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Creators/Authors contains: "McCarthy, Matthew D."

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  1. Abstract

    Photosynthesis in the surface ocean converts atmospheric CO2into organic particles, with the fraction sinking to depth representing a major part of the ocean's biological pump. Although sinking particles are known to be altered by attached‐bacteria during transit, most prior organic geochemical data indicated only minor replacement of plankton‐derived particles by bacterial material. We exploit bacteria‐specific biomarkers (d‐amino acids) in a multi‐year sediment trap in the Pacific Ocean (1,200 m) and suggest a different view. Majord‐amino acids were consistently measured at abundance demonstrating widespread accumulation of bacterial material in sinking particles. Bacterial detritus was estimated to account for up to 19% of particulate organic carbon and up to 36% of particulate nitrogen, much higher than cell count‐based values. The bacterial relative contribution increased with decreasing export production. Our results indicate that bacterial material constitutes an underappreciated component of the biological pump, a role expected to rise as the ocean warms.

     
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  2. Abstract

    Studies of Antarctic paleo‐archives have produced conflicting hypotheses on the relative impact of long‐term climate change and historic exploitation of marine mammals on Southern Ocean krill predator foraging ecology. We disentangle these hypotheses using amino acid stable isotope analysis on a 7000‐yr Holocene archive of Adélie penguin (Pygoscelis adeliae) eggshells to differentiate variation in diet and trophic dynamics from baseline biogeochemical cycling as drivers of the rapid decline in krill predator bulk tissue δ15N values in recent centuries. Contrary to previous hypotheses suggesting solely trophic dynamic mechanisms as drivers of this decline, we identified an abrupt decline in source amino acid δ15N values, indicative of major changes in biogeochemical cycling at the base of the Southern Ocean food web that mirrored the decline in penguin bulk tissue δ15N values. These abrupt shifts in penguin δ15N values and associated biogeochemical cycling aligned with climatic events during the Little Ice Age that decreased surface δ15NNO3−, likely connected to a proposed increase in Ekman upwelling via a southward migration of the Westerlies. This baseline shift was in addition to a long‐term, gradual decline in penguin trophic position over the Holocene that began prior to both recent anthropogenic climate change and a proposed “krill‐surplus” following historic marine mammal exploitation in the 19thand 20thcenturies. In resolving these outstanding hypotheses about drivers of Southern Ocean food web dynamics, this study emphasizes the fundamental importance of climate‐induced variability in biogeochemical cycling on ecological processes and improves the ability of paleo‐archives to inform the ecological consequences of future environmental change in the Southern Ocean.

     
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  3. Abstract

    The composition and cycling dynamics of marine dissolved organic carbon (DOC) have received increased interest in recent years; however, little research has focused on the refractory, low molecular weight (LMW) component that makes up the majority of this massive C pool. We measured stable isotopic (δ13C), radioisotopic (Δ14C), and compositional (C/N,13C solid‐state NMR) properties of separately isolated high molecular weight (HMW) and LMW DOC fractions collected using a coupled ultrafiltration and solid phase extraction approach from throughout the water column in the North Central Pacific and Central North Atlantic. The selective isolation of LMW DOC material allowed the first investigation of the composition and cycling of a previously elusive fraction of the DOC pool. The structural composition of the LMW DOC material was homogeneous throughout the water column and closely matched carboxylic‐rich alicyclic material that has been proposed as a major component of the marine refractory DOC pool. Examination of offsets in the measured parameters between the deep waters of the two basins provides the first direct assessment of changes in the properties of this material with aging and utilization during ocean circulation. While our direct measurements largely confirm hypotheses regarding the relative recalcitrance of HMW and LMW DOC, we also demonstrate a number of novel observations regarding the removal and addition of DOC during global ocean circulation, including additions of fresh carbohydrate‐like HMW DOC to the deep ocean and large‐scale removal of both semilabile HMW and recalcitrant LMW DOC.

     
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  4. Abstract

    Compound‐specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns inAAisotope fractionation in birds. We conducted a controlledCSIAfeeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individualAAcarbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ13CC‐Dand Δ15NC‐D, respectively). We found that essentialAAδ13C values and sourceAAδ15N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessentialAAΔ13CC‐Dvalues reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDFGlu‐Phe= 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ15NC‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDFGlu‐Pheequation with the avian‐specificTDFGlu‐Phevalue from our experiment provided estimates that were more ecologically realistic than estimates using a singleTDFGlu‐Pheof 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use ofCSIAin nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.

     
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