Research

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).