Video: Underwater robot reveals algae under ice
Appearances can be deceptive. At first glance, the sea ice that covers polar regions in winter seems rather barren – but clinging to the bottom of it are remarkable upside-down algal "forests" that support a wide array of creatures.
Not long ago Arctic researchers assumed phytoplankton blooms were restricted to open waters, and for good reason: the algae rely on light to survive and little of this makes it through a couple of metres of ice. A few years ago, however, researchers cruising in the Chukchi Sea between Alaska and Siberia found massive phytoplankton blooms beneath sea ice – with a biomass that was, in places, four times as much as the amount in open waters.
It's now clear that the discovery was just the tip of the iceberg. In 2014, researchers had their first opportunity for a closer look beneath the ice using a new submersible – the Nereid Under Ice Vehicle designed and built at the Woods Hole Oceanographic Institution in Massachusetts. At the American Geophysical Union meeting last month, they revealed that the submersible uncovered diverse animal communities thriving just below the ice.
"With Nereid we saw not only a big bloom of plankton but also subsequent blooms of zooplankton feeding on the phytoplankton. First copepods, then jellies and salps," says Antje Boetius at the Helmholtz Center for Polar and Marine Research in Bremerhaven, Germany. "They were concentrated in the few metres of meltwater under the ice in fantastic abundances."
Forest in bloom
The effect is something like an upside-down forest. Some algae are actually trapped in the ice, growing downwards into the water column and dangling from the surface. Other species drift in the meltwater just below the ice, forming phytoplankton blooms.
"When you think of forests, there are the trees, the shrubs, the ferns, the mosses, all active in different heights above the soil," says Boetius. Likewise, in the Arctic, the primary producers are spread across the ice column, some attached to it, others drifting right below in bits of floating or recently melted ice.
It's that stratified arrangement that might help explain why life in these upside-down forests can be so diverse and abundant. The habitats that algae growing there create soon attract microbes and small grazers – which in turn attract fish and other predators.
"It may very well be that there is undescribed animal life under the ice," says Boetius.
Similar algal communities are also showing up beneath the sea ice around Antarctica – again, thanks to new submersibles that can cruise just below the ice without disturbing it in the way a ship would. Here, though, the communities have been better studied, says Ian Hawes at the University of Canterbury in New Zealand.
Cool shades
The upside-down mats of algae provide nutrients when other options are limited, sustaining the ecosystem through lean times. "Ice algae tend to begin to grow in early spring, shortly after the sun returns and before phytoplankton in the water column below are getting enough light to do so," says Hawes.
So, the ice algal bloom is there at a time when there are few other sources of nutrients, and it is concentrated in patches, making it efficient for animals to harvest it, says Hawes.
Boetius thinks that understanding the ecology of this narrow zone under the ice may hold clues to the future of the Arctic's marine mammals and fish – and also the large predators, like polar bears, that feed on them.
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