McMahon Kelton W.

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Kelton W.

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Now showing 1 - 9 of 9
  • Article
    A review of ecogeochemistry approaches to estimating movements of marine animals
    (Association for the Sciences of Limnology and Oceanography, 2013-03) McMahon, Kelton W. ; Hamady, Li Ling ; Thorrold, Simon R.
    Ecogeochemistry—the application of geochemical techniques to fundamental questions in population and community ecology—has been used in animal migration studies in terrestrial environments for several decades; however, the approach has received far less attention in marine systems. This review includes comprehensive meta-analyses of organic zooplankton δ13C and δ15N values at the base of the food web, dissolved inorganic carbon δ13C values, and seawater δ18O values to create, for the first time, robust isoscapes for the Atlantic Ocean. These isoscapes present far greater geographic variability in multiple geochemical tracers than was previously thought, thus forming the foundation for reconstructions of habitat use and migration patterns of marine organisms. We review several additional tracers, including trace-element-to-calcium ratios and heavy element stable isotopes, to examine anadromous migrations. We highlight the value of the ecogeochemistry approach by examining case studies on three components of connectivity: dispersal and natal homing, functional connectivity, and migratory connectivity. We also discuss recent advances in compound-specific stable carbon and nitrogen isotope analyses for tracking animal movement. A better understanding of isotopic routing and fractionation factors, particularly of individual compound classes, is necessary to realize the full potential of ecogeochemistry.
  • Preprint
    Tracing carbon flow through coral reef food webs using a compound-specific stable isotope approach
    ( 2015-11) McMahon, Kelton W. ; Thorrold, Simon R. ; Houghton, Leah A. ; Berumen, Michael L.
    Coral reefs support spectacularly productive and diverse communities in tropical and sub26 tropical waters throughout the world’s oceans. Debate continues, however, on the degree to which reef biomass is supported by new water column production, benthic primary production, and recycled detrital carbon. We coupled compound-specific δ13C analyses with Bayesian mixing models to quantify carbon flow from primary producers to coral reef fishes across multiple feeding guilds and trophic positions in the Red Sea. Analyses of reef fishes with putative diets composed primarily of zooplankton (Amblyglyphidodon indicus), benthic macroalgae (Stegastes nigricans), reef-associated detritus (Ctenochaetus striatus), and coral tissue (Chaetodon trifascialis) confirmed that δ13C values of essential amino acids from all baseline carbon sources were both isotopically diagnostic and accurately recorded in consumer tissues. While all four source end-members contributed to the production of coral reef fishes in our study, a single source end-member often dominated dietary carbon assimilation of a given species, even for highly mobile, generalist top predators. Microbially-reworked detritus was an important secondary carbon source for most species. Seascape configuration played an important role in structuring resource utilization patterns. For instance, L. ehrenbergii, showed a significant shift from a benthic macroalgal food web on shelf reefs (71 ± 13% of dietary carbon) to a phytoplankton-based food web (72 ± 11%) on oceanic reefs. Our work provides insights into the roles that diverse carbon sources play in the structure and function of coral reef ecosystems and illustrates a powerful fingerprinting method to develop and test nutritional frameworks for understanding resource utilization.
  • Preprint
    Variation in Serripes groenlandicus (Bivalvia) growth in a Norwegian high-Arctic fjord : evidence for local- and large-scale climatic forcing
    ( 2006-03-03) Ambrose, William G. ; Carroll, Michael L. ; Greenacre, Michael ; Thorrold, Simon R. ; McMahon, Kelton W.
    We examined the growth rate of the circumpolar Greenland Cockle (Serripes groenlandicus) over a period of 20 years (1983-2002) from Rijpfjord, a high-Arctic fjord in northeast Svalbard (80º10´N, 22°15´E). This period encompassed different phases of large-scale climatic oscillations with accompanying variations in local physical variables (temperature, atmospheric pressure, precipitation, sea ice cover), allowing us to analyze the linkage between growth rate, climatic oscillations, and their local physical and biological manifestations. Standard Growth Index (SGI), an ontogenetically-adjusted measure of annual growth, ranged from a low of 0.27 in 2002 up to 2.46 in 1996. Interannual variation in growth corresponded to the Arctic Climate Regime Index (ACRI), with high growth rates during the positive ACRI phase characterized by cyclonic ocean circulation and a warmer and wetter climate. Growth rates were influenced by local manifestations of the ACRI: positively correlated with precipitation and to a lesser extent negatively correlated with atmospheric pressure. A multiple regression model explains 65% of the variability in growth rate by the ACRI and precipitation at the nearest meteorological station. There were, however, complexities in the relationship between growth and physical variables, including an apparent 1-year lag between physical forcing changes and biological response. Also, when the last 4 years of poor growth are excluded, there is a very strong negative correlation with ice cover on a pan-arctic scale. Our results suggest that bivalves, as sentinels of climate change on multi-decadal scales, are sensitive to environmental variations associated with large-scale changes in climate, but that the effects will be determined by changes in environmental parameters regulating marine production and food availability on a local scale.
  • Dataset
    Primary producer amino acid nitrogen isotope values from published literature to examine beta variability in trophic position estimates
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact:, 2022-03-24) McMahon, Kelton W. ; Ramirez, Matthew D. ; Besser, Alexi ; Newsome, Seth D.
    This dataset is a meta-analysis of primary producer amino acid δ15N data presented in Ramirez et al. (2021). A literature review provided primary producer amino acid isotope data with ecologically relevant information to examine beta variability in trophic position estimates. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at:
  • Thesis
    Functional connectivity of coral reef fishes in a tropical seascape assessed by compound-specific stable isotope analyses
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2011-02) McMahon, Kelton W.
    The ecological integrity of tropical habitats, including mangroves, seagrass beds and coral reefs, is coming under increasing pressure from human activities. Many coral reef fish species are thought to use mangroves and seagrass beds as juvenile nurseries before migrating to coral reefs as adults. Identifying essential habitats and preserving functional linkages among these habitats is likely necessary to promote ecosystem health and sustainable fisheries on coral reefs. This necessitates quantitative assessment of functional connectivity among essential habitats at the seascape level. This thesis presents the development and first application of a method for tracking fish migration using amino acid (AA) δ13C analysis in otoliths. In a controlled feeding experiment with fish reared on isotopically distinct diets, we showed that essential AAs exhibited minimal trophic fractionation between consumer and diet, providing a δ13C record of the baseline isoscape. We explored the potential for geochemical signatures in otoliths of snapper to act as natural tags of residency in seagrass beds, mangroves and coral reefs in the Red Sea, Caribbean Sea and Eastern Pacific Ocean. The δ13C values of otolith essential AAs varied as a function of habitat type and provided a better tracer of residence in juvenile nursery habitats than conventional bulk stable isotope analyses (SIA). Using our otolith AA SIA approach, we quantified the relative contribution of coastal wetlands and reef habitats to Lutjanus ehrenbergii populations on coastal, shelf and oceanic coral reefs in the Red Sea. L. ehrenbergii made significant ontogenetic migrations, traveling more than 30 km from juvenile nurseries to coral reefs and across deep open water. Coastal wetlands were important nurseries for L. ehrenbergii; however, there was significant plasticity in L. ehrenbergii juvenile habitat requirements. Seascape configuration played an important role in determining the functional connectivity of L. ehrenbergii populations in the Red Sea. The compound-specific SIA approach presented in this thesis will be particularly valuable for tracking the movement of species and life-stages not amenable to conventional tagging techniques. This thesis provides quantitative scientific support for establishing realistic population connectivity models that can be used to design effective marine reserve networks.
  • Article
    Benthic community response to ice algae and phytoplankton in Ny Ålesund, Svalbard
    (Inter-Research, 2006-04-03) McMahon, Kelton W. ; Ambrose, William G. ; Johnson, Beverly J. ; Sun, Ming-Yi ; Lopez, Glenn R. ; Clough, Lisa M. ; Carroll, Michael L.
    We assessed the digestibility and utilization of ice algae and phytoplankton by the shallow, subtidal benthos in Ny Ålesund (Kongsfjord) on Svalbard (79°N, 12°E) using chlorophyll a (chl a), essential fatty acids (EFAs) and stable isotopes as tracers of food consumption and assimilation. Intact benthic communities in sediment cores and individuals of dominant benthic taxa were given ice algae, phytoplankton, 13C-enriched ice algae or a no food addition control for 19 to 32 d. Ice algae and phytoplankton had significantly different isotopic signatures and relative concentrations of fatty acids. In the food addition cores, sediment concentrations of chl a and the EFA C20:5(n-3) were elevated by 80 and 93%, respectively, compared to the control after 12 h, but decreased to background levels by 19 d, suggesting that both ice algae and phytoplankton were rapidly consumed. Whole core respiration rates in the ice algae treatments were 1.4 times greater than in the other treatments within 12 h of food addition. In the ice algae treatment, both suspension and deposit feeding taxa from 3 different phyla (Mollusca, Annelida and Sipuncula) exhibited significant enrichment in δ13C values compared to the control. Deposit feeders (15% uptake), however, exhibited significantly greater uptake of the 13C-enriched ice algae tracer than suspension feeders (3% uptake). Our study demonstrates that ice algae are readily consumed and assimilated by the Arctic benthos, and may be preferentially selected by some benthic species (i.e. deposit feeders) due to their elevated EFA content, thus serving as an important component of the Arctic benthic food web.
  • Preprint
    Bivalves as indicators of environmental variation and potential anthropogenic impacts in the southern Barents Sea
    ( 2009-04) Carroll, Michael L. ; Johnson, Beverly J. ; Henkes, Gregory A. ; McMahon, Kelton W. ; Voronkov, Andrey ; Ambrose, William G. ; Denisenko, Stanislav G.
    Identifying patterns and drivers of natural variability in populations is necessary to gauge potential effects of climatic change and the expected increases in commercial activities in the Arctic on communities and ecosystems. We analyzed growth rates and shell geochemistry of the circumpolar Greenland smooth cockle, Serripes groenlandicus, from the southern Barents Sea over almost 70 years between 1882 and 1968. The datasets were calibrated via annually-deposited growth lines, and growth, stable isotope (δ18O, δ13C), and trace elemental (Mg, Sr, Ba, Mn) patterns were linked to environmental variations on weekly to decadal scales. Standardized growth indices revealed an oscillatory growth pattern with a multi-year periodicity, which was inversely related to the North Atlantic Oscillation Index (NAO), and positively related to local river discharge. Up to 60% of the annual variability in the Ba/Ca could be explained by variations in river discharge at the site closest to the rivers, but the relationship disappeared at a more distant location. Patterns of δ18O, δ13C, and Sr/Ca together provide evidence that bivalve growth ceases at elevated temperatures during the fall and recommences at the coldest temperatures in the early spring, with the implication that food, rather than temperature, is the primary driver of bivalve growth. The multi-proxy approach of combining the annually integrated information from the growth results and higher resolution geochemical results yielded a robust interpretation of biophysical coupling in the region over temporal and spatial scales. We thus demonstrate that sclerochronological proxies can be useful retrospective analytical tools for establishing a baseline of ecosystem variability in assessing potential combined impacts of climatic change and increasing commercial activities on Arctic communities.
  • Preprint
    Linking habitat mosaics and connectivity in a coral reef seascape
    ( 2012-06) McMahon, Kelton W. ; Berumen, Michael L. ; Thorrold, Simon R.
    Tropical marine ecosystems are under mounting anthropogenic pressure from overfishing and habitat destruction, leading to declines in their structure and function on a global scale. While maintaining connectivity among habitats within a seascape is necessary for preserving population resistance and resilience, quantifying movements of individuals within seascapes remains challenging. Traditional methods of identifying and valuing potential coral reef fish nursery habitats are indirect, often relying on visual surveys of abundance and correlations of size and biomass among habitats. We used compound-specific stable isotope analyses to determine movement patterns of commercially important fish populations within a coral reef seascape. This approach allowed us to quantify the relative contributions of individuals from inshore nurseries to reef populations and identify migration corridors among important habitats. Our results provided direct measurements of remarkable migrations by juvenile snapper of over 30 km between nurseries and reefs. We also found significant plasticity in juvenile nursery residency. While a majority of individuals on coastal reefs had used seagrass nurseries as juveniles, many adults on oceanic reefs had settled directly into reef habitats. Moreover, seascape configuration played a critical but heretofore unrecognized role in determining connectivity among habitats. Finally, our approach provides key quantitative data necessary to estimate the value of distinctive habitats to ecosystem services provided by seascapes.
  • Article
    Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator, the gentoo penguin (Pygoscelis papua)
    (John Wiley & Sons, 2015-02-25) McMahon, Kelton W. ; Polito, Michael J. ; Abel, Stephanie ; McCarthy, Matthew D. ; Thorrold, Simon R.
    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 in AA isotope fractionation in birds. We conducted a controlled CSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individual AA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ13CC-D and Δ15NC-D, respectively). We found that essential AA δ13C values and source AA δ15N 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 nonessential AA Δ13CC-D values 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-Phe equation with the avian-specific TDFGlu-Phe value from our experiment provided estimates that were more ecologically realistic than estimates using a single TDFGlu-Phe of 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.