Rivers transport large amounts of organic carbon, trace metals and nutrients from land to coastal oceans. At the interface between land and sea, elemental fluxes and transformations are strongly influenced by processes occurring across the continuum of rivers, wetlands, and estuaries. Increasing anthropogenic pressure (e.g., land use changes) and frequency of extreme events (e.g., hurricane landfalls, fires) are increasingly altering the sources and quality of organic carbon and nutrients exported to coastal ecosystems. Preserving the integrity of coastal aquatic systems is key as these systems provide critical ecosystem services to support societal development. It is therefore crucial to understand the biogeochemical connections of the carbon and nutrient cycles to ecosystem processes and microbial processing and how they are affected by humans. In our session we seek to bring together scientists from all areas of biogeochemistry that cut across boundaries, covering ecosystems from a wide range of latitudinal settings and spatiotemporal scales. Contributions that examine carbon and nutrient fluxes across the land-ocean-continuum, biogeochemical transformations in rivers, coastal wetlands, (subterranean) estuaries, and the fate of terrestrial carbon in the marine environment are particularly welcome. We also encourage submissions that seek to explain mechanisms underlying observed patterns in the distribution or rates of sedimentary, particulate, and dissolved organic matter transformation, their linkage to CO2 outgassing or uptake and microbial community composition across aquatic gradients, as well as approaches to quantify the response of coastal aquatic systems to environmental stressors in order to guide possible conservation and rehabilitation efforts.
Lead Organizer: Michael Seidel, University of Oldenburg (m.seidel@uni-oldenburg.de)
Co-organizers:
Patricia M. Medeiros, University of Georgia, USA (medeiros@uga.edu)
Sasha Wagner, Rensselaer Polytechnic Institute, USA (wagnes3@rpi.edu)
Nicholas D. Ward, Pacific Northwest National Laboratory, USA (nickdward@gmail.com)
Presentations
10:30 AM
Organic matter size continuum: Elucidating patterns and drivers of its chemical composition in river networks (4864)
Primary Presenter: Edurne Estévez, University of Innsbruck (edurne.estevez-cano@uibk.ac.at)
Rivers are important contributors to the global carbon cycle as they actively cycle terrestrial organic matter (OM) during transport to the oceans. However, what controls the degradation of OM, especially at the river network scale, remains largely unknown. OM chemical composition is certainly relevant in this process, yet difficult to describe across the entire OM size continuum from dissolved OM (DOM) to large particulate OM (POM). This is likely needed, however, as OM size likely acts as a master trait, that mediates the retention and transport of OM and selects the target consumer community. At larger network scale, this should give rise to OM size-specific patterns of OM composition in dependence on the dendritic nature of a river network and the diversity of land cover types in its terrestrial matrix. Here, we examined the longitudinal patterns and drivers of OM chemical composition along two river networks differing in topology and land cover. We characterized OM chemical composition by mass spectrometry for DOM and by infrared spectroscopy for individual POM particles. Preliminary results showed an overall less degraded OM in headwaters than main stems and in river sections draining forested sub-catchments compared to those draining agriculture-dominated sub-catchments. However, the OM composition patterns across the river network varied with OM size.
10:45 AM
TEMPORAL VS. SPATIAL CONTROLS ON ISOTOPE SIGNATURES OF CARBON PHASES EXPORTED BY SWISS RIVERS (5345)
Primary Presenter: Timo Rhyner, ETH Zurich (timo.rhyner@erdw.ethz.ch)
Lateral carbon mobilization processes along the freshwater continuum remain poorly constrained despite their fundamental role in the C-cycle. We examine temporal versus spatial variations in isotopic signatures of particulate and dissolved organic carbon and dissolved inorganic carbon transported by Swiss rivers to assess carbon sources and cycling within corresponding watersheds. Prior time-series investigations of small Swiss sub-alpine headwater streams revealed large variations in OC isotopic compositions during baseflow conditions and more homogenous isotopic and generally <sup>14</sup>C-enriched (younger) signatures at elevated discharge. Here, we assess whether these characteristics persist further downstream in higher-order Swiss fluvial systems and explore variations within a seasonal perspective. We also compare isotopic characteristics of a suite of 21 Swiss rivers sampled in summer 2021, a year of extraordinary heavy rainfall, with summer 2022, a year of extreme drought, which drain the five different ecoregions of Switzerland that are characterized by contrasting climatic regimes as well as catchment characteristics. Multivariate Regression Analysis reveals that POC, DOC and DIC <sup>14</sup>C values of streams draining alpine terrain are lower than rivers draining lower elevation terrain, and that <sup>14</sup>C signatures of POC under drought conditions are lower than those under high-flow conditions. Overall, we seek to disentangle the role of different hydrological and drainage basin properties on the flux and composition of carbon exported by Swiss rivers.
11:00 AM
Large-scale nutrient and carbon dynamics along the river-estuary-ocean continuum (5746)
Primary Presenter: Norbert Kamjunke, Helmholtz Centre for Environmental Research (norbert.kamjunke@ufz.de)
Nutrient and carbon dynamics within the river-estuary-coastal water systems are key processes to understand the flux of matter from the terrestrial environment to the ocean. Here, we analysed those dynamics by following a sampling approach based on the travel time of water. We started with a sampling of River Elbe from the source. After a subsequent investigation of the estuary, we followed the plume of the river by raster sampling the German Bight (North Sea) using three ships simultaneously. In the river, we detected intensive growth of phytoplankton connected with high oxygen saturation and pH values and undersaturation of CO2, whereas concentrations of dissolved nutrients declined. In the estuary, the Elbe shifted from an autotrophic to a heterotrophic system: Phytoplankton died off upstream of the salinity gradient causing minima in oxygen saturation and pH, supersaturation of CO2, and a release of nutrients. In the shelf region, phytoplankton and nutrient concentrations were low, oxygen close to saturation, and pH in a typical marine range. We detected a positive relationship between pH and oxygen saturation and a negative one between pCO2 and oxygen saturation. Flux rates of dissolved nutrients from river into estuary were low and determined by depleted concentrations. In contrast, fluxes from the estuary to the coastal waters were higher and the pattern was determined by tidal current. Overall, the approach is appropriate to better understand land-ocean fluxes, particularly if it is performed under different hydrological conditions including extremes.
11:15 AM
Is Abiotic Sulfurization a Mechanism for Recalcitrant Dissolved Organic Matter Formation in the North Sea? (5766)
Primary Presenter: Wiebke Freund, Carl von Ossietzky University of Oldenburg, Oldenburg, Germany (wiebke.freund@uol.de)
660 Pg of carbon is globally stored in marine dissolved organic matter (DOM). This refractory DOM (RDOM) is stable on millennial timescales and contributes to oceanic carbon storage. Dissolved organic sulfur (DOS), as part of the marine DOM, is mainly derived from primary production with contributions of DOS through pore water discharge from abiotic sulfurization in sulfidic environments such as intertidal flat sediments. However, questions remain about the stability of DOS from sulfidic environments once discharged into the water column and how different DOS sources can contribute to the long-term carbon storage as oceanic RDOM. In this project, we use the North Sea as an ideal system for studying DOS formation and fate from various sources, including phytoplankton, porewater as well as riverine outflow, and ultimately, DOS export to the North Atlantic Ocean. Water column and porewater samples were acquired during a research cruise in addition to seasonal intertidal flat, and river sampling (Weser, Ems, and Elbe Rivers). Our quantitative stoichiometric and molecular composition (FT-ICR-MS) analyses of solid-phase extracted DOM revealed non-conservative mixing behavior of carbon-to-sulfur ratios with salinity in the North Sea suggesting export of DOS-enriched porewater from the sulfidic intertidal flats. Our ongoing analyses aims at identifying characteristic DOM molecular fingerprints to trace the different riverine and porewater sources and to constrain the export of DOS to the North Atlantic Ocean.
11:30 AM
Land use change alters the biogeochemistry and dissolved organic matter composition of Malaysian peat-draining rivers (6855)
Primary Presenter: Sarah Bercovici, National Oceanography Centre (sarah.bercovici@noc.ac.uk)
Pristine peat swamp forests in SE Asia act as carbon sinks and store organic matter. However, when drained and logged, carbon may remobilize into aquatic bodies and the coastal ocean. The effects of land use change on the flux of dissolved organic matter (DOM) and its fate (i.e., oceanic sequestration vs. atmospheric release), are largely unknown. Here, we assess the flux and potential fate of DOM in 3 peat-draining rivers in the Sarawak region of Malaysia, representing varying levels of catchment disturbance due to logging and agriculture. We sampled a pristine (Maludam), moderately disturbed (Sebuyau), and disturbed (Simunjan) river system along a fresh to seawater gradient for greenhouse gases, nutrients, DOC, DOM composition (via FT-ICR-MS). Oxygen consumption and nutrient and methane concentrations of the disturbed rivers were higher than those of the pristine Maludam, implying agricultural nutrient input, greenhouse gas emissions, and differences in respiration in these more disturbed systems. However, the DOC concentrations were higher in the Maludam than the disturbed rivers and there were distinctions in DOM composition between sites: the DOM from the Maludam had the highest molecular richness, diversity, and fractions of labile (calculated with a molecular lability boundary) and refractory (identified as CRAM) molecular formulae of the 3 river systems. These results suggest that DOM from the disturbed rivers is less molecularly diverse and contains fewer labile and refractory molecular formulae, which would alter its bioavailability and potential for carbon sequestration.
11:45 AM
Big river energy: Compositional change in DOM exported from a large river links terrestrial and marine ecosystems in the Pacific coastal temperate rainforest. (5199)
Primary Presenter: Allison Oliver, Skeena Fisheries Commission (allisonoliver@skeenafisheries.ca)
The northeast Pacific coastal temperate rainforest (NPCTR) of British Columbia produces large fluxes of freshwater and dissolved organic matter (DOM) to nearshore marine waters. Regionally, these land-ocean linkages provide important DOM contributions to marine ecosystems but current understanding is based largely on freshwater studies of relatively small watersheds, with little attention given to the role of DOM export from large rivers and estuarine processes. We examine freshwater-ocean linkages of DOM exported from a large river (Skeena River) within the NPCTR. We examine dissolved organic carbon concentration and DOM composition using fluorescence spectroscopy and ultra-high resolution Fourier-transform ion cyclotron resonance mass spectrometry. Seasonality is an important control over riverine DOM export, and we observe non-conservative land-ocean mixing behavior with changes across salinity in optical properties and molecular-level metrics, including biological index, modified aromaticity index, heteroatom class, and the relative abundance of highly unsaturated and phenolic, aliphatic and peptide-like compounds. We also investigate source-specific marker formulae to identify and trace unique and dynamic DOM contributions across the land-to-ocean continuum.
SS009B Biogeochemical Cycling Across the Land-Ocean-Continuum
Description
Time: 10:30 AM
Date: 6/6/2023
Room: Auditorium Illes Balears