Melting sea ice changes not only how much light enters the ocean, but also its color, disrupting marine photosynthesis and altering Arctic ecosystems in subtle but profound ways.
As global warming causes sea ice in the polar regions to melt, it is not just the amount of sunlight entering the ocean that changes. The color of the underwater light shifts, too, and this has major consequences for life below the surface.
According to new research published in Nature Communications, these changes could significantly impact tiny but vital organisms like ice algae and phytoplankton. The study was led by marine biologists Monika Soja-Woźniak and Jef Huisman from the University of Amsterdam's Institute for Biodiversity and Ecosystem Dynamics.
The international team of scientists, which included physical chemist Sander Woutersen and collaborators from the Netherlands and Denmark, explored how melting sea ice transforms the underwater light environment. Light behaves very differently in sea ice compared to open water.
Sea ice reflects and scatters most sunlight, letting only a small amount through, but that small amount includes nearly all visible wavelengths. Open seawater, on the other hand, absorbs reds and greens, while allowing blue light to travel deeper. This is why the ocean appears blue to our eyes.
Molecular vibrations of water
Another key difference between ice and liquid water lies in the role of molecular vibrations. In liquid water, H₂O molecules are free to move and vibrate, which leads to the formation of distinct absorption bands at specific wavelengths. These bands selectively remove portions of the light spectrum, creating gaps in the light available for photosynthesis.
Previous research by Maayke Stomp and Prof. Huisman demonstrated that these molecular absorption features create 'spectral niches'—distinct sets of wavelengths available for photosynthetic organisms. Phytoplankton and cyanobacteria have evolved a diversity of pigments tuned to the different spectral niches, shaping their global distribution across oceans, coastal waters, and lakes.
In ice, however, water molecules are locked into a rigid crystal lattice. This fixed structure suppresses their ability for molecular vibrations and thereby alters their absorption features. As a consequence, ice lacks the absorption bands of liquid water, and hence a broader spectrum of light is preserved under sea ice. This fundamental difference plays a key role in the spectral shift that occurs as sea ice melts.
Ecological implications
As sea ice disappears and gives way to open water, the underwater light environment shifts from a broad spectrum of colors to a narrower, blue-dominated spectrum. This spectral change is crucial for photosynthesis.
"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 Monika Soja-Woźniak. "When the ice melts, these organisms suddenly find themselves in a blue-dominated environment, which provides a lesser fit for their pigments."
Using optical models and spectral measurements, the researchers showed that this shift in light color not only alters photosynthetic performance, but may also lead to changes in species composition. Algal species specialized in blue light may gain a strong competitive advantage in comparison to ice algae.
According to Prof. Huisman, these changes can have cascading ecological effects. "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 study highlights that climate change in the polar regions does more than melt ice—it causes fundamental shifts in key processes such as light transmission and energy flow in marine ecosystems.
The results underscore the importance of incorporating light spectra and photosynthesis more explicitly in climate models and ocean forecasts, especially in polar regions where environmental change is accelerating at an unprecedented rate.
Reference: "Loss of sea ice alters light spectra for aquatic photosynthesis" by Monika Soja-Woźniak, Tadzio Holtrop, Sander Woutersen, Hendrik Jan van der Woerd, Lars Chresten Lund-Hansen and Jef Huisman, 30 April 2025, Nature Communications.
DOI: 10.1038/s41467-025-59386-x
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