of phytoplankton, PHYTOPLANKTON A drifting plant that performs photosynthesis. they are measured on the scale of microns (µm). One micron is , times smaller than a centimeter! Phytoplankton typically live in what we call the euphotic zone, simply put, the depths where there is enough light for their photosynthesis.

PHOTOSYNTHESIS
A process in which plants use the sun's energy to convert carbon dioxide and water to oxygen and sugar.
Through photosynthesis, they take in carbon dioxide (CO ) from the atmosphere and produce oxygen, just like plants on land. Together, all the phytoplankton in the world's oceans produce half of the Earth's oxygen. This is an immense amount of oxygen considering that phytoplankton make up < % of the world's plant biomass BIOMASS The total weight of an organism, or group of organisms in a specific region.
[ ]! In comparison, large plants like trees make up around % of global plant biomass yet produce about the same amount of oxygen as microscopic phytoplankton . To demonstrate just how hardy https://www. changingarctic-ocean. ac.uk/ phytoplankton are, it is worthwhile to note that they have been around a pretty long time. The first sign of phytoplankton was preserved in rocks from western Australia around . billion years ago! Diatoms are the largest phytoplankton in our oceans ( Figure ). They DIATOM A large type of phytoplankton that is an important food source for zooplankton.
can be circular or elongated plants and are responsible for almost % of the Earth's photosynthesis. Though diatoms primarily live in the open ocean, they also thrive in bizarre places. Large masses of diatoms have been found within the sea ice of both polar regions, dwelling inside salty, liquid, ice channels that have enough nutrients and light for them to perform photosynthesis. When diatoms are found in ice they are no longer classified as phytoplankton. As they are fixed in one place and do not drift we call them ice-dwelling algae or simply ice algae. They are often trapped in the ice as the top layer of the ocean starts to freeze. To find ice algae, ice cores roughly cm in diameter are drilled out of the ice. Figure shows the bottom of an ice core that has an abundance of algae inside.

WHAT EATS PHYTOPLANKTON?
Zooplankton are the "middlemen" of the Arctic, performing the ZOOPLANKTON A drifting animal unable to swim against an ocean current. essential role of distributing nutrients to creatures throughout the food web as they are eaten by larger predators (Figure ). At some point in their lives, crabs, fish, and squid are all ocean drifters, and therefore termed zooplankton. Diatoms are a major food source for many zooplankton, because they contain many nutrients that give zooplankton the energy and raw materials to carry out activities, such as growing and reproducing.
As sea ice melts in summer, nutrients stored in the ice are released back into the ocean. Light also becomes more available because there is less sea ice to reflect the light back into the atmosphere. These spring changes favor phytoplankton, zooplankton, and everything that consumes these bottom-of-the-food-web residents. Since zooplankton eat diatoms, the zooplankton themselves become nutritious for larger animals, such as fish, seabirds, and whales [ ]. If zooplankton were not present, the rest of the ecosystem, including humans, would face a great loss of food. Humans in Inuit communities have relied on fish (zooplankton predators) and seals (fish predators) in the Arctic for over , years!
There is a dirty aspect of zooplankton that is particularly importanttheir poo. When zooplankton excrete their bodily waste, it becomes a food source for many other creatures. If it does not get eaten, it can end up in the seabed where it stores carbon for millions of kids.frontiersin.org September | Volume | Article |

Figure Figure
An ice core showing sea ice algae (brown layer inside the ice), including diatoms, dwelling within the bottom cm of the ice .
One group of zooplankton found in marine ecosystems worldwide is particularly noteworthy. The copepods were given their name COPEPODS A type of zooplankton with oar shaped feet. A very abundant type of copepod is called Calanus. due to their "pods" (or feet), which are shaped like the oars used for rowing a boat. Their oar-like feet (Figure ) help to give these microscopic animals superhero powers! Copepods could win prizes for having some of the most outstanding features in the animal kingdom. Copepods are the strongest animal, the fastest jumpers, and may be the most numerous type of animal on the planet! The favorite food of many copepods is phytoplankton, which must live near the ocean surface where there is enough sunlight for photosynthesis. Feeding on phytoplankton is good, but not always safe; fish, birds, and other predators also hunt in the well-lit surface water, and they are waiting for copepods to make a mistake. Every day, copepods deal with the threat of predators by only entering the shallow water at night, when there is no light. After eating, copepods quickly migrate down to deeper, darker waters before shallow-water predators can see them. This daily migration of copepods and other zooplankton is the largest migration of biomass on the planet, a humongous daily movement spanning depth of tens, hundreds, or thousands of meters.
Some of the most abundant copepods in the Arctic Ocean are members of a group called Calanus. Packed with nutritious fats after CALANUS Some of the most abundant and nutritious copepods in the Arctic Ocean belong to this group. intense spring and summer feeding, the Calanus copepods are so nutritious that some seabirds, fish, and whales travel massive distances across the oceans every year to gorge on them, typically in spring and summer. When most of the phytoplankton and zooplankton have been eaten, many of the birds, fish, and mammals leave the Arctic, to return the following year (but not all).

Figure Figure
The Arctic's marine food web [ ].
Phytoplankton and ice algae are eaten by zooplankton, and in turn, zooplankton are eaten by polar cod, seabirds, and the bowhead whales. This shows how both phytoplankton and zooplankton are an incredibly important food supply to the rest of the Arctic's ecosystem.

THE DARKNESS
The dark months of winter may not be the best time to be an herbivore dependent solely on photosynthesising plants for survival! Some copepods become omnivores in the winter, while others stop eating altogether and enter hibernation in safe waters far below the sea ice. Intense feeding during peak phytoplankton abundance is crucial for building the necessary fat stores to hibernate during winter. Copepods can look very di erent in February (after a winter of hibernation and starvation) compared to June (after feeding). In the Arctic, waking up before the phytoplankton bloom can be beneficial BLOOM Rapid growth of algae or phytoplankton.
to copepods. It allows them to feed on diatoms that hang and fall o the bottom the sea ice in spring. Following months of hibernation, an individual Calanus typically appears skinny, with limited fat reserves. It is only after feeding in spring and summer that a Calanus copepod can replenish its fat stores to their former glory (Figure ) [ ]! After their return to algae-rich surface waters in the spring, many successful copepods reproduce during the spring ice algal bloom, allowing their o spring to hatch during the phytoplankton bloom that occurs below the ice a few months later [ ]. This may be essential for their o spring to survive.

THE FUTURE
Researchers believe that if Calanus copepods failed to eat ice algae, the size of the copepod population could be drastically reduced. As sea ice declines due to climate change, this important food source for copepods is removed. Over long time scales, sea ice loss and other factors could decrease the availability of nutrients for the kids.frontiersin.org September | Volume | Article | Figure   Figure Calanus copepods sampled in February (left) and June (right). Though the lengths of the two copepods are relatively similar ( . and . mm), the February copepod is smaller overall than the June copepod, and the February animal, which is approaching the end of hibernation, also contains less fat in its oil sac ( ).
phytoplankton that are trying to grow below the ice [ ]. This decrease of food for phytoplankton could mean that smaller phytoplankton would become more numerous than bigger, more nutritious diatoms. So, instead of having an abundance of high-quality food like large diatoms, copepods in a warmer, ice-free Arctic might be forced to eat less nutritious, smaller phytoplankton. Scientists are already seeing smaller-sized organisms in both the copepod and phytoplankton communities [ ].

PROJECTED CHANGES IN THE ARCTIC AND WHAT WE CAN DO TO HELP
As the Arctic region changes, it is likely that we could see food stocks, such as diatoms and other phytoplankton decline, while also becoming smaller and less nutritious. Changes in the lowest part of the food web can have immense consequences for larger animals.
Extinction of species at the bottom of the food web can be terrible news for specialized predators that have evolved to eat them. Changes in the amount and type of plankton a ect humans and animals in many direct and indirect ways, ranging from changes in air quality, to how we interact with the environment and its resources. With less phytoplankton in the Arctic, CO concentrations in the atmosphere would increase causing our planet to continue warming.
As a society, we need to be more aware of the fact that our activities at home, work, or school can all a ect ecosystems in places that are far away from us. Small changes, such as walking or cycling instead of driving can drastically help to limit CO emissions. Research programs like the Changing Arctic Ocean , based in the UK, are providing governments and the public with the most up-to-date information on biological changes in the Arctic. Two groups from Changing Arctic Ocean have collaborated on writing this manuscript, and we have additional resources available if you would like to learn more , .

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.
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