Kelton McMahon


Kelton grew up in coastal Connecticut and has been trying to figure out “how the ocean works” for as long as he can remember. His parents instilled in him a deep sense of stewardship for the ocean and a tremendous appreciation for the power of science. That union drives his passion for ocean research and conservation, both professionally and personally. He received a B.Sc. in Biology from Bates College, studying the impacts of climate change-mediated shifts in ice algae and phytoplankton on the Arctic benthos. He then received his Ph.D. in Biological Oceanography from the MIT-WHOI Joint Program in Oceanography. There he developed cutting edge molecular isotope geochemistry tools to identify essential nursery habitats and migration pathways for coral reef fishes. From there Kelton completed a postdoctoral scholar position at the King Abdullah University of Science and Technology in Saudi Arabia. He used compound-specific stable isotope analysis (CSIA) to examine the sources and cycling of organic matter fueling the tremendous biomass and biodiversity on tropical coral reefs. Kelton then went on to complete a second postdoctoral position at the University of California, Santa Cruz. Here he applied the CSIA tools he had been developing in modern ecosystems to deep sea corals, which provide high resolution, long term records of past regime shifts in phytoplankton community composition biogeochemical cycling on centennial to millennial time scales. Kelton spent two more years at UC Santa Cruz as a research and lecturing faculty before moving to the URI Graduate School of Oceanography in the fall of 2017. At GSO, Kelton is developing a molecular geochemistry lab to explore how climate change and human-environment interactions alter the sources and cycling of organic matter in past and present marine ecosystems around the World. When Kelton is not ensconced in research, education, and outreach, chances are he’s rock climbing, playing beach volleyball, or helping GSO beat WHOI in the annual Niskin Cup hockey game.


Biological Oceanography, Biogeochemistry, Biology, Climate Change, Ecology, Ecosystem Dynamics, Food Web Dynamics, Stable Isotope Geochemistry

Modern oceans are in an era of extraordinary change, reflecting increasing anthropogenic disturbance on top of longer-term natural climate variability. The McMahon lab is actively engaged in basic and applied research to examine the roles that food web architecture play in the structure, function, and resilience of marine ecosystems, and how climate change and human-environment interactions alter those relationships. Through controlled feeding experiments, our lab develops a quantitative understanding of the biochemical and physiological mechanisms that control resource acquisition and allocation. We then apply this knowledge in large-scale field studies to examine a wide range of questions from individual nutritional ecology and population connectivity to ecosystem-scale food web dynamics and the functioning of the biological pump. While the McMahon Lab’s research program is firmly rooted in fundamental oceanographic theory, it extends beyond the basic research questions to address the applied management and practical decision-making needed to resolve current and emerging coastal ocean challenges. This research is directly relevant to the recent decadal review of ocean science by The National Academy of Sciences, which highlighted 1) the relationship between ocean biogeochemical cycling and earth’s climate and 2) changes in food web structure over the next 50 to 100 years as high priority questions in ocean science over the next decade.

Our group is currently involved in a number of concurrent research foci that span a wide range of species, ecosystems, and questions. The unifying theme for all of these projects is the development and application of cutting edge molecular isotope geochemistry tools to explore the sources and cycling of organic matter as it flows through marine ecosystems (termed “ecogeochemistry”).
• Ancient and modern Antarctic Krill predator dynamics: The Southern Ocean is a classic “canary in the coalmine” for the impacts of rapid climate change and anthropogenic disturbance on ecosystem function and resilience. Penguins, which are central nodes in Antarctic food webs, act as sensitive bioindicators of ecosystem health. We use modern field collections, historical museum archives, and paleontological excavations to obtain multi-millennial time series of penguin feather and eggshells. We then apply compound-specific stable isotope analysis (CSIA) to explore how recent climate change and historic overharvesting of marine mammals have influenced foraging and population dynamics of Antarctic penguins. We are also working with colleagues to map the redistribution of predator populations with ArcGIS to provide a history of population movements, which will be overlaid on large-scale isoscapes (geospatial contour maps of stable isotope data) for the Southern Ocean to aid in our understanding of biogeochemical cycling in the region. This research provides an unprecedented look into how past climate change and anthropogenic exploitation altered polar food webs through time, and provides a window into predicted future food web responses in a rapidly warming World. Key collaborators in this project include Dr. Michael Polito (LSU), Ms. Chantel Michelson (Ph.D. student, LSU), and Dr. Steve Emslie (UNC Wilmington)

• Regime shifts and the Biological Pump: Marine food web architecture influences everything from ocean productivity to biogeochemical cycling and the efficacy of the biological pump. Numerous lines of evidence suggest that shifts in climate and ocean condition play a critical role in structuring the plankton community composition that fuels marine food web architecture. However, our understanding of how plankton communities have shifted on centennial and longer time scales has been limited by paleoarchives of sufficient length and resolution. Deep-sea proteinaceous corals act as “living sediment traps,” preserving high resolution, long-term geochemical records of export production and the food web structure of the overlying euphotic zone. In these projects, we are applying CSIA to deep-sea proteinaceous corals to understand past changes in open ocean ecosystem food web structure and biogeochemical cycling. By understanding how and when plankton assemblages at the base of open ocean food webs have changed in the past, we can better understand future changes in food web architecture, biogeochemical cycling, and carbon sequestration in the modern era of rapid climate change. Key collaborators in these projects include Dr. Matthew McCarthy (UCSC), Dr. Thomas Guilderson (LLNL), Dr. Owen Sherwood (Dalhousie U), and Dr. Thomas Larsen (U of Kiel).

• Coastal Ecosystem Ecological Integrity: Coastal marine ecosystems are among the most productive and biodiverse ecosystems on Earth and provide critical goods and services to humans. These systems are also under tremendous pressure from direct and indirect human-environment interactions. Our research looks to address how food web architecture influences the structure, function, and resilience of coastal ecosystems by quantifying the sources and cycling of organic matter supporting coastal food webs. We are particularly interested in how human impacts on coastal ecosystems, either directly (e.g., removal of key species and changes in nutrient inputs) or indirectly (e.g., climate change-mediated shifts in temperature) affect ecosystem productivity, biogeochemical cycling, and food web architecture. A complementary set of projects explores how ontogenetic migration links distant populations in distinct habitats within complex seascape mosaics. Much of this work takes place in tropical seascapes (coral reefs, mangroves, and seagrass beds) and polar seascapes, however, new projects are in the works to explore regional North Atlantic Shelf systems as well. This work provides scientific support to evaluate the effectiveness of management and remediation efforts on restoring ecosystem function. Key collaborators in these projects include Dr. Simon Thorrold (WHOI), Dr. Michael Berumen (KAUST), and Dr. Randi Rotjan (BU).


Ph.D. Biological Oceanography, MIT-WHOI Joint Program in Oceanography, Woods Hole, MA, 2011

B.Sc. Biology (Honors), Bates College, Lewiston, ME, 2005

Selected Publications

(# indicates student author)

Ohkouchi N, Chikaraishi Y, Close HG, Fry B, Larsen T, Madigan DJ, McCarthy MD, McMahon KW, Nagata T, Naito YI, Ogawa NO, Popp BN, Steffan S, Takano Y, Tayasu I, Wyatt ASJ, Yamaguchi YT, Yokoyama Y (2017) Advances in the application of amino acid nitrogen isotopic analysis in ecological and biogeochemical studies. Organic Geochemistry (authors in alphabetical order after first) DOI: 10.1016/j.orggeochem.2017.07.009

McMahon KW, McCarthy MD (2016) Embracing variability in amino acid 15N fractionation: Mechanisms, implications, and applications for trophic ecology. Ecosphere 7:e01511 (Special issue on Biomarkers in Trophic Ecology)

McMahon KW, Thorrold SR, Houghton LA, Berumen ML (2016) Tracing carbon flow through coral reef food webs using a compound-specific carbon isotope approach. Oecologia 180:809-821 (Cover of Issue)

McMahon KW, Guilderson TP, Sherwood OA, Larsen T, McCarthy MD (2015) Millennial-scale plankton regime shifts in the subtropical North Pacific Ocean. Science 350:1530-1533

McMahon KW, Elsdon T, Thorrold SR, McCarthy M (2015). Trophic discrimination of nitrogen stable isotopes in amino acids varies with diet quality in a marine fish. Limnology and Oceanography 60:1076-1087

McMahon KW, Polito M, Abel S, McCarthy MD, Thorrold SR (2015) Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator, the gentoo penguin (Pygoscelis papua). Ecology and Evolution 5:1278-1290

#Schiff JT, Batista F, Sherwood OA, Guilderson TP, Hill TM, Ravelo AC, McMahon KW, McCarthy MD (2014) Compound specific amino acid 13C patterns in a deep-sea proteinaceous coral: implications for reconstructing detailed 13C records of exported primary production. Marine Chemistry 166:82-91

McMahon KW, #Hamady L, Thorrold SR (2013) Ocean ecogeochemistry – A review. Oceanography and Marine Biology: An Annual Review 51:327-374

McMahon KW, #Hamady L, Thorrold SR (2013) A review of ecogeochemistry approaches to estimating movements of marine animals. Limnology and Oceanography 58:697-714

McMahon KW, Berumen ML, Thorrold SR (2012) Linking habitat mosaics and connectivity in a coral reef seascape. Proceedings of the National Academy of Science 109:15372-15376

McMahon KW, Berumen ML, Mateo I, Elsdon TS, Thorrold SR (2011) Carbon isotopes in otolith amino acids identify residency of juvenile snapper (Family: Lutjanidae) in coastal nurseries. Coral Reefs 30:1135-1145 (2011 Best Paper Award finalist)

McMahon KW, Fogel ML, Johnson BJ, Houghton LA, Thorrold SR (2011) A new method to reconstruct fish diet and movement patterns from 13C values in otolith amino acids. Canadian Journal of Fisheries and Aquatic Sciences 68:1330-1340

McMahon KW, Fogel ML, Elsdon T, Thorrold SR (2010) Carbon isotope fractionation of amino acids in fish muscle reflects biosynthesis and isotopic routing from dietary protein. Journal of Animal Ecology 79:1132-1141

Elsdon T, Ayvazian S, McMahon KW, Thorrold SR (2010) Experimental evaluation of stable isotope fractionation in fish muscle and otoliths. Marine Ecology Progress Series 408:195-205

Graham BS, Koch PL, Newsome SD, McMahon KW, and Aurioles D (2010) Using isoscapes to trace the movements and foraging behavior of top predators in oceanic ecosystems. In Isoscapes: Understanding Movement, Pattern and Process on Earth Through Isotope Mapping (eds. J. West, G. J. Bowen, T. E. Dawson, K. P. Tu). pp. 299-318, Springer, New York

Skomal GB, Zeeman SI, Chisholm JH, Summers EL, Walsh HJ, McMahon KW, Thorrold SR. (2009) Mesopelagic trans-equatorial migrations by basking sharks in the western Atlantic Ocean. Current Biology 19:1019-1022 (Cover of Issue)

Carroll M, Johnson B, Henkes G, McMahon KW, Voronkov A, Ambrose W, Denisenko S. (2009). Bivalves as indicators of environmental variation and potential anthropogenic impacts in the southern Barents Sea. Marine Pollution Bulletin 59:193-206

Ambrose WG, Carroll ML, Greenacre M, Thorrold SR, McMahon KW. (2006) Variation in bivalve growth in a Norwegian high-Arctic fjord: Evidence for local- and large-scale climatic forcing. Global Change Biology 12:1595-1607

McMahon KW, Ambrose WG, Johnson BJ, Sun M-Y, Lopez GR, Clough LM, Carroll ML (2006) Benthic community response to ice algae and phytoplankton in Ny Ålesund, Svalbard. Marine Ecology Progress Series 310:1-14 (Issue’s feature article)

McMahon KW, Johnson BJ, Ambrose WG. (2005). Diet and movement of the killifish, Fundulus heteroclitus, in a Maine salt marsh assessed using gut contents and stable isotope analyses. Estuaries 28:966-973


OCG 106G: "You, Me, and Life in the Sea"
This undergraduate grand challenge course explores human-marine organism interactions across diverse disciplines and perspectives.

OCG 550: "Environmental Isotope Geochemistry"
This graduate course covers fundamental concepts in the theory and application of environmental isotope geochemistry.