The western Antarctic Peninsula is experiencing rapid climate change evidenced by warmer air temperatures, increased westerlies, glacial and ice shelf retreat, reduced sea ice cover and delayed time of sea ice formation. The west Antarctic Peninsula food web shows influences of this rapid climate change: reduction in phytoplankton biomass, abundance of Antarctic krill and Adélie penguins have all been associated with changed environmental conditions.
In addition to climate change, the wAP ecosystem is also characterized by high interannual variability. Sea ice variability is governed by fluctuations in the Southern Annular Mode (SAM) – the principal mode of variability in the atmospheric circulation of Antarctica – a positive polarity of SAM in recent years has been associated with a reduction in sea ice cover and length of the sea ice season. Understanding the interactions that link the physical and biological systems in the Southern Ocean is key to interpret the cascade of changes in the ecosystem.
The study from Saba et al. determined how large-scale climate and local physical forcing affected different levels of the food web from phytoplankton to apex predators. Using more than 20 years of observations they described time variability of different parameters that included the physical components (temperature and salinity) of the water column as well as standing stock and rate measurements (chlorophyll, phytoplankton pigments, primary and bacterial productivity). They observed that positive anomalies in chlorophyll a with periods of 4-6 years were forced by physical processes that occurred the previous winter-spring season and that these corresponded with a negative phase of the SAM. Good conditions for phytoplankton (positive chlorophyll a anomalies) included higher sea ice cover and duration, reduced spring-summer winds and enhanced water column stability. More chlorophyll a in summer was also associated with a strong cohort of Antarctic krill, which was also evident in Adélie penguin’s diets, revealing strong trophic coupling. They observed that at present the western Antarctic Peninsula chlorophyll a variability (4-6 years) and Antarctic krill life span (6-7 years) are in sync, but an increase in positive SAM – as is projected by 2100 – could have “catastrophic “consequences for krill populations and correspondingly negative impacts among higher trophic levels including penguins, flighted sea birds, seals and whales.
Large scale climate and local physical forcing in the Antarctic Peninsula food web