As global warming melts more sea ice in the polar regions, the light that enters the ocean is not just increasing in amount—it’s also changing in color.
This shift in the underwater light environment could have serious consequences for the tiny plants and algae that form the base of the Arctic food web.
A new study published in Nature Communications, led by marine biologists Monika Soja-Woźniak and Jef Huisman from the University of Amsterdam, reveals how melting sea ice transforms the quality of light in the ocean and affects the life forms that rely on it for energy.
Sea ice and seawater interact with light in very different ways. Thick sea ice reflects and scatters most sunlight, allowing only a little to pass through.
However, the light that does make it under the ice still includes a wide range of colors, or wavelengths.
In contrast, when ice melts and is replaced by open seawater, the water absorbs much of the red and green light. Only blue light travels far down into the ocean, which is why the sea often looks blue.
Another key difference lies in the behavior of water molecules. In liquid water, molecules are constantly moving and vibrating.
These vibrations absorb light at specific wavelengths, removing some colors from the spectrum. In solid ice, the molecules are locked in place, which limits their vibrations and allows a broader range of light to pass through.
This difference is critical for photosynthetic organisms like ice algae and phytoplankton. These microscopic organisms have pigments adapted to the full range of light found under the ice.
When the ice melts, they suddenly find themselves in a world where mostly blue light remains, and their pigments may no longer be a good match for the available light.
The researchers used models and measurements to show that this shift in light color reduces the ability of some algae to photosynthesize efficiently. It could also change which species thrive in the Arctic Ocean. Algae that are better suited to blue light may outcompete others, disrupting the balance of species.
These changes could have major ripple effects. Algae are the base of the food chain in polar ecosystems. If their productivity drops or if certain species disappear, it can affect everything from fish to birds to seals and whales.
It also impacts how much carbon dioxide the ocean absorbs from the atmosphere, since algae play a key role in this process.
This study shows that melting ice does more than change the landscape—it changes the way life in the ocean works. The authors say it’s crucial to include these light-related effects in climate models, especially as the Arctic faces rapid environmental change.
