• Sea ice loss narrows the spectrum of light in the ocean, shifting it towards a blue-dominated environment.
• This spectral shift impacts photosynthetic microorganisms, potentially altering food webs and carbon cycling.
• Researchers emphasize the need to incorporate light spectra and photosynthesis more explicitly in climate models and ocean forecasts.

The dramatic decrease in sea ice in polar regions, driven by global warming, is causing a significant color change in the light that reaches the ocean. Sea ice scatters light and reflects much of it, allowing only a small amount to penetrate, which still contains almost the full range of visible wavelengths. As sea ice disappears, the underwater light environment shifts from a broad spectrum of colors to a narrower, blue-dominated spectrum. This change has far-reaching consequences for photosynthetic organisms such as ice algae and phytoplankton, which form the base of the marine food web.

• What: The melting of sea ice leads to a shift in the underwater light spectrum, making it narrower and more blue-dominated. This impacts the availability of light for aquatic photosynthesis, especially for ice algae and phytoplankton.
• Who: Key researchers include Monika Soja-Woźniak and Jef Huisman from the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the UvA, as well as collaborators from the Netherlands and Denmark. The primary organisms affected are ice algae and phytoplankton.
• When: The research indicates that the trend of sea ice loss has been ongoing in recent years, with predictions suggesting an ice-free Arctic Ocean in summer within the next few decades. The Nature Communications publication date is April 30, 2025.
• Where: The study focuses on polar regions, including the Arctic and Antarctic oceans. Field measurements were taken in locations such as the Central Arctic Ocean, Baltic Sea, and McMurdo Sound, Antarctica.

The shift in underwater light spectra due to sea ice loss has significant ecological implications. The spectral niches created by water molecule vibrations influence the distribution and composition of phytoplankton communities. As sea ice diminishes, algal species adapted to the broad spectrum of light under the ice may lose their competitive advantage to species specialized in blue light. This change can cascade through the food web, affecting fish, seabirds, and marine mammals, as well as impacting CO2 uptake by the ocean. The research underscores the importance of including light spectra and photosynthesis in climate models to better predict future changes in polar ecosystems.

Shifts in underwater light spectra may alter phytoplankton community composition, with effects on food webs and carbon cycling.

The photosynthetic pigments of algae living under sea ice are adapted to make optimal use of the wide range of colors present in the little amount of light passing through ice and snow. When the ice melts, these organisms suddenly find themselves in a blue-dominated environment, which provides a lesser fit for their pigments.

Photosynthetic algae form the foundation of the Arctic food web. Changes in their productivity or species composition can ripple upward to affect fish, seabirds, and marine mammals. Moreover, photosynthesis plays an important role in natural CO2 uptake by the ocean.

The loss of sea ice in the Arctic and Antarctic is not just a matter of melting ice; it is fundamentally changing the underwater light environment. This spectral shift has the potential to disrupt marine ecosystems by altering the composition and productivity of phytoplankton communities. Incorporating these light-related changes into climate models is crucial for understanding and predicting the full impact of climate change on polar regions and the global carbon cycle. Monitoring light and ecosystem responses is essential for managing future impacts.