[Eoas-seminar] Oceanography Dissertation Defense - Sarah Ellen Johnston - April 15, 9am - OSB 327

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Tue Apr 2 11:47:47 EDT 2019

Oceanography Seminar
Sarah Ellen Johnston
Ph.D. Chemical Oceanography Candidate

Title: Spatial and Temporal Drivers of Arctic and Boreal Dissolved Organic Matter Composition Across Latitudinal Gradients

Major Professor: Dr. Rob Spencer
Defense Date: April 15, 2019
Time: 9:00 AM
Location: OSB 327

Northern high-latitude regions are undergoing rapid changes as the Arctic warms at about twice the rate of mid-latitudes. Climate change is causing permafrost thaw, vegetation and hydrologic shifts, and the increased incidence of wildfire, all of which have major implications for regional and global carbon (C) cycling. In this study, I evaluate dissolved organic matter (DOM) composition across temporal and spatial gradients using chromophoric DOM (CDOM), the biomarker lignin phenol, and Fourier transform ion cyclotron resonance mass spectrometry (FT- ICR MS). The goal of this dissertation was to improve spatial and temporal understanding of DOM composition and cycling across aquatic gradients by improving spatial (Chapter 1) and temporal (Chapter 2) coverage of DOM composition, and using space for time gradients to understand the seasonal and landscape scale controls on DOM composition in lakes and rivers (Chapters 3 and 4, respectively) and how they may change into the future. Finally, an overarching theme of these studies were the utilization of optical measurements to estimate dissolved organic carbon (DOC) concentration and DOM composition for future applications for in situ and remote sensing technology.
By including an understudied, mid-sized watershed in pan-Arctic flux estimates as a model for the unsampled portion of the pan-Arctic watershed (i.e. not encompassed in the major six Arctic rivers from which historic estimates are extrapolated) DOC flux estimates were increased from 27 Tg C to 34 Tg C annually to the Arctic Ocean. Additionally, the residence time of lignin and thus terrestrial DOM was further constrained from previous studies to 0.5 to 1.8 years. This refinement of the pan-Arctic flux estimate and terrestrial DOM residence time is important for the accurate assessment of land-ocean C fluxes and their implications for future change. Temporal DOM dynamics were also evaluated in both rivers and lakes. Diel lake sampling revealed that seasonal
variability accounted for the greatest changes in DOM composition and underscored the need to sample lakes seasonally while regular diel trends were not observed. Finally, using space for time transitions in both lakes and rivers the seasonal and landscape drivers of DOM composition were evaluated to allow future projections in a changing Arctic. In lakes that were relatively hydrologically disconnected there was lower CDOM compared to hydrologically connected lakes and a strong decoupling of DOC from CDOM. Further evidence showed that DOM in these lakes was driven by autochthony and that a future drier climate is unlikely to cause browning in these lakes as suggested by recent research. In rivers, where hydrologic connectivity is high, the watershed relief, soils and vegetation played an important role in determining DOM composition. Lower DOC yields and more aliphatic DOM were mobilized in watersheds underlain by continuous permafrost compared to discontinuous permafrost. Together these studies show the utility of space for time gradients to project future change and the use of CDOM parameters as a proxy for DOC concentration and DOM composition. Taken together these results allow us to make projections for boreal and arctic change, as well as allow future studies to improve spatial and temporal resolution via CDOM parameters.

Sarah Ellen Johnston
PhD Candidate
Earth, Ocean & Atmospheric Science
Florida State University

Phone: 865-386-0141
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