13 July 2014

Strong links between Antarctic climate, phytoplankton, krill and penguins

Adélie penguins on Avian Island
along the West Antarctic Peninsula.
Photo credit: Donna Patterson-Fraser.
ANTARCTICA – A long-term study of the links between climate and marine life along the rapidly warming West Antarctic Peninsula reveals how changes in physical factors such as wind speed and sea-ice cover send ripples up the food web – from single-celled phytoplankton to Adélie penguins.

The study, published on 7 July in Nature Communications, is authored by members of the Palmer Long-Term Ecological Research (PAL-LTER) programme, which conducts annual shipboard surveys along the western side of the Antarctic Peninsula. PAL-LTER scientists began studying the fast-changing region in 1990.

One of the study's authors, Deborah Steinberg, is a professor at William & Mary's Virginia Institute of Marine Science (VIMS) and has been a lead scientist of PAL-LTER since 2008. She said the current study provides one of the few instances where marine researchers have a dataset of sufficient length and detail to reveal how climate signals can reverberate through a polar food web.

"That's the importance of long-term ecosystem monitoring. It provides the data needed to separate a signal from the noise, and to determine how plants and animals interact with both their physical environment and each other. That knowledge is critical as climate warming continues to impact this polar ocean ecosystem," she said.

The West Antarctic Peninsula is one of the fastest warming regions on Earth, with annual winter temperatures increasing by 11°F (approximately 6°C) during the last 50 years. But how is this linked to Antarctic penguin populations?

Krill recovered from a penguin's stomach
Photo credit: Donna Patterson-Fraser
The polar food web starts with phytoplankton – tiny drifting plants. The team's research shows that phytoplankton populations peak every 4 to 6 years in the waters along the West Antarctic Peninsula. These blooms correlate with a negative phase of the Southern Annular Mode (SAM), a see-saw shift in atmospheric pressure between mid-latitudes and Antarctica.

During a negative phase of SAM, cold southerly winds blow across the Peninsula in winter, increasing the extent of sea-ice. From spring into summer, winds are significantly reduced, delaying ice retreat.

"The combination of a windy winter with heavy sea-ice followed by a calm spring favors the development and persistence of a stable water column in the summer along the West Antarctic Peninsula," said another of the study's authors, Dr Grace Saba of Rutgers University.

This stable water column encourages phytoplankton growth, probably by keeping the tiny plants nearer the sunlit surface and close to the iron-rich glacial meltwater they need to thrive.

Moving up the food web, the team's sampling shows that the area's periodic, climate-driven phytoplankton blooms are a key to krill "recruitment" – the addition of new, young individuals into the krill population. Adélie penguins and other top Antarctic predators rely on a robust population of krill prey for their own health and reproductive success.

"When climate conditions – a negative SAM and stable water column – lead to peaks in the abundance of phytoplankton and krill, Adélie penguins don't have to go far to forage," said Saba.

"But when SAM is positive, warm northwesterly winds blow over the Peninsula region, bringing less sea ice and a less-stable water column – factors that discourage the large blooms of phytoplankton on which krill rely.

"Penguins then have to forage further, and thus end up delivering less food to their chicks. That can decrease their reproductive success."

Ongoing work by Palmer scientists shows that the population of Adélie penguins near Palmer Station has fallen by 85% since 1974. The researchers stop short of attributing this decline solely to a climate-related shortage of krill, but they are concerned for the future, given that climate models project an increase in the occurrence of positive SAM episodes during the coming century.

"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," said Saba.

"If even one positive SAM episode lasted longer than the krill lifespan – 4 or 6 years with decreased phytoplankton abundance and krill recruitment – it could be catastrophic to the krill population."

As well as Adélie penguins, krill are the main food source for macaroni and gentoo penguins, albatross and Antarctic fur seals. They are also eaten by baleen whales such as humpbacks.

Other institutes involved in the study include National Oceanic and Atmospheric Administration Fisheries, the Polar Oceans Research Group, Lamont-Doherty Earth Observatory, Woods Hole Oceanographic Institution and the University of Colorado at Boulder. 

Source
Study reveals strong links between Antarctic climate, food web [media release], 7 July 2014, Virginia Institute of Marine Science

Nature Communications citation
Saba, G.K., Fraser, W.R., Saba, V.S., Iannuzzi, R.A., Coleman, K.E., Doney, S.C., ... Schofield, O.M. (2014) Winter and spring controls on the summer food web of the coastal West Antarctic Peninsula Nature Communications 5. doi:10.1038/ncomms5318

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