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Abstract
Organic matter (OM) is a key component of local and global carbon cycles, serving as sources of carbon, nutrients and energy for aquatic ecosystems. Despite extensive efforts, a full understanding of OM dynamics and its role in carbon cycling remains challenging due to its complexity and multiple sources. This dissertation employed multiple approaches, including particulate and dissolved organic carbon concentrations (POC and DOC), stable carbon isotopic ratios (δ13C) and molecular analyses (GC-MS and FT-ICR MS) to investigate changes in bulk organic carbon and organic matter composition across dissimilar aquatic ecosystems. In CHAPTER 2, spatiotemporal changes in bulk measurements and molecular biomarkers of suspended and sinking POC were analyzed under high and low discharge conditions of the Amazon River. Results showed that suspended POC composition at the surface was primarily driven by phytoplankton, while trap POC was influenced by zooplankton and bacteria, with contributions varying seasonally. Disparities between coexisting POC and DOM compositions indicated that OM pools are shaped by distinct processes. In CHAPTER 3, seasonal changes in DOM composition were examined during a transition from monomictic to meromictic conditions in Mono Lake (California). Under monomictic conditions, DOM composition was primarily influenced by DOM sources and redox conditions. Following prolonged stratification, the main changes in DOM were associated with shifts in phytoplankton community structure and strong redox gradient in the lake. Lastly, in CHAPTER 4, changes in bulk OC and DOM composition were characterized along a transect in a coastal polynya off West Antarctica during the austral summer. Surface bulk POC measurements suggested that the POC pool was associated with phytoplankton blooms. Meanwhile, DOM pool was influenced by algal blooms early in the season. As the end of summer approached, DOM became less affected by phytoplankton and strongly shaped by microbial degradation. Compositional differences in deep DOM suggested that mixing of water masses, biodegradation, and possibly onshore transport of Circumpolar Deep Water influenced DOM composition in bottom waters. Ultimately, this dissertation highlights the use of targeted and untargeted chemical analyses to unravel the complexity of OM, providing valuable insights into the biogeochemical processes that regulate OM dynamics across diverse aquatic ecosystems.