New research published in Nature Communications reveals that the disappearance of sea ice due to global warming doesn’t just expose the ocean to more sunlight—it also fundamentally changes the color of the light that enters the water. This shift in the underwater light spectrum has critical consequences for marine life, particularly photosynthetic organisms such as ice algae and phytoplankton.
The international study, led by marine biologists Monika Soja-Woźniak and Jef Huisman from the University of Amsterdam’s Institute for Biodiversity and Ecosystem Dynamics (IBED), shows that as the ice melts, the underwater environment transitions from a broad light spectrum to a blue-dominated one. This subtle but powerful change can disrupt the delicate ecological balance at the foundation of the Arctic food web.
From Scattered White to Deep Blue
Sea ice and seawater interact with light in fundamentally different ways. Sea ice reflects and scatters most of the incoming sunlight, allowing only a small but broad-spectrum amount of light to pass through. This means organisms living under sea ice have adapted to make efficient use of a wide range of colors.
In contrast, open seawater absorbs red and green wavelengths, while blue light penetrates the deepest. This is why the open ocean appears blue—and it’s this blue light that increasingly dominates the underwater world as ice cover diminishes.
The Role of Water Molecule Vibrations
One of the key findings relates to how water absorbs light on a molecular level. In liquid water, the free movement and vibration of H₂O molecules create distinct absorption bands that selectively remove parts of the light spectrum. These bands are absent in solid ice, where molecules are locked into a rigid crystal structure and can’t vibrate freely.
These unique absorption features in water help define what researchers call “spectral niches”—specific sets of light wavelengths available to different photosynthetic organisms. Previous work by Maayke Stomp and Prof. Huisman showed that various phytoplankton and cyanobacteria have evolved to fill these niches, developing pigments tuned to the specific colors of light in their environment.
A Shock to the Arctic Algae
When sea ice melts, organisms adapted to the broad, soft light spectrum beneath the ice are suddenly thrust into a sharp, blue-heavy light environment. “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,” says lead author Soja-Woźniak. “When the ice melts, these organisms suddenly find themselves in a blue-dominated environment, which provides a lesser fit for their pigments.”
The research team used optical models and spectral measurements to demonstrate that this shift could impair the photosynthetic efficiency of ice algae. Meanwhile, algal species that are better adapted to blue light—often found in open water—may gain a competitive edge, potentially leading to significant shifts in species composition.
Ripple Effects Through the Arctic Food Web
According to Professor Huisman, these changes could have cascading impacts across the entire Arctic ecosystem. “Photosynthetic algae form the foundation of the Arctic food web,” he explains. “Changes in their productivity or species composition can ripple upward to affect fish, seabirds, and marine mammals.”
Beyond ecological implications, there are climate consequences as well. Photosynthetic organisms in the ocean play a crucial role in natural CO₂ uptake, helping regulate the Earth’s climate. Disruptions to this system could therefore affect global carbon cycles and climate regulation.
A Call for Climate Models to Evolve
The study’s findings highlight that climate change in polar regions goes far beyond rising temperatures and melting ice. It triggers deep, structural changes in how light and energy move through the ocean. The researchers stress the importance of integrating light spectrum data and photosynthetic dynamics more explicitly into climate models and ocean forecasts, especially in the rapidly changing Arctic.
As environmental change accelerates in these fragile regions, understanding the full scope of its effects—from the color of light to the survival of algae—is vital to anticipating and mitigating the broader consequences for Earth’s climate and marine ecosystems.
