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Trophic cascades in the western Ross Sea, Antarctica: revisited Public Deposited

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  • We investigated mesopredator effects on prey availability in the Ross Sea, Antarctica, assessing the reasons why Adélie penguin Pygoscelis adeliae foraging trip duration (FTD) increases and diet changes from krill to fish as numbers of foraging penguins and competing cetaceans increase in the penguins’ foraging area. To investigate penguins’ seasonally changing FTD as a function of foraging- population size—previously investigated indirectly— we used bio-logging to determine the penguins’ 3-dimensional foraging volume, while an autonomous glider quantified the depth, abundance, and distribution of potential prey. As numbers of foraging penguins and cetaceans increased, penguins spent more time on foraging trips, traveling farther and deeper, and their diet included more fish, as average maximum depth of krill increased from 45 to 65 m, and that of small fish also deepened, but only from 51 to 57 m. With a need to forage at greater depths for increasingly over lapping prey, the penguins consumed more of the energy-dense fish. Krill depth was negatively correlated with chlorophyll (a proxy for krill food), indicating an uncoupling between the two and the overwhelming importance of predation avoidance by the krill relative to food acquisition. Results support the hypotheses that (1) predators remove the grazers from Ross Sea surface waters, controlling their vertical distributions; and (2) the food web has a ‘wasp-waist’ structure, in which middle- and upper-trophic levels are controlled top-down, whereas phytoplankton production and accumulation are regulated bottom- up, largely independent of grazer control. Ross Sea models need revision to reflect this food web structure.
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  • Ainley, D. G., Ballard, G., Jones, R. M., Jongsomjit, D., Pierce, S. D., Smith, W. O., & Veloz, S. (2015). Trophic cascades in the western Ross Sea, Antarctica: revisited. Marine Ecology Progress Series, 534, 1-16. doi:10.3354/meps11394
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  • This project was supported financially by NSF grants ANT 1141948 and ANT 1142174, and logistically by the U.S. Antarctic Program, including helicopters to support glider deployment and the RV 'Nathaniel B. Palmer' for glider retrieval. Access to the Cape Crozier Specially Protected Area and work on penguins was approved under ACA Permit 2011-002 and Animal Welfare Permit 4130 through Oregon State University. We acknowledge use of imagery from the Land Atmosphere Near-real time Capability for EOS (LANCE) system operated by the NASA/GSFC Earth Science Data and Information System (ESDIS) with funding provided by NASA/HQ. The manuscript benefitted from review by K. Dugger, A. Lescroel, and B. Saenz. This is VIMS contribution number 3492, and Point Blue Conservation Science contribution number 2038.
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