because of condensation of sea ice ArcticSunlight penetrates deeper and deeper into the ocean and eventually Lower plankton production above the surface. This too, very negative consequences for seals, polar bears and whalesTheir food depends on these tiny organisms.
The response of marine zooplankton to available light also changes their behavior, particularly the way small organisms move up and down the water column.
As an international research team led by the Alfred Wegener Institute in Germany has shown, this future more frequent food shortages for zooplankton and adverse effects on larger species such as seals and whales, published in the journal ‘Nature Climate Change’.
The extent and thickness of Arctic sea ice is decreasing in response to anthropogenic climate change; The average amount of sea ice is currently decreasing by 13% per decade. Already in 2030, as recent studies and simulations show, The North Pole could see its first summer without ice.
As a result, the physical conditions of organisms in the Arctic Ocean are also noticeably changing. For example, because of the shrinking and thinning of sea ice, sunlight can penetrate far below the surface.
As a result, the primary production (ie growth) of microalgae in water and ice can be significantly increased under certain conditions. How these changing light conditions affect higher trophic levels in the food chain, such as zooplankton that partially feeds on microalgae, is not yet fully understood.
In this context, an international team of researchers led by Dr Hauke Flores of the Alfred Wegener Institute of the Helmholtz Center for Polar and Marine Studies (AWI) has now acquired valuable data.
Mass movement of microorganisms
According to Flores, “the largest amount every day mass movement of organisms The state of our planet: the daily migration of zooplankton, including small copepods and krill. At night, zooplankton come to the surface to feed, and when daylight comes they return to the depths, sheltered from predators.”
“Although individual organisms are small, together they create the enormous daily vertical movement of biomass in the water column,” he continued, “but migration is different in polar regions: they are seasonal, or zooplankton. They follow a seasonal cycle. they stay in the depths for months; During the dark months of the polar night in winter, some zooplankton rise and remain in near-surface waters.just under the ice,” he explains.
Both daily migration in low latitudes and seasonal migration in polar regions mainly determined by sunlight. Tiny organisms generally prefer twilight conditions. They like to stay under a certain light intensity (critical radiation), which is usually quite low and in the twilight range.
When the intensity of sunlight changes throughout the day or throughout the seasons, zooplankton move to where they find their preferred light conditionsThis means that they eventually move up or down the water column.
“There was no data on zooplankton available, particularly for the upper 20 meters of the water column just below the sea ice,” he stressed, “but this certainly hard-to-access area is the most interesting because microalgae that feed on zooplankton grow inside and just below the ice.”
To be able to take readings there, the team designed and built a self-contained biophysical observatory that AWI anchored under the ice at the end of the MOSAiC expedition with the research icebreaker Polarstern in September 2020. There, away from any light pollution from activities. For humans, the system was able to continuously measure the intensity of light under the ice and the movements of zooplankton.
Light depends on the thickness of the ice
“From our readings We have determined an extremely low critical radiation for zooplankton: 0.00024 watts per square metre. –To explain–. We then added this parameter to computer simulation models of the sea ice system. This has allowed us to predict, for a range of climate scenarios, how the depth of this radiation level will change by the middle of this century if sea ice becomes progressively thinner due to climate change.”
Here’s what the experts discovered: continuous reduction in ice thicknesscritical radiation will sink to greater depths earlier and earlier in the year and will not return to the surface layer until later in the year. Since zooplankton mainly resides in waters below this critical level, movements will reflect this change.
So in these future scenarios they will stay deeper and deeper for longer and longerIn winter, the time they stay close to the surface under the ice will be getting shorter and shorter.
Flores explains: “In future warmer climates, ice will form later in the fall, which will reduce algae production on the ice.” “This, along with the delay in its rise to the surface, may result in more frequent ice formation.” food shortages for zooplankton in winter”.
At the same time, “If zooplankton rises earlier in the spring, larvae of ecologically important zooplankton species living at deeper levels may be endangered, and many of these may be eaten by adults.”
“Taken together, our study points to a previously overlooked mechanism. further reduces the chances of survival of arctic zooplankton in the near future,” says Flores.
“If this happens, Deadly 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 is met than if greenhouse gas emissions get out of control. As a result, every tenth of the anthropogenic warming that could have been avoided is critical to the Arctic ecosystem.”
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