Krill, a major component in the diets of many Antarctic species such as whales, penguins and seals, appears to be undergoing a major decline.
Krill: An important link in the food web
Since the 1970s, numbers of the shrimp-like creatures have fallen by 80% in waters near the Antarctic Peninsula, UK scientists tell Nature magazine.
The crustacean feeds on algae under the ice so the fall may be linked to recent warming that has reduced sea-ice cover.
The change could have a big impact on the whole Southern Ocean food web.
"Krill are a central species [to this] web; they are a major food item for species such as penguins, seals, albatrosses and whales," Dr Angus Atkinson, from British Antarctic Survey, told BBC News.
Dr Atkinson and his colleagues established the krill trend by analysing 12,000 net hauls taken during periods from 1926 to 2003.
They found krill were concentrated in the southwest Atlantic sector of the Southern Ocean and this was where the decline was most marked.
Their statistical analysis suggests the most likely explanation is the reduction in sea-ice cover. This has been pronounced around the Antarctic Peninsula, one of the three fastest-warming regions on Earth.
"Krill feed on the algae found under the surface of the sea ice, which acts as a kind of 'nursery'," Dr Atkinson said.
"The Antarctic Peninsula, a key breeding ground for the krill, has warmed by 2.5C in the last 50 years, with a striking decrease in sea ice. We don't fully understand how the loss of sea-ice here is connected to the warming, but we believe that it could be behind the decline in krill."
This would fit with a corresponding increase in the numbers of transparent tube-like creatures known as salps. These animals tend to live in warmer, less food-rich areas.
The implications of the changes are commercial as well as scientific, the Bas team believes, because krill are becoming an increasingly valuable resource for fisheries.
The krill thrive on algae under the ice
"It is not a problem for the fisheries at the moment because they're catching a relatively small amount of krill, but the fisheries could expand quite dramatically in the future.
"We've got to monitor the situation, work out what's causing the decline in krill and predict the future for the food web."