Food web dynamics in the Scotia Sea in summer: a stable isotope study

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Stowasser, G., Atkinson, A., McGill, R. A. R., Phillips, R. A., Collins, M. A., Pond, D. W.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2012
• Subjects: Antarctic; Scotia Sea; Southern Ocean; The Antarctic; Antarc*; Euphausia superba; Copepods
• Online Access: https://oceanrep.geomar.de/id/eprint/53831/; https://oceanrep.geomar.de/id/eprint/53831/1/4487.pdf; https://doi.org/10.1016/j.dsr2.2011.08.004

The pelagic food web of the Scotia Sea was studied by analysing natural abundances of nitrogen and carbon stable isotopes of primary producers and pelagic consumers, sampled from the seasonal ice edge in the south to the Antarctic Polar Front in the north. The analysis covered, within a single mid-summer period, particulate organic matter (POM) and 38 taxa, ranging from suspension feeding copepods and salps to omnivorous euphausiids, pelagic fish and higher, land-based predators including fur seals, penguins and flying birds. Spatial variation in δ15N of POM correlated well with nutrient availability and primary productivity. Latitudinal differences in δ13C of POM were closely linked to variations in temperature, nutrients and productivity depending on the frontal region sampled. This translated to equivalent (although smaller) regional δ13C differences among higher trophic levels. The trophic positions of species based on isotope values broadly agreed with previously published dietary data with three important exceptions. First, the carnivorous amphipod Themisto gaudichaudii had anomalously low δ15N values. Second, Euphausia superba had δ15N values that were also surprisingly low, considering the abundant literature suggesting its omnivory. Third, the copepod Rhincalanus gigas, considered a suspension feeder, had unexpectedly high δ15N values rather more in keeping with omnivorous feeding. The consumer δ15N values ranged from 1.2‰ (min.) measured in Salpa thompsoni (designated here as trophic level (TL) 2 across all regions) to 15.2‰ (max.) measured in white-chinned petrels (Procellaria aequinoctialis, calculated as TL5 relative to the TL2 of salps). Excluding seabirds, the resulting food chain length of 3.7 TL (above POM at TL1) was lower than in most other Southern Ocean and temperate marine pelagic ecosystems. The majority (60%) of vertebrate predators occupied only 1–1.5 trophic levels above the herbivorous suspension feeders such as krill. This indicates the existence of the classic short food chain of ...