I drafted this while looking north over the frozen
Lincoln Sea, at the northernmost tip of Ellesmere
Island in Canada. I was at Alert, a Canadian Forces
Station which, at 82°N, is the most northerly
permanently inhabited place on Earth. Just 815km away,
across the Arctic Ocean, lay the North Pole.
It was May, and the sea should have still been frozen,
but this year the bridge of sea ice between Ellesmere
and Greenland broke up early, and Arctic ice began
flowing down the narrow Nares Channel and south into
Baffin Bay. All across the Arctic Ocean, the amount
and persistence of sea ice is declining – September
ice cover has fallen around 30% since 1980.
The Arctic is warming at twice the rate of the rest of
the planet, and images of polar bears on small ice
floes capture the imagination. But those images
represent (excusing the pun) only the tip of the
iceberg – the consequences of ice loss are profound
and start from the very bottom of the food chain, in
the microbial processes that drive the biology of the
ocean.
Arctic food chains sometimes start in sea ice
Sea ice forms when seawater temperature falls below
-1.8℃. As the ice crystals form, salt is forced out
and ice brines and other dissolved constituents become
trapped in a honeycomb of small channels in the ice.
Cold salty water draining from the ice also sinks deep
to the bottom of the oceans and drives water
circulation across the globe.
As the air grows colder, the ice thickens downwards
and, in the brine channels and across the ice bottom,
specialised algae and bacteria grow. When sunlight
returns to the Arctic in the spring and penetrates
through the ice (which is rarely more than a few
metres thick) these ice-algal communities start to
photosynthesise, producing algal biomass and abundant
dissolved organic matter.
This feeds a wide range of microscopic creatures known
as zooplankton, which graze across the bottom of the
ice. These zooplankton in turn feed larger animals and
drive the food chain throughout spring.
When the ice melts more of this material flows out
into the seas, providing more food resources at the
bottom of food chains. In a recent study published in
Nature Climate Change, colleagues and I showed how the
different components of this organic matter derived
from ice-algae are used by different species of
bacteria and at different rates in underlying
seawater, so that more melting ice will change the
patterns of organic matter turnover in surface waters
during spring.
Different food chains may develop
Not all sea ice melts each summer – or at least it
didn’t. Multi-year ice can go through a number of
years of melting and growing, getting thicker and more
structurally complex. But, over time, this multi-year
ice has become rarer. In the 1980s, around one-third
of the Arctic’s ice cover was more than four years old
– today, such ice is almost nonexistent. Instead, more
first-year ice will form and completely melt each
year, providing new food inputs into areas of ocean
that were previously permanently covered in ice.
This has significant consequences. Less ice cover in
summer means more open ocean water, which – as it is
darker – absorbs more sunlight and heat, making it
harder to freeze in the autumn. Open water also means
the wind can stir up the sea and slow the process of
refreezing. More open water in summer will change the
plankton communities, and then the animals that feed
on them.
Some species are moving north. Already the Barents Sea
between Norway and Svalbard is now rarely covered in
ice in winter – and North Atlantic species such as cod
and top predators such as orca are moving in.
Specialist species that rely on ice such as polar
bears, ringed seals, walrus and Arctic cod are losing
their habitats, while non-indigenous species are
expanding their range.
Different food chains may develop
Not all sea ice melts each summer – or at least it
didn’t. Multi-year ice can go through a number of
years of melting and growing, getting thicker and more
structurally complex. But, over time, this multi-year
ice has become rarer. In the 1980s, around one-third
of the Arctic’s ice cover was more than four years old
– today, such ice is almost nonexistent. Instead, more
first-year ice will form and completely melt each
year, providing new food inputs into areas of ocean
that were previously permanently covered in ice.
This has significant consequences. Less ice cover in
summer means more open ocean water, which – as it is
darker – absorbs more sunlight and heat, making it
harder to freeze in the autumn. Open water also means
the wind can stir up the sea and slow the process of
refreezing. More open water in summer will change the
plankton communities, and then the animals that feed
on them.
Some species are moving north. Already the Barents Sea
between Norway and Svalbard is now rarely covered in
ice in winter – and North Atlantic species such as cod
and top predators such as orca are moving in.
Specialist species that rely on ice such as polar
bears, ringed seals, walrus and Arctic cod are losing
their habitats, while non-indigenous species are
expanding their range.
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