Unraveling the Enigma: The Influence of Melting Sea Ice on Oceanic Fauna

Photo Caption: Arctic amphipod Themisto libellula. Source: Alfred Wegener Institute / Barbara Niehoff

A recent scientific investigation has revealed that climate change is altering the traditional migratory patterns of Arctic zooplankton.

The accelerated melting of Arctic sea ice is facilitating deeper penetration of sunlight into ocean waters, consequently affecting the migratory habits of marine zooplankton. A study spearheaded by the Alfred Wegener Institute suggests that this could lead to recurrent scarcity of food for zooplankton, with subsequent implications for larger Arctic organisms. The research emphasizes the need to limit global warming to a 1.5-degree target for the conservation of the Arctic ecosystem.

Escalating Arctic Ice Loss and Its Consequences for Aquatic Species

The enhanced loss of sea ice in the Arctic region allows for sunlight to permeate increasingly deeper layers of oceanic water. Marine zooplankton, being responsive to changes in light conditions, are subsequently altering their behaviors, specifically in relation to their vertical movement within the ocean’s water column. An international consortium of researchers directed by the Alfred Wegener Institute has found that such behavioral changes could result in a more frequent dearth of food for zooplankton, thereby adversely affecting larger animals such as seals and whales.

The study was formally published on August 28, 2023, in the scientific journal Nature Climate Change.

Transformations in the Arctic Ecosystem Induced by Climate Change

Anthropogenic climate change is causing both a reduction in the extent and a thinning of Arctic sea ice; on average, sea ice coverage is decreasing at a rate of 13 percent per decade. Forecasts and computational models suggest that as early as 2030, the Arctic could experience its inaugural ice-free summer. Consequently, these changes are also affecting the marine organisms residing in the Arctic Ocean.

For instance, diminished sea ice allows for more extensive penetration of sunlight below the surface. This has the potential to substantially elevate the primary production—or growth—of microalgae present in water and ice. However, the ramifications of these altered lighting conditions on higher levels of the food chain, such as zooplankton that partially feed on microalgae, remain inadequately understood. In this context, a global team of scientists led by Dr. Hauke Flores of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, has provided valuable insights.

An Elucidation of Zooplankton Migratory Patterns

Dr. Flores elaborates, “The world’s largest mass movement of organisms occurs daily within the ocean due to zooplankton migration. These minute organisms ascend to the ocean’s surface to feed at night and retreat into deeper waters during the daytime to evade predators. However, in polar regions, this migratory pattern is largely seasonal.”

Sunlight predominantly controls both the daily and seasonal migratory activities of these microscopic entities. When changes in sunlight intensity occur due to daily or seasonal variations, zooplankton adjust their positions within the water column to find their optimal light conditions.

The research team devised an autonomous biophysical observatory, moored beneath the ice, to consistently measure light intensity and zooplankton movements. This was conducted as part of the MOSAiC expedition with the AWI research icebreaker Polarstern in September 2020.

New Insights and Future Implications

Dr. Flores states, “Based on our measurements, we determined an exceptionally low critical irradiance for zooplankton at 0.00024 watts per square meter. We then used this data to project future conditions, revealing that due to diminishing ice thickness, the critical light level would descend deeper earlier in the year and return to surface layers later.”

As per the projections, zooplankton would linger in deeper waters for increasingly longer periods, while their time near the ice surface during winters would be curtailed. This could result in increased food scarcity for zooplankton and risks for other zooplankton species.

“In summary, our study identifies a previously underappreciated mechanism that may severely jeopardize the survival prospects of Arctic zooplankton in the near future,” concludes Dr. Flores. “Should this come to fruition, the implications for the broader ecosystem, including seals, whales, and polar bears, would be devastating. However, our models indicate that adhering to the 1.5-degree global warming target would mitigate such impacts.”

Reference: “Declining Sea Ice May Lead to Prolonged Deep Water Residence for Zooplankton,” by Hauke Flores, et al., published on August 28, 2023, in Nature Climate Change. DOI: 10.1038/s41558-023-01779-1

Frequently Asked Questions (FAQs) about Arctic zooplankton migration

More about Arctic zooplankton migration

• Alfred Wegener Institute’s Official Publication
• Nature Climate Change Journal
• The 1.5-degree Global Warming Target
• Overview of Arctic Sea Ice
• Introduction to Marine Zooplankton
• Impacts of Climate Change on Marine Ecosystems
• Anthropogenic Climate Change
• The MOSAiC Expedition
• Global Climate Models and Projections