The Arctic sea-ice retreat changes the light conditions in the water and thus disturbs the behavior of the zooplankton, researchers report. As a result, the ecologically important animals will remain longer and longer in deep water layers, according to the data. According to the models, this threatens them with food shortages. Since zooplankton forms one of the foundations of the Arctic food chain, this disruption could have far-reaching consequences, the scientists say.
This is a particularly drastic impact of man-made climate change: the extent and thickness of sea ice in the Arctic Ocean is shrinking as a result of strong warming. In the summer, the coverage has continued to reach record lows in recent years and then the floating ice spreads out again later and weaker. According to current forecasts, the North Pole could become ice-free in summer as early as 2030. But the loss of sea ice is not just a drastic sign of global warming: the associated changes to the Arctic environment could once again cause complex climatic and ecological problems, researchers warn.
One of these is the change in lighting conditions in the Arctic Sea, to whose previous patterns the living creatures there have adapted. Specifically, due to the shrinking and thinning ice sheet, sunlight can now penetrate deeper and deeper water areas. For example, this can critically disrupt algal development, which forms a basis of the food chain. However, less is known about the reactions of the next link in this chain. An international research team led by Hauke Flores from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven has now examined in more detail how the changed light conditions affect the zooplankton.
How zooplankton align themselves with the light
As the researchers explain, there is a seasonal vertical migration of zooplankton in the polar regions, which includes copepods. “The animals there follow a seasonal cycle: During the month-long brightness of the summer polar day, the zooplankton stays permanently at greater depths. “In the months-long darkness of the polar night in winter, however, some of the zooplankton permanently ends up in the near-surface water directly under the ice,” says Flores. In principle, it is already known that this vertical migration is determined by the lighting conditions. The tiny animals remain in areas below a certain intensity of the twilight. As sunlight intensity changes over the course of the day or the seasons, zooplankton follow suit, ultimately causing them to rise and fall in the water column.
As part of their study, the scientists have now examined the behavior of the animals in the upper 20 meters of the water column directly beneath the sea ice. “It is precisely this area, which is difficult to reach for measurements, that is the most exciting because it is where the microalgae that the zooplankton feed on grow in and under the ice,” says Flores. In order to measure there, the team constructed a special measuring observatory, which they anchored under the ice from the research icebreaker Polarstern. The system was then able to continuously measure the light intensity and the movements of the zooplankton there.
Lack of food due to disturbed hiking behavior
“We found a very low critical light intensity for the zooplankton of 0.00024 watts per square meter,” reports Flores. The researchers then integrated this value into computer models that simulate the sea ice system. In this way, they were able to calculate for different climate scenarios how the depth of the critical light intensity will change as the sea ice becomes thinner as a result of advancing climate change. The results made it clear how the critical brightness penetrates to greater depths earlier and earlier in the year and reaches the surface layer again later and later in the year. The researchers say that the zooplankton will then react accordingly: they will remain below the critical light conditions. This means that in future scenarios it will stay at greater depths for longer and longer and its winter presence under the ice will become shorter and shorter.
The problem is that this could make it more difficult for the organisms to access their food, explains Flores: “In the future, in a warmer climate, the ice will form later in the fall, which will lead to lower ice algae production. In combination with the later ascent, this can often lead to a lack of food for zooplankton in winter. In return, an earlier decline of the zooplankton in spring can endanger the deeper-living juvenile stages of ecologically important zooplankton species, which could then be increasingly eaten by the adults,” says Flores.
According to the researchers, a new mechanism is now emerging that could harm the chances of survival of zooplankton, which is important for the food chain, in the Arctic. “This would have fatal effects on the entire ecosystem, including seals, whales and polar bears. “Every tenth of a degree less anthropogenic warming is therefore crucial for the Arctic ecosystem,” concludes Flores.
Source: Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, specialist article: Nature Climate Change, doi: 10.1038/s41558-023-01779-1