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The melting of Arctic ice may reduce the duration of winter feeding for zooplankton.


The Sun rising above Arctic sea ice; view of a meltwater pond and pressure ridges in the foreground

Hauke Flores has a vivid recollection of a day in late September from his initial Arctic voyage 11 years ago. On that day, the nets used for sampling on the ship were covered in bioluminescence, which appeared to be flashing. The captured creatures were a multitude of small copepods that had started their annual journey from the depths to spend the winter feeding on the underside of the sea ice.

A polar ecologist, Flores from the Alfred Wegener Institute in Bremerhaven, Germany, has collaborated on a research that cautions about the potential impact of ongoing ice loss in the Arctic on copepods and other zooplankton. The study suggests that these organisms may be forced to remain at deeper depths for extended periods, resulting in a decrease of their winter feeding time by as much as a month.

A Calanus copepod with a size bar indicating it is about 2 millimeters long

Most of the zooplankton in the Arctic is composed of copepods, like this Calanus specimen. Attributed to: Kim Vane.

In the Arctic, the amount of sea ice has decreased by approximately 13% every decade since satellite observation began. This results in smaller and thinner ice floes, allowing more sunlight to penetrate into the ocean.

Small, floating animals in the water, known as zooplankton, have a precise method of moving up and down the water column. This allows them to both find tiny bits of food under the ice and avoid predators in the dark. According to Flores, the level of light is a crucial factor in this behavior, which occurs in oceans around the globe. During the day, zooplankton drift down, and at night they rise, making it one of the largest coordinated movements of living matter on Earth.

In the Arctic region, the migration of winter-active zooplankton is seasonal due to the Sun setting only once a year. During the polar night, these zooplankton move upwards to consume ice algae and microbes. They are most active during twilight and only rise above a certain level of light known as critical irradiance when the Sun is no longer visible on the horizon.

A Changing Arctic Lightscape

Previous research monitored the movement of Arctic zooplankton during their migration, indicating that their upward movement was highly affected by low levels of light. However, they did not gather data on light levels at the same time.

Scientists on the German icebreaker Polarstern attached a self-sufficient set of instruments to an ice floe in 2020. The instruments gathered data as the ice floe traveled over 1,000 kilometers in the polar night, from near the North Pole to off the coast of Greenland. Over a period of 6 months, the instruments monitored the density of zooplankton in the top 50 meters of the water and also measured the amount of light below the ice during the Arctic fall and spring twilights.

Two expedition members use an auger to drill a hole into sea ice to install an autonomous observatory.

The MOSAiC expedition team, consisting of researchers from various disciplines, are using tools to create a gap in the sea ice and set up a self-sufficient observatory for studying Arctic climate. The research vessel Polarstern can be seen in the background. Photo credit: Alfred-Wegener-Institut/Folke Mehrtens.

The information revealed that the intensity of light needs to be extremely low, specifically 0.00024 watt per square meter, in order for the zooplankton to begin their ascent. According to Gaëlle Veyssière, a coauthor of the study and a sea ice physicist at the British Antarctic Survey, this level of light can only be achieved during twilight and with a significant amount of snow and sea ice to filter out incoming light.

Map showing the drift path of the ice floe with the observatory

For half a year, the self-sufficient observatory moved through the middle of the Arctic Ocean. Attribution: Flores et al., 2023, CC BY 4.0.

Veyssière, Flores, and their team utilized four global climate models to simulate the potential decrease in sea ice and its effect on light transmission in the future. The scenarios used were a 2°C maximum warming through the end of the century with reduced carbon dioxide emissions, and a 4.4°C warmer average temperature.

According to the models, if the melting of ice in the Arctic continues without interruption, zooplankton will face a longer period of time in deeper waters and less time for feeding by the year 2050. This could pose a challenge for the winter survival of certain species.

Even though the Sun is below the horizon, light is still entering.

Tom Langbehn, a polar scientist at the University of Bergen who was not part of the study, stated that previous observations were confined to specific moments and locations. He also mentioned that observations such as these are crucial in bridging gaps in our knowledge and establishing a foundation for understanding ecological interactions.

According to him, they also enhance the accuracy of data. He explained that when we collect samples from research vessels that are well-lit and noisy, there is a bias that is often overlooked. This bias can be reduced by utilizing self-governing platforms.

According to Veyssière, the group had to be creative in their climate simulations as the models are unable to factor in light levels below 0.5 watt per square meter. To address this, the researchers created a separate model to accurately depict irradiance during twilight, as they were aware that light is still present even when the Sun is below the horizon.

Zone

Rewording:

Food networks in the deep sea layer known as the Twilight Zone.

Think of sea ice as a sponge.

In the beginning of the Arctic winter, when the sun is setting, the organic material created by a last-minute growth of ice algae becomes an important source of carbon for zooplankton that remain active during the winter. Flores explained, “Think of sea ice as a sponge. It has small passages and holes filled with salty water where debris can gather and provide food for microorganisms and some larger creatures.”

Zooplankton species that migrate are unable to store enough fat to survive the winter in a dormant state. They must continue to feed during this time. However, with thinner sea ice and changes in light, these zooplankton will arrive at the ice later, causing them to miss out on a significant amount of food. This is expected to have consequences for the Arctic food chain and carbon levels.

Copepods are tiny crustaceans that make up one of the largest groups of zooplankton. The bioluminescent copepods Flores saw on his first voyage, Metridia longa, are one of the most abundant species in the Arctic Ocean. Another important seasonal migrant is the centimeter-sized Calanus spp. “[Calanus copepods are] the main food of the polar cod, which is the main food of seals and so on,” Flores said.

He stated that the entire food chain could be affected if the zooplankton suffers any negative consequences, as they are responsible for transferring carbon to larger predators.

Veronika Meduna, a science writer, tweeted.

According to a study by Meduna (2023), the melting of Arctic ice may result in a decrease in the duration of winter feeding for zooplankton. This was published in Eos on 2 October 2023 and can be accessed at https://doi.org/10.1029/2023EO230373.

Text © 2023. The authors. CC BY-NC-ND 3.0

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