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1. Behavioral observations and stable isotopes reveal high individual variation and little seasonal variation in sea otter diets in Southeast Alaska

   https://doi.org/10.3354/meps13871  

   LaRoche, NL ; King, SL ; Rogers, MC ; Eckert, GL ; Pearson, HC ( October 2021 , Marine Ecology Progress Series)

   Two complementary approaches were used to assess year-round variation in the diet of sea otters Enhydra lutris around Prince of Wales Island (POW) in southern Southeast Alaska, a region characterized by mixed-bottom habitat. We observed sea otters foraging to determine diet composition during the spring and summer. Then, we obtained sea otter vibrissae, which record temporal foraging patterns as they grow, from subsistence hunters to identify year-round changes in sea otter diets via stable isotope analysis of carbon (δ 13 C) and nitrogen (δ 15 N). We compared the stable isotopes from sea otter vibrissae and sea otter prey items that were collected during spring, summer, and winter. Overall, year-round sea otter diet estimates from stable isotope signatures and visual observations from spring and summer were dominated by clams in terms of biomass, with butter clams Saxidomus gigantea the most common clam species seen during visual observations. Our results indicate that these sea otters, when considered together at a regional level around POW, do not exhibit shifts in the main prey source by season or location. However, sea otter diets identified by stable isotopes had a strong individual-level variation. Behavioral variation among individual sea otters may be a primary drivingmore »factor in diet composition. This study provides quantitative diet composition data for modeling predictions of invertebrate population estimates that may aid in the future management of shellfisheries and subsistence hunting and the development of co-management strategies for this protected species.« less
2. Advancing the Sea Ice Hypothesis: Trophic Interactions Among Breeding Pygoscelis Penguins With Divergent Population Trends Throughout the Western Antarctic Peninsula

   https://doi.org/10.3389/fmars.2021.526092  

   Gorman, Kristen B. ; Ruck, Kate E. ; Williams, Tony D. ; Fraser, William R. ( September 2021 , Frontiers in Marine Science)

   We evaluated annual and regional variation in the dietary niche of Pygoscelis penguins including the sea ice-obligate Adélie penguin ( Pygoscelis adeliae ), and sea ice-intolerant chinstrap ( Pygoscelis antarcticus ) and gentoo ( Pygoscelis papua ) penguins, three species that nest throughout the western Antarctic Peninsula (AP) to test the sea ice trophic interaction hypothesis , which posits that penguin breeding populations with divergent trends, i.e., declining or increasing, are reliant on differing food webs. Our study relies on values of naturally occurring carbon ( 13 C/ 12 C, δ 13 C) and nitrogen ( 15 N/ 14 N, δ 15 N) stable isotopes as integrated proxies of penguin food webs measured over three years at three different breeding colonies. At Anvers Island in the north, where reductions in sea ice and changes in breeding population trends among sympatric sea ice-obligate (Adélie) and sea ice-intolerant (chinstrap and gentoo) penguins have been most notable, our analyses show that all three species of Pygoscelis penguins became more similar isotopically over the reproductive period. By late chick-rearing at Anvers Island, crèched chicks at 5-weeks-old for all species occupied similar trophic positions. Isotopic mixing models indicated that the proportions of prey provisioned bymore »adult penguins to 5-week-old chicks at Anvers Island were generally similar across species within years, consisting primarily of Antarctic krill ( Euphausia superba ). Crèched Adélie chicks had higher δ 13 C and δ 15 N values at Avian and Charcot Islands, southern breeding colonies where sea ice is more prominent and populations of Adélie penguins have increased or remain stable. Trophic position increased with latitude, while the proportions of prey provisioned by Adélie penguin adults to chicks at southern breeding colonies included species typical of high Antarctic marine food webs, especially crystal krill ( Euphausia crystallorophias ). A Bayesian metric for dietary niche width, standard ellipse area (SEA-B), indicated that Pygoscelis penguins with greater population changes in the north had more variability in dietary niche width than stable populations further south. Our results lend insight on marine food web drivers of Pygoscelis penguin reproduction at the regional scale and question the long-standing paradigm that Antarctic krill are the only food web component critical to penguin reproductive survival in this region of the Southern Ocean.« less
3. Changes in Organic Matter Deposition Can Impact Benthic Marine Meiofauna in Karst Subterranean Estuaries

   https://doi.org/10.3389/fenvs.2021.670914  

   Brankovits, David ; Little, Shawna N. ; Winkler, Tyler S. ; Tamalavage, Anne E. ; Mejía-Ortíz, Luis M. ; Maupin, Christopher R. ; Yáñez-Mendoza, German ; van Hengstum, Peter J. ( May 2021 , Frontiers in Environmental Science)

   Subsurface mixing of seawater and terrestrial-borne meteoric waters on carbonate landscapes creates karst subterranean estuaries, an area of the coastal aquifer with poorly understood carbon cycling, ecosystem functioning, and impact on submarine groundwater discharge. Caves in karst platforms facilitate water and material exchange between the marine and terrestrial environments, and their internal sedimentation patterns document long-term environmental change. Sediment records from a flooded coastal cave in Cozumel Island (Mexico) document decreasing terrestrial organic matter (OM) deposition within the karst subterranean estuary over the last ∼1,000 years, with older sediment likely exported out of the cave by intense storm events. While stable carbon isotopic values (δ 13 C org ranging from −22.5 to −27.1‰) and C:N ratios (ranging from 9.9 to 18.9) indicate that mangrove and other terrestrial detritus surrounding an inland sinkhole are the primarily sedimentary OM supply, an upcore decrease in bulk OM and enrichment of δ 13 C org values are observed. These patterns suggest that a reduction in the local mangrove habitat decreased the terrestrial particulate OM input to the cave over time. The benthic foraminiferal community in basal core sediment have higher proportions of infaunal taxa (i.e., Bolivina ) and Ammonia , and assemblages shift to increasedmore »miliolids and less infaunal taxa at the core-top sediment. The combined results suggest that a decrease in terrestrial OM through time had a concomitant impact on benthic meiofaunal habitats, potentially by impacting dissolved oxygen availability at the microhabitat scale or resource partitioning by foraminifera. The evidence presented here indicates that landscape and watershed level changes can impact ecosystem functioning within adjacent subterranean estuaries.« less
4. Sea otter effects on trophic structure of seagrass communities in southeast Alaska

   https://doi.org/10.3354/meps13819  

   Raymond, WW ; Schram, JB ; Eckert, GL ; Galloway, AWE ( September 2021 , Marine Ecology Progress Series)

   Previous research in southeast Alaska on the effects of sea otters Enhydra lutris in seagrass Zostera marina communities identified many but not all of the trophic relationships that were predicted by a sea otter-mediated trophic cascade. To further resolve these trophic connections, we compared biomass, carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope (SI), and fatty acid (FA) data from 16 taxa at 3 sites with high and 3 sites with low sea otter density (8.2 and 0.1 sea otters km -2 , respectively). We found lower crab and clam biomass in the high sea otter region but did not detect a difference in biomass of other seagrass community taxa or the overall community isotopic niche space between sea otter regions. Only staghorn sculpin differed in δ 13 C between regions, and Fucus , sugar kelp, butter clams, dock shrimp, and shiner perch differed in δ 15 N. FA analysis indicated multivariate dissimilarity in 11 of the 15 conspecifics between sea otter regions. FA analysis found essential FAs, which consumers must obtain from their diet, including 20:5ω3 (EPA) and 22:6ω3 (DHA), were common in discriminating conspecifics between sea otter regions, suggesting differences in consumer diets. Further FAmore »analysis indicated that many consumers rely on diverse diets, regardless of sea otter region, potentially buffering these consumers from sea otter-mediated changes to diet availability. While sea otters are major consumers in this system, further studies are needed to understand the mechanisms responsible for the differences in biomarkers between regions with and without sea otters.« less
5. Lipid normalization and stable isotope discrimination in Pacific walrus tissues

   https://doi.org/10.1038/s41598-019-42095-z  

   Clark, Casey T. ; Horstmann, Lara ; Misarti, Nicole ( April 2019 , Scientific Reports)

   Analysis of stable carbon and nitrogen isotope values (δ¹³C and δ¹⁵N) of animal tissues can provide important information about diet, physiology, and movements. Interpretation of δ¹³C and δ¹⁵N values, however, is influenced by factors such as sample lipid content, tissue-specific isotope discrimination, and tissue turnover rates, which are typically species- and tissue-specific. In this study, we generated lipid normalization models for δ¹³C and investigated the effects of chemical lipid extractions on δ¹³C and δ¹⁵N in Pacific walrus (Odobenus rosmarus divergens) muscle, liver, and skin. We also evaluated tissue-specific isotope discrimination in walrus muscle, liver, skin, and bone collagen. Mean δ¹³C_lipid-freeof skin and bone collagen were similar, as were mean δ¹⁵N of muscle and liver. All other tissues differed significantly for both isotopes. Differences in δ¹³C_lipid-freeand δ¹⁵N among tissues agreed with published estimates of marine mammal tissue-specific isotope discrimination factors, with the exception of skin. The results of this work will allow researchers to gain a clearer understanding of walrus diet and the structure of Arctic food webs, while also making it possible to directly compare the results of contemporary walrus isotope research with those of historic and paleoecological studies.