Trends, cycles, interannual variability for three pelagic species west of the Antarctic Peninsula 19932008

The Palmer Long Term Ecological Research study region west of the Antarctic Penin- sula is experiencing warming and changing seasonal sea ice dynamics. Abundance patterns of 3 species of pelagic secondary producers were analyzed for trends, cycles, range extensions or shifts in the location of highest density, and for changes in population dynamics over a 16 yr period (1993�2008). Species analyzed represented different hydrographic regimes and are known to have contrasting responses to seasonal sea ice dynamics: krill Euphausia superba, seasonal sea ice zone; tunicates Salpa thompsoni, warmer waters with minimal sea ice; and larval Antarctic silver- fish Pleuragramma antarcticum, cold continental shelf waters. Cycles were observed in grid-wide abundance and recruitment for E. superba. Maximum grid-wide densities did not decrease, but the location of highest densities shifted southward 200 km, away from Adelie penguin rookeries at the northern end. A distinct change post-1999 was apparent in the frequency of occurrence and abundance of S. thompsoni. Mixtures of krill and salps became common, but neither peak densi- ties nor the frequency of peak years for salps increased. As with Antarctic krill, highest salp den- sities shifted southward alongshore. Larval P. antarcticum were abundant in the northern coastal region in the early 1990s, but virtually disappeared in that region after 1999/2000. Possible mech- anisms underlying these observations include the southerly movement of the sea ice edge during spring, changes in proximity of source populations (salps), and changes in transport pathways (larval P. antarcticum). Patterns are compared to those in the SW Atlantic.