Polar sea ice loss beginning to affect the bottom of the food chain

Dramatic loss of sea ice due to global heating is changing the quality of light reaching algae at the base of the polar marine food chain, report Dutch and Danish marine biologists and chemists.  This could have potentially disastrous effects on polar marine ecosystems.

Although sea ice reflects between 20 and 90% of sunlight, most light wavelengths do penetrate the ocean depths, says marine biologist and lead author of the new research, Dr Monika Soja-Woźniak of the University of Amsterdam. 

But melt the ice and that same sunlight encounters seawater which removes the longer wavelengths (orange, red and infrared). Those left are at the shorter end of the spectrum (violet and blue), which is why the sea is blue-green.

Algae growing on the underside of the sea ice and in the adjacent water column (phytoplankton) have evolved pigments over millions of years that absorb light across the visible spectrum. Limiting these organisms to the bluer-greener wavelengths of the open ocean is going to have major impacts, completely outside their evolutionary experience.  

An international team, led by Soja-Woźniak and marine biologist Dr Jef Huisman from the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the University of Amsterdam have now explored what such algae face with the loss of the polar sea ice, and the implications for marine food chains. 

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

The team, using optical models and spectral measurements, showed that this shift in light colour may lead to changes in species composition. Ice algae would be outcompeted by open ocean species, says Soja-Woźniak.

“I think it’s really important contribution, says polar marine ecologist, Dr Rebecca Duncan, formerly of University of Technology, Sydney, now at Linnaeus University, Sweden. We talk a lot about light increasing as sea ice melts. But I think they’ve captured how light quality changes as well.” Duncan was not involved in the research.

Such changes will 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,” says Huisman.

For example, shrimp-like crustaceans called ’krill’ are the major food source for penguins and baleen whales in the Antarctic.  

These krilll, particularly the juveniles, feed directly on the algae on the underside of the ice, where algal blooms form within the ice itself, Duncan told Cosmos. Then, as the ice melts in the Spring, the algae are released into the water column where the krill again pick them off. 

The problem is that krill are dependent on larger algal species, like diatoms, she says, whereas the predicted changes in algal species composition, with sea ice melting, are towards smaller species, as the light shifts to blue. “We are definitely seeing this in the Antarctic, ”says Duncan. 

This means “krill just don’t graze as efficiently, so they aren’t getting the nutrients they need to reproduce. And of course, that can fairly rapidly reduce their numbers in the ocean.” 

Krill are not evenly distributed around Antarctica. Penguin breeding colonies rely on a krill within swimming distance and could be in trouble if that population crashed, says Duncan.

“Ice algae are also important because of the seasonal timing of blooms,” she says.

Ice algae bloom earlier than open water phytoplankton, and it’s this bloom that gives the krill and other zooplankton the nutrients they need to produce eggs, she says. The juveniles that grow from those hatchlings join the zooplankton and feed on the later open-water algal bloom. “So, we don’t know what will happen if there is no sea ice bloom. Will we just get an earlier open water bloom or not?” 

There is more to sea ice melting than meets the eye. Its loss will ripple through entire marine ecosystems.

Soja-Woźniak says light spectra and photosynthesis must be more explicitly included in climate models and ocean forecasts, especially in polar regions where environmental change is accelerating at an unprecedented rate.

The research was published in Nature Communications. 

Sea ice at record lows