Compiled seabird densities and distance to fronts in the Southern Ocean

We conducted a Southern Ocean seabird data synthesis using previously collected Antarctic-wide cruise data to determine seabird species assemblages and quantitative relationships to fronts as a way to provide context to the long-term Palmer Long Term Ecological Research (LTER) program and the winter Southern Ocean Global Oceans Ecosystems Dynamics (GLOBEC) studies in the eastern Bellingshausen Sea. We combined 9 cruises conducted in areas around the Antarctic continent during the summer (October-March) for an Antarctic-wide summer data set with 2348 transects and 63 species seen at least once. We combined seven cruises conducted in areas around the Antarctic continent during the winter (April-September) for an Antarctic-wide winter data set with 1591 transects and 56 species seen at least once. We used 6 Palmer LTER cruises which, combined, had 1446 transects and 29 species seen at least once. The 4 winter Southern Ocean GLOBEC cruises combined had 652 transects and 16 species seen at least once. 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. Using GIS, distances to the different fronts were measured for all transects and water mass determined; these variables were added to the seabird density files for 4 combined density-environmental files (Antarctic-wide-summer, Antarctic-wide-winter, PALTER-summer, SOGLOBEC-winter).

Methods
The Southern Ocean GLOBEC cruises will be referred to as the Bellingshausen winter cruises and the Palmer LTER cruises as the Bellingshausen summer cruises.

Field and Analysis Methods

All cruises used strip transects to collect data on species occurring along the cruise track. In general, two observers carried out surveys from one side of the ship, usually from the flying bridge. Strip width was 300 m wide. On all cruises, observers noted time of sighting, species identity, whether the bird was stationary (e.g., standing on ice, sitting on water) or flying, and, if flying, direction of the flying bird in relation to the ship. Species sightings were made continuously on all but 3 cruises (winter Aurora Australis cruises) where 10-minute intervals were used. Survey effort was partitioned into 30-minute bins, sequentially, for surveys with continuous counts. All counts were adjusted for bird flux (movement of birds relative to that of the ship) using ship speed, wind speed and direction, and direction of flying birds in relation to the ship following Spear et al. (1992). Species densities by transect were the adjusted number of birds by species/area surveyed (square kilometers). Transect area varied due to differences in transect length, which depends on ship speed (which, in turn, reflects environmental conditions), and transect time interval.

GIS files defining land, the Antarctic coast, and the fronts (southern boundary of the ACC and Antarctic Polar Front) were obtained from the Australian Antarctic Data Centre. The 1000 m isobath was obtained by creating a bathymetric lattice contour GIS file using gridded ETOPO1 data from NOAA’s National Geospatial Data Center, then edited to produce one continuous line defining the position of the shelf break. Using GIS, we measured the following set of environmental variables for every transect:

distance to the ice edge defined by 15% ice cover (km)
distance to the ice edge defined by 50% ice cover (km)
distance to the Antarctic Polar Front (km)
distance to the southern boundary of the Antarctic Circumpolar Current (ACC) (km)
distance to the 1000 m isobath (km) (which represents the Shelf Break Front)
distance to the Antarctic coast (km)
distance to nearest land (nearest mainland or island) (km)

All transects were placed into one of three water masses (Subantarctic, Low Antarctic, High Antarctic). The water masses are separated by the Antarctic Polar Front and the southern boundary of the ACC. The Subantarctic water mass is north of the Antarctic Polar Front, the Low Antarctic water mass is south of the Antarctic Polar Front but north of the southern boundary of the ACC, and the High Antarctic water mass is south of southern boundary of the ACC. Transects with a positive distance to the Antarctic Polar Front (i.e., north of the polar front) were placed in the Subantarctic water mass, transects with a negative distance to the Antarctic Polar Front (i.e., south of the polar front) but a positive distance to the Southern Boundary of the ACC (i.e., north of the southern boundary) were placed in the Low Antarctic water mass, and transects with a negative distance to the southern boundary of the ACC (i.e., south of the southern boundary) were placed in the high Antarctic water mass. See Ribic et al. (2011) for maps of the cruise tracks and fronts by season.

Cruises were divided seasonally between winter (April–September) and summer (October–March). For each season, Antarctic-wide cruises were combined into one data set, resulting in 2348 transects in summer (Antarctic-wide-summer-dens-env-final-021025.xlsx) and 1591 transects in winter (Antarctic-wide-winter-dens-env-final-021025.xlsx). Species listed in the combined data sets are all species seen at least once over all the cruises for the specific season. If a species was not seen on a specific cruise, a zero was entered for the species density value. In the Antarctic-wide-summer file, there are 63 species seen at least once. In the Antarctic-wide-winter file, there are 56 species seen at least once.

Note Ribic et al. (2011) removed 17 transects from Antarctic-wide summer cruises that overlapped the Bellingshausen summer cruise grid, resulting in 2331 transects for analysis in the paper. The removed transects have Summer.Cruise.IDs 2332-2348 in the Antarctic-wide-summer-dens-env-final-021025.xlsx file. No transects in the Antarctic-wide winter cruises overlapped the Bellingshausen winter cruise grid.

While the Bellingshausen summer cruises were analyzed separately, they were combined into a single file for publishing, resulting in a file with 1446 transects (PALTER-summer-dens-env-final-021025.xlsx). While the Bellingshausen winter cruises were analyzed separately, they were combined into a single file for publishing, resulting in a file with 652 transects (SOGLOBEC-winter-dens-env-final-021025.xlsx). Similar to the Antarctic-wide seasonal data files, species listed in the PALTER-summer file are all species seen at least once over the 6 Palmer LTER cruises and species listed in the SOGLOBEC-winter file are all species seen at least once over the 4 Southern Ocean Globec cruises. If a species was not seen on a specific cruise, a zero was entered for the species density value. In the PALTER-summer file, there are 29 species seen at least once. In the SOGLOBEC-winter file, there are 16 species seen at least once.

References: 
Ribic, C. A., D. G. Ainley, R. G. Ford, W. R. Fraser, C. T. Tynan, and E. J. Woehler. 2011. Water masses, ocean fronts, and the structure of Antarctic seabird communities: putting the eastern Bellingshausen Sea in perspective. Deep-Sea Research II 58:1695-1709. https://doi.org/10.1016/j.dsr2.2009.09.017

Spear, L. B., N. Nur, and D. G. Ainley. 1992. Estimating absolute densities of flying seabirds using analyses of relative movement. Auk 109:385–389.