As sea ice in the Arctic continues to melt at an accelerated pace, sunlight is now reaching deeper into the ocean. Since marine zooplankton – tiny organisms that drift with the ocean currents – react to the available light, this phenomenon is also changing their behavior, particularly how they rise and fall within the water column.
According to a recent study led by the Alfred Wegener Institute for Polar and Marine Research (AWI), thinning sea ice could soon lead to more frequent food shortages for the zooplankton, as well as negative effects for larger species such as seals or whales.
Due to climate change, the extent and thickness of sea ice are currently declining. As the most recent measurements show, the mean sea ice extent is decreasing at a rate of 13 percent per decade and, by 2030, the North Pole could see its first ice-free summer.
As a result, the physical conditions for a variety of animal species inhabiting the Arctic Ocean are also radically changing. For instance, since due to less and thinner sea ice sunlight can penetrate much farther below the ocean surface, the growth of microalgae in the water and ice can increase substantially.
However, how such changing light conditions are impacting higher trophic levels, such as zooplankton (which feed in part on microalgae) remains poorly understood.
“Every day, the largest-scale mass movement of organisms on our planet takes place in the ocean – the daily migration of the zooplankton, which include tiny copepods and krill,” said study study lead author Hauke Flores, a biologist at AWI.
“At night, the zooplankton rise near the water’s surface to feed. When day comes, they migrate back to the deep, keeping them safe from predators.”
“Although the individual organisms are minuscule, taken together this constitutes a tremendous daily vertical motion of biomass within the water column.”
“But in the polar regions, the migration is different – it’s seasonal; in other words, the zooplankton follow a seasonal cycle.”
“During the months-long brightness of the Polar Day in summer, they remain in the deep; during the months-long darkness of the Polar Night in winter, part of the zooplankton rise and remain in the near-surface water just below the ice.”
According to the experts, both the daily migration at lower latitudes and the seasonal migration at the poles are mainly driven by sunlight, with zooplankton usually enjoying twilight conditions, and preferring to stay below a certain light intensity (known as critical irradiance).
When the intensity of sunlight changes – either in the course of a day or during seasonal shifts – the zooplankton migrate to places where they can find their preferred light conditions, rising or sinking in the water column.
“Particularly when it comes to the topmost 20 meters of the water column, just below the sea ice, there was no available data on the zooplankton,” Flores said.
“But it’s precisely this hard-to-reach area that’s most interesting, because it’s in and just below the ice where the microalgae that the zooplankton feed on grow.”
To investigate this region, the scientists designed and built an autonomous biophysical observatory, which they then moored below the ice during the MOSAiC expedition with the research icebreaker Polarstern in September 2020.
Far from any light pollution caused by human activities, this system succeeded in continually measuring the light intensity below the ice, along with the movements of zooplankton.
“Based on our readings, we identified an extremely low critical irradiance for the zooplankton: 0.00024 watts per square meter,” Flores reported. “We then fed this parameter into our computer models for simulating the sea-ice system.”
“This allowed us to project, for a range of climate scenarios, how the depth of this irradiance level would change by the middle of this century if the sea ice grew thinner and thinner due to climate change.”
The investigation revealed that, due to steady declines in ice thickness, the critical irradiance level would drop to greater depths earlier in the year, and would take a longer time to return to the surface layer.
Since the zooplankton typically remain in waters below this critical level, their movements would mirror such changes.
Thus, in the future, these tiny creatures will likely remain at greater depths for longer periods of time, with the time spent near the surface below the ice in winter considerably decreasing.
“In warmer future climates, the ice will form later in the autumn, resulting in reduced ice-algae production. This, in combination with their delayed rise to the surface, could lead to more frequent food shortages for the zooplankton in winter,” said Flores.
“At the same time, if the zooplankton rise earlier in the spring, it could endanger the larvae of ecologically important zooplankton species living at deeper levels, more of which could then be eaten by the adults.”
“Altogether, our study points to a previously overlooked mechanism that could further reduce Arctic zooplankton’s chances of survival in the near future. If that comes to pass, it will have fatal consequences for the entire ecosystem, including seals, whales and polar bears.”
“But our simulations also show that the impact on vertical migration will be much less pronounced if the 1.5-degree target can be reached than if greenhouse-gas emissions rise unchecked. Accordingly, every tenth of a degree of anthropogenic warming that can be avoided is critical for the Arctic ecosystem.”
The study is published in the journal Nature Climate Change.