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Abstract
Dissolved organic matter (DOM) is a critical component of the global carbon cycle; it has multiple sources including terrestrial runoff, riverine input, phytoplankton excretion, viral lysis, among others. These inputs have varying levels of contribution depending on temporal and spatial scales as well as environmental variables, making the characterization of the DOM increasingly complex. This dissertation used primarily molecular (ultrahigh resolution mass spectrometry - FT-ICR MS) and satellite-derived ocean-color techniques to investigate the transformation and transport of DOM in two important aquatic systems. In CHAPTER 2, FT-ICR MS in hand with metatranscriptomics were used to characterize the dominant patterns of variability modifying the DOM composition in a typical estuary off the Southeastern U.S. Results showed that after seasonal variations, long-term biodegradation was comparatively more important in the fall, while tidal variability was the second most important factor correlated to DOM compositional changes in the spring, when the freshwater content in the estuary was high. Microbial data revealed a similar pattern, with variability in gene expression occurring first at the seasonal scale, then by tidal influence. Over shorter time scales, however, the influence of microbial processing on DOM compositional changes was small. In CHAPTER 3, two decades of ocean color observations from satellite were used to describe seasonal and interannual variability in terrigenous DOC (tDOC) content in the Amazon River plume. Seasonality accounted for 40% of the total variance in the plume core, and interannual variability in tDOC accounted for 15% of the total variance, likely associated with hydrological changes. Overall, we found MODIS-tDOC to be an effective tracer for the Amazon River plume, with great potential to be used for identifying events of increased off-shelf tDOC transport. Finally, in CHAPTER 4, MODIS-tDOC was used to estimate tDOC degradation over the shelf. A clear seasonal pattern was observed, with increased degradation occurring during the high discharge season. Also, a correlation was found between onshelf degradation and tDOC anomalies offshore with a lag-time of ~40 days. Overall, this dissertation uncovered the relative importance of various factors on DOM composition and degradation, especially in the context of combining various research techniques.