Water masses, ocean fronts, and the structure of Antarctic seabird communities: Putting the eastern Bellingshausen Sea in perspective
2011
Waters off the western Antarctic Peninsula (i.e., the eastern Bellingshausen Sea) are unusually complex
owing to the convergence of several major fronts. Determining the relative influence of fronts on
occurrence patterns of top-trophic species in that area, therefore, has been challenging. In one of the few
ocean-wide seabird data syntheses, in this case for the Southern Ocean, we analyzed ample, previously
collected cruise data, Antarctic-wide, to determine seabird species assemblages and quantitative relation-
ships to fronts as a way to provide context to the long-term Palmer LTER and the winter Southern Ocean
GLOBEC studies in the eastern Bellingshausen Sea. Fronts investigated during both winter (April–
September) and summer (October–March) were the southern boundary of the Antarctic Circumpolar
Current (ACC), which separates the High Antarctic from the Low Antarctic water mass, and within which
are embedded the marginal ice zone and Antarctic Shelf Break Front; and the Antarctic Polar Front, which
separates the Low Antarctic and the Subantarctic water masses. We used clustering to determine species’
groupings with water masses, and generalized additive models to relate species’ densities, biomass and
diversity to distance to respective fronts. Antarctic-wide, in both periods, highest seabird densities and
lowest species diversity were found in the High Antarctic water mass. In the eastern Bellingshausen,
seabird density in the High Antarctic water mass was lower (as low as half that of winter) than found in
other Antarctic regions. During winter, Antarctic-wide, two significant species groups were evident: one
dominated by Ade´ lie penguins (Pygoscelis adeliae) (High Antarctic water mass) and the other by petrels and
prions (no differentiation among water masses); in eastern Bellingshausen waters during winter, the one
significant species group was composed of species from both Antarctic-wide groups. In summer, Antarctic-
wide, a High Antarctic group dominated by Ade´ lie penguins, a Low Antarctic group dominated by petrels,
and a Subantarctic group dominated by albatross were evident. In eastern Bellingshausen waters during
summer, groups were inconsistent. With regard to frontal features, Antarctic-wide in winter, distance to
the ice edge was an important explanatory factor for nine of 14 species, distance to the Antarctic Polar Front
for six species and distance to the Shelf Break Front for six species; however, these Antarctic-wide models
could not successfully predict spatial relationships of winter seabird density (individual species or total)
and biomass in the eastern Bellingshausen. Antarctic-wide in summer, distance to land/Antarctic continent
was important for 10 of 18 species, not a surprising result for these summer-time Antarctic breeders, as
colonies are associated with ice-free areas of coastal land. Distance to the Shelf Break Front was important
for 8 and distance to the southern boundary of the ACC was important for 7 species. These summer models
were more successful in predicting eastern Bellingshausen species density and species diversity but failed
to predict total seabird density or biomass. Antarctic seabirds appear to respond to fronts in a way similar
to that observed along the well-studied upwelling front of the California Current. To understand fully the
seabird patterns found in this synthesis, multi-disciplinary at-sea investigations, including a quantified prey
field, are needed.
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