At Yale Environment 360 Fen Montaigne provides a fascinating, if disturbing, report on the findings of scientists working on the effects of sea ice retreat on the polar marine food chain. Montaigne is the author of the book Fraser’s Penguins which I reviewed earlier this year and of an earlier article at Yale Environment about the melting at the periphery of the Antarctic ice dome discussed here.
This is how it has been in the Antarctic Peninsula for a very long time:
’Marine algae, or phytoplankton, trapped in the ice and floating in the water column have burst into life as the sun catalyzes the photosynthetic process. In addition, melting freshwater from the sea ice formed a buoyant cap atop the heavier salt water, trapping the algae in the upper layer of the ocean, where it was exposed to the sun’s rays and bloomed.
’These seasonal phytoplankton blooms have fed the entire food web around Antarctica: shrimp-like krill and fish ate the phytoplankton, and virtually everything else – from AdÃ©lie penguins to humpback whales – ate the krill, which are part of the crustacean family.’
It has changed in recent decades. Due to warming, sea ice now blankets the Southern Ocean off the western Antarctic Peninsula nearly three fewer months a year than 30 years ago. Martin Montes-Hugo, an oceanographer and remote sensing expert at the University of Quebec who has studied the impact of sea ice declines on the marine food web along the western Antarctic Peninsula, observes that over the last three decades phytoplankton production declined by nearly 90 percent in the northwestern Antarctic Peninsula. He puts the decline down to the shorter duration of the sea ice season.
’Less sea ice has meant two things. First, it has reduced the size of the freshwater layer that kept phytoplankton trapped in the upper layer of the Southern Ocean, where they bloomed when exposed to sunlight. Second, the region’s nearly three-month decline in sea ice duration means that the ocean’s surface is more exposed to winds, which stir up the water and mix the upper layers with lower layers. The upshot is that phytoplankton blooms have been sharply reduced as the algae are driven deeper into the water column, where they are not exposed to the bloom-producing energy of the sun.’
The decline in phytoplankton is believed to be a major reason for a regional decrease in Antarctic krill. And that loss of krill, coupled with the shrinking of the sea-ice that AdÃ©lies use as a feeding platform in winter, is a major reason why AdÃ©lie penguin populations in the northwestern Antarctic Peninsula have plummeted by more than 80 percent in the last 35 years.
It’s a different story further south where the waters off the Antarctic Peninsula were covered in extensive sea ice nearly year-round, inhibiting phytoplankton growth. But as the region has warmed and sea ice has retreated, the growing expanses of open water have actually led to an increase in phytoplankton production of more than 60 percent, which may well boost production of krill and the creatures that feed upon them.
There are other complications. Montaigne reports another researcher in the area, Hugh Ducklow, a biological oceanographer from the Marine Biological Laboratory in Woods Hole, Mass. He and his colleagues are observing another important change related to the loss of sea ice. Antarctica has long been characterized by a relatively simple marine ecosystem with few links in the food chain, which is dominated by large species at every level. A large type of phytoplankton, called diatoms, has prevailed, as has the large Antarctic krill.
’Now, however, Ducklow said that smaller phytoplankton – which Antarctic krill do not customarily eat – are becoming more prevalent, which he says is related to rising water temperatures and changes in ocean stratification. In addition, researchers in the region are finding that Antarctic krill are beginning to be replaced by a creature known as a salp – a clear, barrel-shaped, jellyfish-like tunicate that is 99 percent water and therefore has little nutritional value. These developments, said Ducklow, could bode ill for the penguins, seals, and whales so dependent on Antarctic krill.’
Montaigne provides summary quotes from the two experts:
Ducklow: ’This indicates fundamental changes in the food chain as a result of the loss of sea ice.’
Montes-Hugo: ’There is definitely large-scale ecosystem change, and where it’s heading, nobody knows.’
I won’t pursue the detail of Montaigne’s article into the Arctic, where he reports how the much earlier phytoplankton blooms are causing problems for whales and seabirds which have long timed their seasonal arrival in the Arctic to coincide with the peak of zooplankton, whose populations eat phytoplankton. Complex interrelationships between zooplankton and phytoplankton populations related to the sea ice are also affecting the food supply of clams and other bottom-dwelling organisms. Populations of spectacled eiders and marine mammals which eat this bounty resting on the bottom in the shallow waters of the continental shelf are consequently diminished. Other species, however, are favoured by the ice decline.
The ultimate effects of these changes at the base of the food chain in the polar regions are not yet clear, but it is already apparent that some individual populations and species are being relentlessly pressured and that long-established ecosystems are severely jolted. The discoveries of the scientists Montaigne reports are yet another indication of the far-ranging turmoil our continuing rapid release of greenhouse gases into the atmosphere is causing. We are playing havoc with the intricate web of life.