Study reveals how melting ice changes ocean light, affecting arctic food chain

The Arctic is changing – and not just in the ways you might think. Once covered in thick, bright sea ice, the region is now seeing more open water. It’s not just changing the climate or opening new shipping routes. It’s also affecting the light that reaches the ocean, which scientists say could harm tiny sea creatures that are key to the food chain.

In a recent study published in Nature Communications, researchers from the University of Amsterdam, along with teams from Denmark and the Netherlands, found that as Arctic ice melts, the sunlight reaching the ocean is changing. Instead of a broad range of light, it's becoming more limited and dominated by blue. This shift may seem minor, but it's enough to affect microscopic organisms that rely on certain light wavelengths to grow and survive.

Under sea ice, sunlight passes through layers of snow and ice in a scattered but full mix of colours, from red and green to blue. Over time, polar algae and phytoplankton have adapted to use this wide range of light, even though it’s not very bright.

But when the ice melts, open water soaks up much of the red and green light, allowing mostly blue light to pass through. This creates a new light environment that’s narrower and different, and not all marine life can adjust to it.

“The photosynthetic pigments of algae living under sea ice are adapted to make optimal use of the wide range of colours present in the little amount of light passing through ice and snow,” said Monika Soja-Woźniak, lead author of the study, as quoted by earth(dot)com. “When the ice melts, these organisms suddenly find themselves in a blue-dominated environment, which provides a lesser fit for their pigments.”

To understand how light changes as ice melts, the researchers used computer simulations. Their models showed that under sea ice, the main light wavelength was around 550 nanometres– mostly green light. But in open water, it shifted to 472 nanometres, where blue light is strongest.

This change doesn’t just make the light dimmer– it also limits the range of colours that marine organisms can use. The study explains that solid ice lets more light through because its molecules are tightly packed. But in liquid water, the moving molecules absorb more light, especially the longer wavelengths like red and green.

“Ice has a much smoother absorption spectrum,” the researchers explained, as quoted by earth(dot)com. In contrast, water quickly absorbs many of the wavelengths that algae once depended on.

Some algae, like Arctic diatoms and Phaeocystis, have a wide range of pigments, including one called fucoxanthin, that help them absorb red and green light well. These species have done well under sea ice, but now they’re facing light conditions their pigments aren’t built for.

Meanwhile, species like Micromonas– tiny phytoplankton that absorb blue and violet light– may be better adapted to this new environment. With their simpler, blue-light-friendly pigments, they could have an advantage and begin to spread more easily.

According to the study, coastal Antarctic waters are already showing signs of this change. “Cryptophytes that are better suited to blue light are increasing in number as pigment-rich species decline,” the researchers observed.

At first, this might seem like just a small technical change– a shift in light wavelengths. But the impact could go far beyond that. These tiny algae are at the very base of the polar food chain. So, any change in their numbers or how well they grow can affect everything above them, from fish to seabirds to marine mammals.

“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,” said Professor Jef Huisman, co-author of the study, as quoted by the earth(dot)com.

If fewer algae can adjust to the new light conditions, or if their ability to photosynthesise drops, the Arctic may also take in less carbon dioxide from the atmosphere, putting even more strain on an already stressed climate.

One key finding from the study is that many climate models don’t consider the specific colours of light underwater. While they measure overall light levels, they often overlook how different wavelengths affect marine life. The researchers say that adding this kind of light data to climate models could lead to better predictions about biodiversity, changes in ecosystems, and how much carbon the ocean can absorb.

Some species, like Phaeocystis, may be able to adjust their pigments to suit their environment. However, the researchers warn that this ability may not be common or quick enough to keep up with the rapid melting of ice.

As the oceans become bluer, the underwater ecosystem is changing– not only because of temperature or salinity, but because of the light itself. For many organisms that evolved in the more colourful world under the ice, the challenge now is adapting to a world that is becoming more and more blue.