Abstract
Antarctic krill Euphausia superba is a predominant species in the Southern Ocean, it is very sensitive to climate change, and it supports large stocks of fishes, seabirds, seals and whales in Antarctic marine ecosystems. Modern krill stocks have been estimated directly by net hauls and acoustic surveys; the historical krill density especially the long-term one in the Southern Ocean, however, is unknown. Here we inferred the relative krill population changes along the West Antarctic Peninsula (WAP) over the 20th century from the trophic level change of Antarctic fur seal Arctocephalus gazella using stable carbon (δ13C) and nitrogen (δ15N) isotopes of archival seal hairs. Since Antarctic fur seals feed preferentially on krill, the variation of δ15N in seal hair indicates a change in the proportion of krill in the seal's diets and thus the krill availability in local seawater. For the past century, enriching fur seal δ15N values indicated decreasing krill availability. This is agreement with direct observation for the past ∼30 years and suggests that the recently documented decline in krill populations began in the early parts of the 20th century. This novel method makes it possible to infer past krill population changes from ancient tissues of krill predators.
Highlights
Over the past 50 years, the West Antarctic Peninsula (WAP) has experienced rapid regional warming associated with significant sea-ice and krill stock reductions [1,2,3]
We propose a novel stable isotope methodology for deducing long-term relative krill population dynamics based upon the trophic level of krill predators, and inferred the relative krill population change of the 20th century using the d15N of Antarctic fur seal hair
The d15N values of seal hairs range from 10.26% to 11.94% with a mean of 11.24% (Fig 2c); There is an obvious difference in the rates of change between 1924–1957 and after 1957, that is the amplitude of variation in hair d15N in 1924–1957 (10.30%–11.78%, a 1.48% difference) is larger than that in 1958– 1997 (11.12%–11.94%, a 0.82% difference)
Summary
Over the past 50 years, the West Antarctic Peninsula (WAP) has experienced rapid regional warming associated with significant sea-ice and krill stock reductions [1,2,3]. Environmental variability throughout the WAP could lead to cascading trophic level changes [7], for krill predators such as the Antarctic fur seal. From a predator’s perspective, trophic level and dietary change of krill predators is a reflection of relative krill population changes [8]. Stable isotope analyses of animal tissues are increasingly recognized as a powerful tool for quantifying animal’s foraging habitat and trophic level [9]. Stable carbon (d13C) and nitrogen (d15N) isotope signatures of hair and whisker have been used to infer animal diets [10,11]. Well-preserved hairs and droppings in lake sediments have been used to infer past populations of seals and penguins [12,13,14]
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