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

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 delta N-15 of POM correlated well with nutrient availability and primary productivity. Latitudinal differences in delta C-13 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 delta C-13 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 delta N-15 values. Second, Euphausia superba had delta N-15 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 delta N-15 values rather more in keeping with omnivorous feeding. The consumer delta N-15 values ranged from 1.2 parts per thousand (min.) measured in Salpa thompsoni (designated here as trophic level (TL) 2 across all regions) to 15.2 parts per thousand (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 POM-suspension feeder-vertebrate predator. However the presence of trophic levels 4 and above indicates the existence of alternative trophic pathways, for example involving myctophid fish or carrion, and that some wide-ranging predators which breed at South Georgia also feed outside the region. This conclusion is supported first by the continuum of delta N-15 values between krill, suspension feeding copepods and myctophid fish, and secondly by higher trophic levels in several of the myctophid species in the low-krill region of the northern Scotia Sea, suggesting latitudinal differences in food web structure and food chain length. (C) 2011 Elsevier Ltd. All rights reserved.