Weddell seal pups

Weddell seal mothers with pups are a dependable sight from October onwards in front of Scott Base. Photo: Anthony Powell

Sea ice and ecosystems

11 May 2023

The annual patterns of sea-ice formation and disappearance play a crucial role in the life cycles of many Antarctic organisms, from the algae at the base of food chains to seals and penguins at the top. For some it is a solid habitat floating on a cold ocean, for others it determines how fast they can grow or where their next meal is coming from. The impacts of climate change on sea ice dynamics will have flow-on effects on Antarctic ecosystems that we do not yet fully understand.

An essential habitat

For many organisms, sea ice is an essential habitat for all or parts of their lifecycles. Most Emperor Penguins depend on land-fast sea ice as a breeding habitat, and it serves similar purposes for many other animals. Weddell seal pups are born early in the summer on fast ice — mothers with pups in colonies are a dependable sight from October onwards in front of Scott Base. Crabeater seals, Antarctica’s most common seal, spend their lives in pack ice, where the floes offer a place to rest, warm up and give birth to pups.

The top surface of the sea ice is exposed to extreme winter temperatures and is habitable only by warm-blooded organisms. The underside, however, offers a home to a whole ecosystem of algae, invertebrates and fish. When the sun returns to Antarctica, and the light penetrating through the sea ice is sufficient to support photosynthetic growth, blooms of algae – mostly diatoms – develop inside and on the underside of the ice. Thick films growing under the ice serve as a vital spring food source for overwintering invertebrates, including the krill that are such critical components of the food chains that support fish, seals, whales and seabirds.

Like krill, silverfish are critical elements of Antarctic food webs – these tasty packets of energy are eaten by a wide range of high level predators. They too are dependent on sea ice. The platelet ice layer under some areas of fast ice around Terra Nova Bay is the only location in the Ross Sea where the eggs and larvae of silverfish have been recorded. Other animals associated with the underside of sea ice include fish that are specialised to allow them to contact ice without freezing solid, a vast range of crustaceans, some that burrow into the ice, as well as the delicate sea butterflies that patrol under the ice (watch the video above). Material sinking out of the ice provides a rich food source for animals that live on the seafloor.

A view of the underside of ice shelf

A view of the underside of ice shelf affected sea ice which is coated in ice crystals. This environment provides a habitat for algae, invertebrates and fish. Photo: Brett Grant/NIWA

A control on productivity

While many Antarctic organisms have adapted to exploit sea ice, for phytoplankton – the unicellular plants floating in the water and the main producers of organic carbon in the Southern Ocean – it can be a mixed blessing. Sea ice, and its associated biota, reflect or absorb most of the light incident to its surface. For phytoplankton, it is when the ice is gone that their growth can really start. Melting sea ice helps to stabilise the water column and can be a source of iron, an essential nutrient often in short supply in Antarctic waters. Intense summer blooms of algae can then form, which make the Ross Sea one of the most productive waters of the Southern Ocean.

What impacts will climate change bring?

Changes to sea ice as a result of climate change will have major impacts on Antarctic marine ecosystems. Some impacts can be readily foreseen, such as for animals that rely on sea ice to breed at the right time, like emperor penguins. Projecting what is likely to happen to ecosystem processes is much less certain. Complex interactions between ice and ocean are superimposed on even more complex interactions within food webs.

The 2022 ACCE report was inconclusive on the response of phytoplankton to changed climate. It is uncertain how changing growth conditions will affect dominance by different types of phytoplankton. We also don’t know how changing phytoplankton dynamics will affect higher levels of food webs, and the efficiency with which the biological pump that sequesters carbon deep in the ocean will work.

A better understanding of how changing sea-ice cover will affect ecosystem composition and functioning is a core driver of Antarctic Science Platform research in the Ross Sea. It is critical for projecting how the system will change in the long term, and therefore how our management needs to adapt. In the shorter term, a better understanding of changing sea-ice cover is fundamental to our efforts to understand how any changes observed in coming decades within the Ross Sea region Marine Protected Area can be attributed to direct (fishing) or indirect (climate change) human activities.

Sea ice

Adélie penguins sitting on sea ice. Photo: Craig Potton

Article prepared by Ian Hawes and Rowan Howard-Williams.