Changing Life: Sea Ice

Sea ice is more than just frozen ocean. Below the surface it is a vibrant ocean oasis, and a vital habitat for life in Antarctica.

Billions of microscopic sea plants live underneath the ice, providing food for Antarctic krill and other small crustaceans, which form the foundation of the Antarctic food chain.

The sea ice surface is also a critical breeding ground for emperor penguins.

In some parts of Antarctica, winter sea ice cover is declining dramatically.

Reduced sea ice cover has a domino effect, impacting Antarctic ecosystems from tiny plants to penguins, seals and whales.

Antarctic seasonal ice change. Image credit: NASA Goddard Space Flight Center Scientific Visualization Studio/ NASA Earth Observatory/ Jesse Allen

Sea ice is more than just a layer of frozen ocean: it is a vibrant, essential habitat for many Antarctic species.

Each winter, an area of the Southern Ocean roughly the size of the United States freezes around the Antarctic coastline. This sea ice provides a protective layer over the ocean: a cover that insulates it against the strong winds and cold temperatures above, and creates a sheltered habitat for life to flourish.

Antarctic sea ice — Phytoplankton bloom from space. Image credit: NASA Earth Observatory, Joshua Stevens.

As with life in the Southern Ocean, the sea ice ecosystem begins with single-celled species, including bacteria, protozoa and microscopic phytoplankton (sea plants) such as algae. Sea ice algae grows on the underside of the sea ice, tingeing the ice a browny-green color.

When enough light penetrates the sea ice, individual algae can clump into dense algal mats, which provide a banquet for tiny animals – zooplankton such as copepods and krill – which graze on the sea plants. These tiny animals form the basis of the food web, providing nutrients and energy to seals, penguins, whales and other marine creatures.

Sea ice is an important habitat for Antarctic krill, particularly in the larval and juvenile stages of life. Krill larvae have been seen scraping sea ice algae from the underside of the ice to eat, and juvenile krill find shelter from predators and strong currents in small ridges and caverns under the ice. Krill are an essential food source for Antarctic penguins, seals, whales, and many other marine creatures.

While the underside of the sea ice is an important habitat for Antarctic algae, krill and other small plants and animals, the sea ice surface is an important breeding ground for the largest penguins on the planet: emperor penguins.

Emperor penguin colonies are often located on thick plates of sea ice or fast ice, which is sea ice stuck ‘fast’ to the coast or sea floor. Emperor penguins breed, incubate, hatch and raise their chicks on platforms of sea ice, a process that takes around 9 months.

Seasonal sea ice cover has declined dramatically in some parts of Antarctica, where it is forming later and melting sooner.

This has had a range of impacts on life, both above and below the ice.

emperor penguin chicks — Emperor penguin chicks

Emperor penguins breed, incubate their eggs and raise their young on the sea ice. They need a solid, stable platform of sea ice for around 9 months each year, to allow their chicks to mature and become independent before the sea ice breaks apart.

Any change in sea ice can be catastrophic for emperor penguin colonies. Too little sea ice means there is not enough space to breed, raise chicks, hide from predators and rest while molting (replacing their damaged feathers). Too much sea ice, and the distance between their colony and the open water where they feed becomes too large. A longer journey between the colony and feeding grounds can lead to exhaustion and starvation for emperor penguins whose job is to feed themselves and their young. Even when the amount of sea ice is just right, if it breaks up before the chicks are fledged and independent, entire generations can be lost.

Some emperor penguin colonies are being abandoned after repeated seasons of breeding failure due to inconsistent sea ice cover.

Halley Bay in the Weddell Sea used to be the site of the second-largest emperor penguin colony in Antarctica. Between 2015 and 2018, almost no chicks survived the breeding season.

Very high resolution satellite images showed that sea ice cover in Halley Bay reached record lows between 2015 and 2019. It formed later, covered less area and broke up earlier than in previous years. This appears to have been driven by warmer, windier weather in the area, which has been linked to changing global climate patterns.

Brunt Ice Shelf _ Halley Bay, Antarctica — Brunt Ice Shelf and Halley Bay, Antarctica. Observational Data courtesy of Landsat 8 satellite & USGS. Data processed by Paul Quast.

By 2019, almost all of the penguins had abandoned the colony. While it’s normal for penguin colonies to fluctuate in size from year to year, change on this scale is extremely unusual, and unprecedented at Halley Bay.

Emperor penguins face significant threats to their survival due to the warming climate, and other human activities. If we continue with ‘business as usual’ greenhouse gas emissions, 80 percent of Antarctica’s emperor penguins will be gone by 2100, and they will be doomed to eventual extinction.

In 2021 the U.S. Fish and Wildlife Service proposed the emperor penguin for protection under the Endangered Species Act. It is currently recognised as Near Threatened on the IUCN Red List.

Penguins are known as an indicator species. This means that the health and size of their population reflects overall ecosystem health.

When penguin colonies shrink over time, it suggests there could be underlying issues affecting their habitat, and all the species around them.

adelie and chinstrap penguins — Adélie (left) and Chinstrap penguin (right) with chicks.

It’s not just emperor penguins that are being affected. On the Antarctic Peninsula, many Adélie penguin colonies have declined by 50-80 percent since the 1970s. Some Adélie penguin colonies in the northern part of the Peninsula have vanished entirely.

Chinstrap penguin colonies have faced similar declines on the nearby South Shetland Islands, according to a study released in February 2020. The collapse of a number of penguin colonies in this area appears to be linked to reduced availability of krill in nearby waters, which may be related to diminishing sea ice cover across the region. This is an area of current research. Both Adélie and chinstrap penguin populations remain strong in other areas of Antarctica and the subantarctic, and they are not at imminent risk of extinction.

gentoo penguin with chicks — Gentoo penguin with chicks.

While Adélie and chinstrap populations are in decline across the northern parts of Antarctica, gentoo penguins are thriving and their colonies are expanding. As Adélie penguins vacate accessible rocky terrain suitable for nesting, gentoo penguins have moved in to fill the gap.

It is not entirely clear why gentoo penguins are flourishing as chinstrap and Adélie penguins are in decline. Researchers are investigating evidence that the gentoo penguin’s diverse diet makes them more resilient to changes in Southern Ocean zooplankton communities. Unlike Adélie and chinstrap penguins, whose diet consists primarily of krill, gentoo penguins eat fish and squid as well. This flexibility allows them to thrive even as sea ice declines and krill populations shift and shrink.

The loss of Antarctic sea ice doesn’t only affect penguins. It will have impacts across the Antarctic marine environment, which could extend to humans as well.

Whale pump — The Whale Pump. Image credit: Roman J, McCarthy JJ.

In the global ocean, everything is connected. Antarctic life interacts with the rest of the ocean, including sea ice, in many ways that benefit humans. For example, sea ice algae absorbs carbon from the atmosphere, helping to reduce the greenhouse effect. Great whales feed on the Antarctic krill that grow under sea ice. As they travel north to breed they fertilize the ocean with their lifegiving feces, which add vital nutrients to the ocean.

Exactly how changes in Antarctic sea ice and the broader Antarctic marine environment will affect humans remains a great unknown. This uncertainty is part of why it is so important that we protect these fragile ecosystems from the unpredictable changes caused by the climate crisis: we may not know yet what we’ve got to lose. It’s not too late to act – now is the time.

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