The Arctic Food Supply That Grows Under The Ice

Arctic sea ice can look white from above and brown from below. That brown coating can be a layer of ice algae, organisms so small you need a microscope to see them one by one. Together, they can color the underside of an ice sheet. They also feed tiny animals at a time when there may not be much other food in the water.

Frozen Seawater Still Holds Liquid:

As seawater freezes, the growing ice crystals leave much of the salt behind. Some of that salt drains back into the ocean, but some stays trapped in pockets of extra-salty water called brine. Because salt lowers the freezing point of water, those pockets can stay liquid even while the ice around them is solid. Some pockets connect to form tiny channels.

Ice algae can live in these channels as well as on the underside of the ice. Many are diatoms, a type of microscopic alga. Like plants, they use light to make sugars. The lower edge of the ice gives them access to nutrients from the seawater below, as long as enough light gets through the snow and ice above.

An Early Meal For Small Grazers:

As daylight returns after the Arctic winter, ice algae can multiply before large numbers of free-floating algae build up in the water. Small crustaceans called amphipods graze near the ice, and some copepods feed on ice algae in the water below. The timing matters as much as the amount: even a thin layer can provide food before other sources become plentiful.

Not all of that food gets eaten near the surface. As ice melts, some algae break loose and sink. On Arctic continental shelves, that falling material can feed animals on the seafloor. Researchers have used chemical traces from ice-dwelling diatoms to follow this food into animals living well below the ice.

Two Kinds Of Algae Can Share The Same Shade:

“Under the ice” can mean two different things. Ice algae live on or inside the ice itself. Phytoplankton float in the water, and they can multiply beneath ice too if enough light reaches them. A brown patch attached to the ice and a cloud of algae drifting below it aren’t the same thing.

That distinction helps explain why more light in the water doesn’t simply replace what’s lost when sea ice melts. Open water can support phytoplankton, but it can’t provide the icy surface where ice algae grow. The two food sources can also peak at different times, so a grazer may not be able to switch from one to the other just when it needs to eat.

Look At The Ice From The Water Side:

A view from above shows how much of the ocean is covered. A view from below shows what that cover can hold. The brown coating is easy to miss, but it marks a place where sunlight becomes food. Some of that food gets eaten within inches of the ice. Some travels all the way to the seafloor.