Abstract
Long-term studies of pelagic nekton in the Southern Ocean and their responses to ongoing environmental change are rare. Using stable isotope ratios measured in squid beaks recovered from diet samples of wandering albatrosses Diomedea exulans, we assessed decadal variation (from 1976 to 2016) in the habitat (δ13C) and trophic level (δ15N) of five important Southern Ocean squid species in relation to indices of environmental conditions—Southern Oscillation Index (SOI) and Southern Annular Mode (SAM). Based on δ13C values, corrected for the Suess effect, habitat had changed over the last 50 years for Taonius sp. B (Voss), Gonatus antarcticus, Galiteuthis glacialis and Histioteuthis atlantica but not Moroteuthopsis longimana. By comparison, mean δ15N values were similar across decades for all five species, suggesting minimal changes in trophic levels. Both SAM and SOI have increased in strength and frequency over the study period but, of the five species, only in Taonius sp. B (Voss) did these indices correlate with, δ13C and δ15N values, indicating direct relationships between environmental conditions, habitat and trophic level. The five cephalopod species therefore changed their habitats with changing environmental conditions over the last 50 years but maintained similar trophic levels. Hence, cephalopods are likely to remain important prey for top predators in Southern Ocean food webs, despite ongoing climate change.
Highlights
Long-term studies of pelagic nekton in the Southern Ocean and their responses to ongoing environmental change are rare
Decadal variation in stable isotope ratios. δ13C values in beaks ranged from − 17.39‰ in Gonatus antarcticus to − 25.95‰ in Galiteuthis glacialis (Fig. 1)
B, Gonatus antarcticus and Galiteuthis glacialis, suggesting a northward shift in habitat in subsequent decades (2006 and 2016), whereas there was no consistent trend over time for Histioteuthis atlantica (Fig. 1). δ15N values in beaks ranged from 4.77‰ in—M. longimana to 12.43‰ in—Taonius sp
Summary
Long-term studies of pelagic nekton in the Southern Ocean and their responses to ongoing environmental change are rare. The two leading modes of environmental variability in the region—the Southern Oscillation Index (SOI) and the Southern Annular Mode (SAM)—have grown in strength and frequency, associated with more and stronger El Niño events and increasing intensity of westerly w inds[6,7,8] Both SOI and SAM have tangible climatic influences on biological processes[2,9]. In the Southern Ocean, documented effects of climate variability on ecological processes, include changes in diet, foraging areas, breeding success or abundance, at higher trophic levels, such as in penguins and a lbatrosses[24,25,26,27] The only such studies on key pelagic zooplankton or nekton are those on k rill[28,29]. Stable isotope ratios of carbon (13C/12C; δ13C) and nitrogen (15N/14N; δ15N) in these beaks can provide invaluable information on, respectively, the carbon source (habitat) and trophic position[37,42,43]
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