ABSTRACT: 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 overlapping 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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