Abstract
The expected reduction of ice algae with declining sea ice may prove to be detrimental to the Pacific Arctic ecosystem. Benthic organisms that rely on sea ice organic carbon (iPOC) sustain benthic predators such as the Pacific walrus (Odobenus rosmarus divergens). The ability to track the trophic transfer of iPOC is critical to understanding its value in the food web, but prior methods have lacked the required source specificity. We analyzed the H-Print index, based on biomarkers of ice algae versus phytoplankton contributions to organic carbon in marine predators, in Pacific walrus livers collected in 2012, 2014 and 2016 from the Northern Bering Sea (NBS) and Chukchi Sea. We paired these measurements with stable nitrogen isotopes (δ15N) to estimate trophic position. We observed differences in the contribution of iPOC in Pacific walrus diet between regions, sexes, and age classes. Specifically, the contribution of iPOC to the diet of Pacific walruses was higher in the Chukchi Sea (52%) compared to the NBS (30%). This regional difference is consistent with longer annual sea ice persistence in the Chukchi Sea. Within the NBS, the contribution of iPOC to walrus spring diet was higher in females (~45%) compared to males (~30%) for each year (p < 0.001), likely due to specific foraging behavior of females to support energetic demands associated with pregnancy and lactation. Within the Chukchi Sea, the iPOC contribution was similar between males and females, yet higher in juveniles than in adults. Despite differences in the origin of organic carbon fueling the system (sea ice versus pelagic derived carbon), the trophic position of adult female Pacific walruses was similar between the NBS and Chukchi Sea (3.2 and 3.5, respectively), supporting similar diets (i.e. clams). Given the higher quality of organic carbon from ice algae, the retreat of seasonal sea ice in recent decades may create an additional vulnerability for female and juvenile Pacific walruses and should be considered in management of the species.
Highlights
Primary production in the Pacific Arctic region is partitioned between sympagic and pelagic sources [1]
Sea ice organic carbon was higher in walruses that migrated into the Chukchi Sea and were collected in the summer and autumn months compared to those harvested from St
Lawrence Island in the Northern Bering Sea (NBS) in the spring. This regional distinction aligns with sea ice organic carbon levels measured in walrus prey items, in addition to the general distribution of highly branched isoprenoids (HBIs) deposited on the seafloor throughout the Pacific Arctic [76]
Summary
Primary production in the Pacific Arctic region is partitioned between sympagic (sea ice) and pelagic (open water) sources [1]. On the continental shelf there is a tight sympagic-pelagic-benthic coupling resulting in high benthic biomass [2], which supports a variety of specialized benthic predators such as the Pacific walrus (Odobenus rosmarus divergens) [3,4] This ecosystem is undergoing a shift from a benthic-dominated system to a more pelagic-based one due in part to changes in ice algal and phytoplankton production [5]. While ice algae are likely decreasing in some locations, phytoplankton blooms are increasing in response to sea ice declines [7,8] It is unclear how this shift will impact food webs in the Pacific Arctic [9,10]. Monitoring sea ice organic carbon contributions to Pacific walrus diets has the potential to provide another informative tool in assessing one of several ways in which climate change will impact this species
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