Climate change is affecting the seasonal vertical migration of zooplankton in the Arctic, according to a new study.
Increasing melting of Arctic sea ice allows sunlight to penetrate deeper into the ocean, affecting the migratory behavior of marine zooplankton. Research by the Alfred Wegener Institute suggests this could lead to frequent zooplankton food shortages in the future, potentially affecting larger Arctic species. The study shows that keeping global warming to a 1.5-degree target is critical to protecting the Arctic ecosystem.
Increasing melting of arctic ice and its impact on marine life
Due to the increasing melting of sea ice in the Arctic, sunlight is now penetrating deeper into the ocean. Marine zooplankton change their behavior as they respond to available light; especially how small organisms rise and fall in the water column. As an international research team led by the Alfred Wegener Institute has shown, this could lead to more frequent food shortages for zooplankton in the future and have negative consequences for large species, including seals and whales.
The study was published today (August 28, 2023) in the journalism. Nature Climate Change .
Scientists place buoys on an ice floe in the Central Arctic Ocean at the end of MOSAiC 5th phase from RV Polarstern in September 2020. The buoys work as an autonomous biophysical observatory on sea ice and in the ocean. Author: Alfred Wegener Institute / Folke Mertens
Changes in the Arctic ecosystem due to climate change
Arctic sea ice is decreasing in volume and thickness in response to anthropogenic climate change; The average sea ice area is currently decreasing by 13 percent per decade. According to the latest studies and simulations, the first ice-free summer in the Arctic could be experienced as early as 2030. As a result, the physical conditions of organisms in the Arctic Ocean are changing equally markedly.
For example, because sea ice is smaller and thinner, sunlight can penetrate much deeper than the surface. As a result, under certain conditions, the primary production, i.e. growth, of microalgae in water and ice can increase significantly. How these changing light conditions affect higher trophic levels of the food chain, such as zooplankton, which partially feeds on microalgae, is not yet fully understood. In this context, Dr. from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Studies (AWI). An international team of researchers led by Hauke Flores now has valuable information.
Scientists place buoys on an ice floe in the Central Arctic Ocean at the end of MOSAiC 5th phase from RV Polarstern in September 2020. The buoys work as an autonomous biophysical observatory on sea ice and in the ocean. The observatory consisted, among other components, of an Acoustic Zooplankton and Fish Profiler (AZFP) measuring acoustic backscattering in the upper 50 m of the ocean, and a radiation station equipped with hyperspectral light sensors measuring radiation from 350 m at the bottom of the ice. Up to 920 nm25 and float CTD (conductivity, temperature, depth). Author: Alfred Wegener Institute / Folke Mertens
Explaining the migratory behavior of zooplankton
According to Flores: “Every day, the largest mass movement of organisms on our planet takes place in the ocean, the daily migration of zooplankton including tiny copepods and krill. At night, zooplankton rise near the water surface to feed. When the day comes, they migrate back to the depths, sheltered from predators. Although as small as individual organisms are, in aggregate this creates a large daily vertical movement of biomass in the water column.
“But in the polar regions, migration is different; it’s seasonal, meaning that zooplankton follow a seasonal cycle. They stay deep during the months-long brightness of the polar day in summer; during the months-long darkness of the polar night in winter, some of the zooplankton rises and stays in near-surface waters directly under the ice.”
Both the diurnal migration at low latitudes and the seasonal migration in the polar regions are largely determined by sunlight. Tiny organisms generally prefer twilight conditions. They like to stay under a certain light intensity (critical light), which is usually quite low and extends into the twilight range. As the intensity of sunlight changes throughout the day or season, zooplankton gravitate towards where they can find the light conditions they want, which means they eventually rise and fall in the water column.
“Zooplankton data were not available, especially when it came to the top 20 meters of the water column, just below the sea ice,” Flores explains. “But what’s most interesting is this hard-to-reach area, because the microalgae that the zooplankton feed on are growing in and under the ice.”
The team designed and built a stand-alone biophysical observatory to take readings there; They docked under the ice at the end of the MOSAiC expedition with the AWI Polarstern research icebreaker in September 2020. This place is far from any light pollution caused by humans. activity – the system was able to continuously measure the intensity of light under the ice and the movements of zooplankton.
New discoveries and implications for the future
“Based on our readings, we identified an extremely low critical radiation for zooplankton: 0.00024 W per square meter,” says the AWI researcher. “We then entered this parameter into our computer models to simulate the sea ice system. This allowed us to predict, for a number of climate scenarios, how the depth of this exposure level would change by the middle of this century if sea ice became increasingly thinner due to climate change.”
Experts have found that, due to the continued reduction in ice thickness, the critical radiation level falls to greater and greater depths early in the year and does not return to the surface layer until later in the year. Their movements will reflect this change, as zooplankton remain in waters almost below this critical level. Accordingly, in these future scenarios, they stay at great depths for ever longer periods of time, while the time they are near the surface under the ice in winter becomes progressively shorter.
“In a warmer climate in the future, ice will form in late autumn, which will lead to less ice algae production,” explains Flores. “This, coupled with the delay in surfacing, can lead to more frequent food shortages for zooplankton in the 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, many of which may be eaten by adults.”
“Overall, our work points to a previously overlooked mechanism that could further reduce the chances of survival of Arctic zooplankton in the near future,” says Flores. “If this happens, it will have deadly consequences for the entire ecosystem, including seals, whales and polar bears. But our simulations also show that if the 1.5 degree target is met, the impact on vertical migration will be much greater if greenhouse gas emissions continue to rise uncontrollably. “It also shows that it will be less obvious. Accordingly, every tenth anthropogenic warming that can be prevented is critical for the Arctic ecosystem.” Source
Source: Port Altele
