Dissolved organic matter plays a critical role in aquatic biogeochemical cycles, fuels microbial metabolism, and stores as much carbon as the atmospheric carbon reservoir. Because the reactivity and fate of dissolved organic matter is largely controlled by its chemical composition, a wide range of analytical approaches including ultra-high resolution mass spectrometry, nuclear magnetic resonance spectroscopy, natural abundance isotopic analyses, and fluorescence spectroscopy, have been employed to trace its sources and transformations along the land–ocean continuum. Applying these approaches to environmental samples requires overcoming significant logistical challenges, including high salinity, sample volatility and degradation, and low organic carbon concentrations. In addition, each method captures only a subset of the total dissolved organic matter pool. As a result of these challenges and others, our understanding of the environmental role of dissolved organic matter within aquatic biogeochemistry remains limited.

In this session, we welcome studies that integrate various analytical approaches to characterize the composition and/or fate of dissolved organic matter along the land–ocean continuum. We also welcome contributions focused on novel techniques for isolating, concentrating, and purifying organic matter prior to analysis. We hope that by highlighting analytical advances made throughout the aquatic continuum across salinity and organic carbon concentration gradients, the insights gained may inform future efforts as a community to characterize dissolved organic matter in a changing environment. Submissions by students and early career researchers, and researchers from BIPOC, LGBTQIA+, and other underrepresented identities are highly encouraged.

Lead Organizer: Jennifer Bowen, Stanford University (bowen.jenniferc@gmail.com)

Co-organizers:

Benjamin Granzow, Scripps Institution of Oceanography (bgranzow@ucsd.edu)

Margot White, ETH Zurich (margot.white@eaps.ethz.ch)

Presentations

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06:00 PM

LONGITUDINAL DYNAMICS OF DOM WITHIN HEADWATER STREAMS OF A TROPICAL EVERGREEN FOREST (9268)

Primary Presenter: Samantha Sullivan, Old Dominion University (ssulliva@odu.edu)

Dissolved organic matter (DOM) in tropical stream ecosystems contributes to the tropical rivers that dominate the global budget of riverine runoff to the ocean. Yet, the processes that control downstream DOM quality and quantity in the tropics are much less understood compared to higher latitudes. Here, we investigate seasonal and downstream changes in DOM in the dry evergreen forest of the Rio Tempisquito watershed in Costa Rica using UV-Vis, fluorescence, and FT-ICR-MS. We found that seasonal differences in litterfall and daily changes in rainfall controlled the input of terrestrial, humic-like DOM into the stream and led to higher DOC concentrations. By analyzing downstream changes in DOM in three streams during the wet and dry seasons, we show that the average DOM molecular composition consistently shifted from reduced to more oxidized downstream. These longitudinal increases in the oxidation state of DOM coincided with an increase in a pool of humic-like DOM that is thought to come from microbial oxidation. Given that the quality of DOM input laterally should be similar across the longitudinal gradient and shading and high flow velocities should limit in-stream primary production and photodegradation, we propose that downstream changes in DOM quality in dry tropical headwater streams are driven by the preferential microbial metabolism of a pool of labile, reduced DOM compounds. These findings suggest that downstream increases in DOM oxidation state may scale with water depth, velocity, and discharge – all factors that increase the uptake length for DOM in streams.  

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06:00 PM

CHEMICAL CHARACTERIZATION OF DISSOLVED ORGANIC MATTER AT THE LAND-OCEAN CONTINUUM IN A CHILEAN PATAGONIA FJORD (9516)

Primary Presenter: Lucia Cancelada, University of California San Diego (lcancela@ucsd.edu)

In the central Chilean Patagonia, the Martínez-Baker Channel system links terrestrial water inputs with the southeast Pacific Ocean. This system is surrounded by the Pascua, Huemules, and Baker rivers—the latter being Chile’s largest by volume—and the Jorge Montt and Steffen glaciers and fjords. Ocean forcing is driving the rapid retreat of both tidewater glaciers. For the first time, dissolved organic matter (DOM) fractions from this unique transitional aquatic environment have been characterized to the molecular level. During Spring 2024, samples were collected at eleven locations across the Martínez-Baker Channel system at multiple depths aboard the scientific vessel Sur-Austral. In addition to total dissolved organic carbon measurements, the fluorescent fraction of DOM was analyzed via excitation-emission matrix spectroscopy. DOM was further isolated using solid-phase extraction and analyzed by high-resolution liquid chromatography tandem mass spectrometry in positive and negative mode electrospray ionization. A cheminformatics workflow was used to process chromatographic data and assess the chemical composition at a molecular and structural level. This study aims to provide insights into the transformation and fate of terrestrial carbon transported to the ocean. Understanding how these inputs shape the marine ecosystem at the land-ocean continuum is crucial, especially as climate forcing is rapidly altering the environment.

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06:00 PM

Characterization of Algal Dissolved Organic Matter Utilized by Marine Sponges (8938)

Primary Presenter: Courtney Clevenger, University North Carolina Wilmington (ctk6717@uncw.edu)

Coral populations on reefs around the world continue to suffer catastrophic declines. As they die off, there has been a documented rise in the proliferation of sponges and turf algae as dominant reef features. With the expansion of marine sponge and algal populations, it is important to understand their interaction with the surrounding environment, shedding light on the potential consequences for the entire ecosystem. Emergent sponges containing high microbial abundances (HMA) of associated bacteria can feed on dissolved organic matter (DOM) from algae as they filter seawater. To investigate cycling of DOM production by macroalgae and subsequent uptake by sponges, a series of targeted and untargeted analytical analyses, both on the reef and in the laboratory, were employed to characterize algal DOM before and after passing through a sponge. Targeted analyses of bulk dissolved organic carbon (DOC) were correlated with dissolved free carbohydrates (DFCH) and dissolved free amine containing metabolites (DFA) using a combination of gas (GC-MS/MS) and liquid chromatography (LC-MS/MS) hyphenated approaches. In parallel, untargeted HRMS-MS metabolomics investigations were performed to begin to understand alterations to the DOM pool on a molecular level. Experimental design included both in situ sampling and laboratory incubation experiments of multiple brown macroalgae species and high/low microbial abundance (HMA)/(LMA) sponges. Results show distinct trends correlating emergent HMA sponges and LMA sponges with DOM modifications.

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