New clue to Antarctic food-web puzzle: phytoplankton blooms, krill, penguin guano

by Tim Radford, Climate News Network, July 16, 2014

A landmark research study that shows one species of penguin is thriving while other populations are in rapid decline offers new insight into how climate change is affecting Antarctica.

LONDON - Good news from Antarctica: the continent may be warming, the ice shelf may be at risk, and the food chain may ultimately become precarious, but the Adélie penguin population – at least for the moment − is higher than ever before.

The news does not suggest that global warming and climate change are actually good for this important indicator species, which has certainly been in decline on the Antarctic Peninsula. But it does represent an advance: for the first time, a comprehensive study has concluded with a full census of the species.

Heather Lynch, assistant professor of ecology and evelotion at Stony Brook University in New York, and Michelle La Rue, research fellow at the University of Minnesota’s Polar Geopspatial Center, used high resolution satellite imagery to measure levels of penguin guano – the fertiliser industry’s preferred term for seabird excrement – on the continent.

They then used that as the basis for calculating the numbers of birds in a colony necessary to account for all that digested and evacuated seafood.

They report in a journal called The Auk: Ornithological Advances that they identified at least 17 populations of Adélie penguins not previously known to exist, but failed to pinpoint 13 already-recorded colonies, and declared 8 of them eradicated.

Their estimate for the total Adélie population in and around the Southern Ocean stands at 3.79 million, which is 53% higher than all previous estimates.

Useful evidence

The researchers call their work a “landmark” study, and see it not as evidence that climate change is going to work for the benefit of one particular species, but more as a useful piece of the great food-web puzzle in a changing climate.

Penguins have been in rapid decline in the West Antarctic Peninsula, which has become one of the fastest-warming regions on the planet. Warmer weather and increased rain have already started to take toll of Magellanic penguins in Argentina, and researchers recently predicted long-term decline for the iconic Emperor penguin on Antarctica itself.

But this is only long-term decline. As long as Antarctica stays cold and the ice shelf stays stable, the researchers say, the population could, in the short term, actually rise.

That is because what matters most to the species that nest in Antarctica is the supply of fish and krill around the continent’s edge. The health and resilience of the Adélie population – and the Emperor penguin, the leopard seal, the cetaceans, and so on – ultimately depend on how the krill and fish populations respond to climate change.

Humans, too, fish for commercial supplies of Antarctic krill, which provides a source of food for fish farms.

“Our finding of a 53% increase in Adélie penguin breeding abundance, compared to 20 years ago, suggests that estimates of krill consumption by this species may be seriously underestimated,” Dr Lynch said. “Leaving enough prey for natural krill predators is an important element in ensuring fisheries proceed sustainably.”

But a second team confirms in Nature Communications that there are strong links between climate and marine life, and that changes in factors such as wind speed and sea ice can have knock-on effects right around the Antarctic food web.

Since 1990, scientists aboard US research vessels have been conducting annual surveys along the western side of the Antarctic Peninsula, measuring populations of photosynthetic algae.

These peak every 4-6 years, according to changes in atmospheric pressure between the mid-latitudes and Antarctica itself.

Glacial meltwater

In winter, when cold southerly winds blow across the Peninsula, the winter ice extends. Winds drop from spring to summer, reducing the retreat of the ice. So the water column in summer then is stable, and the phytoplankton multiply, fed by iron-rich glacial meltwater.

The blooms of phytoplankton are what the krill need to multiply, and when the krill are around in huge volumes, the Adélie and other penguins, fur seals, baleen whales and albatross don’t have to go so far to find food.

But marine scientist Grace Saba, who did her research while with the Virginia Institute of Marine Science, before moving to Rutgers University, New Jersey, reports that these ideal conditions – negative phases of the Southern Annular Mode (SAM), to give it the technical terminology – are not guaranteed in future. If the world goes on burning fossil fuels, conditions will probably change.

“Projections from global climate models under business-as-usual emission scenarios up to the year 2100 suggest a further increase in temperature and in the occurrence of positive-SAM conditions,” Dr Saba said.
“If even one positive SAM episode lasted longer than the krill lifespan – 4-6 years with decreased phytoplankton abundance and krill recruitment – it could be catastrophic to the krill population.”