Warm-Up Kills Krill, Endangers Food Chain

The food chain is only as strong as its weakest link.

And it appears that the antarctic krill may be reaching the breaking point, thanks to a warm spell in the region since 1940.

The little shrimplike crustaceans are a crucial intermediary between primary producers (microcritters that make their own food, such as algae) and larger animals that feed on krill, including fish, birds, seals and whales.

Krill seem to flourish when sea-ice formation is greatest, Valerie Loeb of Moss Landing Marine Laboratories in California and colleagues report in the June 26 Nature.

When ice cover is low and air temperature relatively high (frequently the case in recent decades), populations drop by a factor of 10.

Often one species’ misfortune is a competitor’s good news. The lucky parties in this case are salps, whose numbers have increased as krill counts plummeted. These tiny planktonic creatures with cylindrical, flow-through bodies are more abundant when air is warmer and ice cover smaller.

But their high-density “blooms” hurt krill reproduction, and they aren’t a popular diet item for antarctic predators.

So if the 50-year warm-up continues, the scientists warn, declining krill abundance (perhaps already responsible for shrinking numbers of Adelie penguins) could derange the entire marine food web in the region - including commercially important fish stocks.