As Antarctic sea ice vanishes, algae threaten to outcompete seafloor life

In Antarctica’s coastal waters, a quiet struggle is unfolding.

Each winter, sea ice spreads across the ocean’s surface, shrouding the seafloor in darkness. This darkness is important for marine invertebrates—creatures like sea urchins, brittle stars and sponges—that thrive in low light. But when summer comes and the sea ice recedes, sunlight returns, fuelling the growth of algae.

In recent years, however, sea-ice cover has experienced record-breaking lows, with dramatic declines in coverage allowing more light to reach the ocean floor for longer periods. This shift raises a critical question: what happens to the balance between the light-dependent algae and the dark-dwelling marine invertebrates?

Elise Mills, a SAEF research assistant at Queensland University of Technology recently led a study that developed a new modelling framework to help answer this question. The results suggest this balance is already being disrupted, and has the potential to alter these ecosystems in profound and irreversible ways.

“The most surprising aspect of these results is that the modelling indicates that sites in the study have already passed, or are about to pass, a tipping point,” Elise said.

“This is where the ecosystems shift from invertebrate dominance to algae dominance—the algae takes up the available space and forces the invertebrates out so that they have nowhere to grow.”

To undertake the research, the team constructed three mathematical models that simulate how a population size responds to different levels of light. “We used statistics to match the models to data obtained from several sites around Casey Station in Antarctica, and looked at what the models predicted the populations to be for a range of light values,” Elise said.

When applied to modelling the population dynamics beneath the sea ice, the models predicted that if sea ice continues to decline, more light will reach the seafloor for longer periods and give algae the upper hand. With more access to light, algae can outcompete invertebrate species.

“If the invertebrate communities are replaced by algae, which our results indicate may already be occurring, this could impact the coastal diversity and functioning of these ecosystems,” Elise said. “With changing conditions in Antarctica, this effect will continue further into the future.”

The study adds to a growing body of evidence that Antarctica’s coastal ecosystems, once buffered by stable sea ice, are becoming increasingly vulnerable to a warming climate. As species decline, the ocean warms and sea ice wanes, these ecosystems may lose the resilience needed to withstand further change. Elise and her SAEF colleagues hope their findings will shine a light on this world—that depends on darkness to endure.

Mills, E., Clark, G. F., Simpson, M. J., Baird, M., & Adams, M. P. (2025) A generalised sigmoid population growth model with energy dependence: Application to quantify the tipping point for Antarctic shallow seabed algae. Environmental Modelling & Software, 188, 106397. https://doi.org/10.1016/j.envsoft.2025.106397

As Antarctic sea ice vanishes, algae threaten to outcompete seafloor life

New modelling suggests that the balance between algae and invertebrates is shifting, with the potential to alter these ecosystems in profound and irreversible ways.

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