There has been climatic warming in the Western Antarctic Peninsula (WAP), with cooling or stable conditions in other sectors of the continent. Adélie penguins (Pygoscelis adeliae) have been declining in northern sectors of the WAP since the 1970s, leading to multiple hypotheses for their decline. The food web of the WAP marine ecosystem is trophically short; krill feed on phytoplankton and penguins mainly feed on krill. Therefore, it is often assumed that any climate-driven change in lower trophic levels (phytoplankton) would be reflected in higher trophic levels. A shift towards smaller phytoplankton cells under warmer climates was the hypothesized explanation for krill declines in the northern WAP. These fundamental changes in the lower trophic levels could be transferred up the short WAP food web, resulting in a decline in Adélie penguin populations through a bottom-up trophic cascade or through competition for krill with other krill predators. However, analyses of Adélie penguin foraging dynamics in their oceanographic context, and local and regional krill surveys suggest that overall krill declines or competition for krill resources may not be the onlly determinants of Adélie penguin declines in the WAP. A warming climate appears to be acting on Adélie penguin chicks directly through increased wet precipitation on unprotected chicks leading to higher thermoregulatory costs as an additional population stressor, indicating that atmospheric forcing may be more important than food web dynamics in determining the success of Adélie penguins, even though there are clear climate-related signals in lower trophic levels.

Dr. Matthew Oliver is the Patricia & Charles Robertson Associate Professor of Marine Science & Policy at the School of Marine Science and Policy and Department of Geography at the University of Delaware. Dr. Oliver completed his doctoral work at Rutgers University. His primary research focuses on biological oceanography, working both in the Mid-Atlantic region and in Antarctica, and is supported by NASA, NOAA, BOEM, NSF and NGO's.

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