Abstract

The Southern Ocean maintains a complex marine food web based on its stock of photosynthesizing phytoplankton. For the same reason, it is our most significant sink of carbon, vital to the functioning of our global atmospheric systems. However, this key polar ocean is also responsive to atmospheric variability dominated by the Southern Annular Mode (SAM). In response to ozone depletion, the SAM has exhibited a significant trend to its positive phase since the 1970s. Consequently, the westerlies driving circulation have strengthened and contracted poleward, changing mixing regimes and shifting fronts further south. Inevitably, these changes must impact on phytoplankton patterns, with ramifications for the Antarctic marine food web. Based on satellite measures of [Chl-a] and light attenuation (Zeu) which were first validated by fluorescence and light data collected by tagged southern elephant seals, this talk addresses the possibility that changes to summer phytoplankton patterns in the Southern Ocean can already be measured in the 12-year MODIS record. Trends suggest overall declines and shifts in the timing of blooms. In some regions, changes also appear to be associated with the SAM . However, declines in surface [Chl-a] may point to increases in deep chlorophyll maxima rather than net losses to biomass, and, ultimately, it is the vertical distribution of phytoplankton that structures marine food webs. Southern elephant seals are tertiary consumers, yet their foraging patterns do not appear to be independent of vertically-integrated phytoplankton abundance. This suggests that perturbations to the base of the marine food web will persist through to the top.