Change in isotopic signatures suggest food web shift off the western Antarctic Peninsula
The local warming occurring within the Western Antarctic Peninsula (WAP) is causing some of the greatest environmental shifts on the planet. Over the past 140 years there have been profound biological and physical perturbations to the wildlife of the WAP. This includes the removal and then return of krill-eating whale (humpback and southern right) and seal (fur seals) populations, fin-fish and krill fisheries, as well as the local warming. We examined whether trophic level shifts were evident in the top predators within this system and use stable isotope signatures as a proxy. We examined the bulk δ15N tissue values from 167 leopard seals, sampled from four disparate Antarctic locations between 1880 and 2011, along with values of specialist predators within the system, to account for the uncertainty due to baseline isotopic shifts.
Contemporary WAP leopard seals have different nitrogen isotopic values (δ15N) to other leopard seal populations, however historically WAP leopard seals had δ15N values within the range of other Western and Eastern Antarctic populations. From our 140yr record of leopard seal tissues, we show that the δ15N values of WAP leopard seal tissues have dropped significantly, and that this change occurs as a step around the 1980s. The magnitude (2.6‰ δ15N) of change is ecologically significant as it reflects a drop of a trophic level within the contemporary WAP food web. Values suggest that leopard seal have shifted from eating vertebrates to krill. This is supported by concurrent fatty acid and scat analysis. Over this same time period there was no shift in δ15N values of the WAP krill-feeding specialist, the crabeater seal, which supports the idea that the change in leopard seal δ15N values is not due to a baseline shift in nitrogen isotope values. Since the 1880s, there has been no significant change in δ15N values in other leopard seal populations from regions experiencing less warming.
Despite the century-long perturbations in the WAP the top predators, the leopard seals, show a trophic downshift only in recent times, post 1980s. The trophic shift by this top predator, seen in concert with changes at all levels lower in the food web, suggests that the WAP may have moved through a regime shift in the 1970-80s and today reflects an alternate stable state.