Food availability for seabirds, such as albatrosses, which breed on sub-Antarctic islands, may decline as the nutrient-rich seasonal sea ice zone continues to retreat due to global warming. This retreat results in increasing distances between the birds’ breeding sites and their foraging grounds.
Albatrosses and petrels travel huge distances over the Southern Ocean when foraging. However, until recently, little was known about the significance of the seasonal sea ice zone around Antarctica as a feeding ground for these birds. A study, led jointly by Durham University and the British Antarctic Survey, now shows that seabirds breeding on sub-Antarctic islands are closely connected to sea ice dynamics. To find food, they travel to this region, thousands of kilometers from their breeding sites. With Antarctic sea ice predicted to retreat further, the effects of human activity on these already vulnerable populations could be exacerbated.
The results of the study, published in August in Progress in Oceanography, suggest that seabirds may face challenges in meeting their food requirements if sea ice continues to retreat due to global warming. As sea ice diminishes, birds will likely have to travel longer distances to find food. Additionally, the spatial and temporal patterns of food availability, which the birds have historically relied on, may shift.
“Every winter the sea freezes around Antarctica, with sea ice covering tens of millions of square miles. We found that albatrosses and large petrels travel hundreds of miles, some far into the area covered by this sea ice and we think that they do this to feed,” explains Dr. Ewan Wakefield, a researcher in the Department of Geography at the University of Durham and lead author of the study, in a university press release. “In that case, Antarctica’s receding sea ice, driven by climate change, could affect not just the penguins, familiar to many people, that breed on the continent, but also huge numbers of seabirds breeding hundreds or thousands of miles away.”
The research team analyzed tracking data of seven seabird species from 1992 to 2023. All seven species – White-chinned petrel (Procellaria aequinoctialis), Northern giant petrel (Macronectes halli), Southern giant petrel (Macronectes giganteus), Light-mantled albatross (Phoebetria palpebrata), Black-browed albatross (Thalassarche melanophris), Grey-headed albatross (Thalassarche chrysostoma) and Wandering albatross (Diomedea exulans) – breed on Bird Island near South Georgia.
The analysis of a total of 2,497 foraging flights undertaken by 1,289 birds during the breeding season showed that all seven species regularly forage in the sea ice zone. However, there are differences between the species. Most albatrosses and White-chinned petrels tend to avoid ice-covered areas, preferring areas where ice melt enriches the ocean with nutrients in late summer and fall, when sea ice extent is at its lowest. On the other hand, Southern giant petrels, which feed mainly on carrion, sometimes migrate hundreds of kilometers into the pack ice zone. Grey-mantled albatrosses also use sea ice areas for foraging during a large part of the breeding season, but not the pack ice.
The researchers also found that species that prefer open water follow the melting sea ice southwards with a delay of a few weeks over the course of the summer. They believe the birds are tracking their prey, which increases following the phytoplankton bloom and gradually moves further south. This migration behavior is known as “Green Wave Surfing,” referring to the birds following the wave of resources created by the bloom. Only two ice-associated species, the Southern Giant Petrel and the Grey-headed Albatross, track the sea ice habitats without delay throughout the year.
Can no longer rely on the sea ice
The Antarctic sea ice is one of the most seasonally dynamic and largest habitats on Earth and the entire food web in the Southern Ocean is adapted to its fluctuations: In September, it covers approximately 40 percent of the Southern Ocean with a maximum extent of around 20 million square kilometers and in February it shrinks to an area of around 4 million square kilometers. The extent of the sea ice has been stable for a long time despite global warming. However, in the last five years it has retreated ever faster and reached record minima.
The maximum (left) and minimum (right) sea ice extent in September and February from 2003 to 2023 and 2024. Animation: Sea ice portal
“Given that all seven species of albatross and petrel we looked at travelled to the Antarctic seasonal sea ice zone, it is likely that they, and many other sub-Antarctic breeding seabirds, are linked to sea ice dynamics. Declines in Antarctic sea ice predicted under climate change could exacerbate the already unsustainable human impacts being experienced by these populations,” says Professor Richard Phillips, head of the Higher Predators and Conservation Group at the British Antarctic Survey and co-author of the study.
Other anthropogenic threats include longline fishing, which kills countless birds as bycatch, and plastic pollution.
Changes in the ecosystems
The researchers also suggest that during their north-south migration, the birds transport nutrients with their excretions to sub-Antarctic islands, where they enhance primary production. Similarly, in the seasonal sea ice zone, excreted nutrients are directly available to primary producers, subsequently supporting the entire food web.
If the populations of seabirds decline due to receding sea ice, the nutrient cycles and thus the ecosystems around the breeding areas and in the seasonal sea ice zone could also change.
Julia Hager, Polar Journal AG
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