Measurements being taken under sea ice by one of the Danish researchers in Greenland. [Image: Photo by Lars Chresten Lund-Hansen]
The extent of sea ice that forms, expands and melts in the ocean is a key indicator of climate change. In recent years, ice cover has decreased dramatically, with predictions suggesting an ice-free Arctic Ocean in summer within the next few decades. The consequences of this steep decline are wide-ranging, such as less sunlight reflected back into space and disruption of ocean currents.
Now, researchers from the Netherlands and Denmark have identified another impact of sea-ice loss: a dramatic color change in the light that reaches the ocean and its inhabitants (Nat. Commun., doi: 10.1038/s41467-025-59386-x). Such a change will likely cause shifts in the populations of numerous photosynthetic microorganisms, including ice algae and phytoplankton, which form the basis for the marine food web.
Ice vs. liquid water
Ice and liquid water have inherent differences in their light scattering and absorption properties. Ice has a much higher albedo and shows stronger scattering than liquid water. In addition, ice has a much smoother absorption spectrum across the visible range. Liquid water has a series of small peaks in the visible and near-infrared range, creating “spectral niches” exploited by different photosynthetic pigments of phytoplankton.
With these disparities in mind, Monika Soja-Woźniak and her colleagues sought to investigate how the ongoing loss of sea ice would affect the underwater light environment, particularly in terms of the spectral quality of light available for aquatic photosynthesis.
“We used radiative transfer modeling combined with field-derived optical properties—such as absorption and scattering coefficients of sea ice, snow and water—to simulate changes in spectral irradiance under varying ice conditions and assess their potential impacts on photosynthesis,” said Soja-Woźniak, University of Amsterdam, Netherlands.
The bottom of a cut-out block of 2-m-thick ice covered with ice algae, placed atop the sea ice of McMurdo Sound, Antarctica. [Image: Photo by Lars Chresten Lund-Hansen]
Implications for aquatic photosynthesis
The researchers focused their analysis on the euphotic zone—the topmost layer of ocean where most photosynthetic activity takes place. They employed a radiative transfer model to compare irradiance spectra in ice-covered water and in open water without sea ice for three different marine ecosystems.
The findings suggest that sea-ice loss leads to significant changes in the spectral composition of underwater light. Specifically, the light environment shifts from a broad spectrum to a narrower, blue-dominated spectrum as ice cover decreases. These spectral shifts impact the availability of light for different phytoplankton groups, which rely on pigments tuned to specific wavelengths.
“Shifts in underwater light spectra may alter phytoplankton community composition, with effects on food webs and carbon cycling,” said Soja-Woźniak. “Mitigating these consequences is difficult, as sea-ice loss is primarily driven by global climate change. However, reducing greenhouse gas emissions and improving monitoring of light and ecosystem responses can help manage future impacts.”