Sea ice loss alters the color of ocean light, and life struggles to adapt

Sunlight gives life to oceans. Yet in the icy polar seas, light barely enters. It reaches the water through a thick shield of snow and ice, scattered and dimmed but still biologically vital.

For countless generations, algae and phytoplankton have adapted to these low-light, multi-colored conditions. But that delicate balance is now vanishing.

Driven by climate change, sea ice is disappearing at alarming rates. This isn’t only opening new shipping routes or endangering polar bears. It’s rewriting the rules of underwater life.

New research led by Monika Soja-Woźniak and Jef Huisman at the University of Amsterdam, in collaboration with Danish and Dutch scientists, has uncovered a major consequence of this change. Melting ice shifts the ocean’s color and light spectrum, which can have cascading effects across marine ecosystems.

The shifting color of ocean light

Ice and water do not transmit light in the same way. Sea ice reflects most incoming sunlight and scatters the rest. Still, the small fraction that makes it through contains nearly the entire visible spectrum. That means red, green, and blue wavelengths all reach the life below the surface.

Seawater, however, tells a different story. It quickly absorbs red and green light. Blue light alone travels deep. This selective filtering of color is what gives the ocean its rich blue appearance.

As ice melts and gives way to liquid water, organisms beneath the surface receive light with less color variety. They are exposed to a narrower, blue-dominated spectrum.

This subtle shift matters greatly for photosynthetic organisms. Algae and phytoplankton harvest light using pigments that are finely tuned to specific wavelengths of the spectrum. A change in available colors alters which species thrive – and which decline.

How ice and ocean water absorb light

Light absorption depends on more than just the medium – it also depends on molecular movement. In liquid water, H₂O molecules vibrate freely.

These vibrations absorb light at specific wavelengths, creating gaps in the light spectrum. The gaps shape the available “spectral niches,” small bands of light that different algal species have evolved to use.

“Ice has a much smoother absorption spectrum,” noted the researchers. In solid form, water molecules lock into a crystalline structure. Their ability to vibrate disappears. Without these vibrations, fewer wavelengths are absorbed. More colors remain in the light that penetrates.

This key difference means that under sea ice, life enjoys a fuller spectrum of light. Phytoplankton adapted to these conditions use a diverse range of pigments to capture energy. As the ice disappears, the richness of their light environment vanishes with it.

Sea ice loss narrows light range

To measure the exact changes, the team developed optical models using radiative transfer simulations. They analyzed different polar environments, from the pristine Arctic Ocean to murky coastal zones. Their models showed that open water absorbs nearly all longer wavelengths, leaving available light that is shifted toward blue.

In clear Arctic waters, the dominant wavelength dropped from about 550 nanometers (green) under ice to 472 nanometers (blue) in ice-free water. This shift also compresses the range of usable light, making the environment less diverse in spectral terms.

This spectral narrowing carries significant biological consequences. Organisms adapted to broad-spectrum light will likely lose out to those specialized for blue wavelengths.

“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,” said Soja-Woźniak, lead author of the study.

“When the ice melts, these organisms suddenly find themselves in a blue-dominated environment, which provides a lesser fit for their pigments.”

Key species will decline

The researchers examined how these spectral changes affect key photosynthetic species.

Ice algae, like many Arctic diatoms and the genus Phaeocystis, use a wide array of pigments, including fucoxanthin, which absorbs green and red light. These pigments allow them to photosynthesize efficiently in the colorful but dim light beneath sea ice.

In contrast, blue specialists like Micromonas – a genus of pico-phytoplankton – possess simpler pigment systems that are tuned to absorb blue and violet light. These species can thrive in the narrow-spectrum environment of open water.

The modeling results showed that as the underwater spectrum shifts, blue specialists may gain a competitive edge. Algal diversity could shrink. Coastal Antarctic waters have already seen signs of this: cryptophytes that are better suited to blue light, are increasing in number as pigment-rich species decline.

Consequences of algae loss

At first glance, this may seem like a technical shift of color bands in light. But it holds deeper implications for marine life. Photosynthetic algae sit at the base of the Arctic food web. Changes in their abundance, productivity, or species mix may have consequences further up the chain.

“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,” explained Professor Huisman.

If fewer algae can thrive, or if they photosynthesize less efficiently, the polar ocean’s ability to sequester carbon will suffer. Climate change, in this way, not only warms the planet but weakens its natural buffers.

Future models must capture new details

Current climate models largely ignore the fine details of light spectra underwater. Yet this research shows how crucial those details are.

A change in color can reshape entire ecosystems. Understanding the interactions between light transmission, pigment adaptation, and photosynthesis is essential for accurate ocean forecasts.

The team suggests incorporating these spectral dynamics into future models. Doing so could improve predictions of ecological change, carbon cycling, and biodiversity in polar regions. As sea ice continues to shrink, these predictions grow more urgent.

The power of ocean light

Some organisms may adapt. Species like Phaeocystis show signs of pigment flexibility – shifting pigment use depending on conditions. Whether that adaptability can keep pace with melting remains to be seen. Evolution is slow, but climate change is not.

As polar waters grow bluer, the underwater world is not just getting brighter. It is changing in quality. The transformation in light affects what grows, what survives, and what disappears.

This is the quiet power of light – a force that shapes life from the microscopic to the megafaunal. And as this study reveals, that power is now being rewritten by melting ice.

The study is published in the journal Nature Communications.

Image Credit: Lars Chresten Lund-Hansen

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