Rivers transport large amounts of organic carbon, trace metals, and nutrients from land to coastal oceans. At the land-sea interface, elemental fluxes and transformations are influenced by processes occurring in rivers, wetlands, and estuaries. Increasing anthropogenic pressure, such as land-use change and increasing frequency of extreme events (e.g., hurricanes, fires), are altering the sources and quality of organic carbon and nutrients exported to coastal ecosystems. Maintaining the integrity of coastal aquatic systems is critical because they provide essential ecosystem services. Effectively adapting to meet these challenges requires us to understand the biogeochemical linkages of carbon and nutrient cycles to ecosystem processes and microbial activity, and how they are altered by human activities. This session aims to bring together scientists from all fields of biogeochemistry, covering ecosystems at different latitudes and spatio-temporal scales. We welcome contributions that address carbon and nutrient fluxes across the land-ocean continuum, biogeochemical transformations in rivers, coastal wetlands and estuaries, as well as the fate of terrestrial carbon in marine environments. We also encourage submissions that explain mechanisms underlying observed patterns in the distribution or transformation of sedimentary, particulate, and dissolved organic matter, their relationship to CO2 outgassing or uptake, and microbial community composition across aquatic gradients. In addition, we are interested in approaches to quantify the response of coastal aquatic systems to environmental stressors to guide conservation and restoration efforts.
Lead Organizer: Michael Seidel, University of Oldenburg (m.seidel@uni-oldenburg.de)
Co-organizers:
Sairah Malkin, University of Maryland (smalkin@umces.edu)
Patricia M. Medeiros, University of Georgia (medeiros@uga.edu)
Sasha Wagner, Rensselaer Polytechnic Institute (wagnes3@rpi.edu)
Nicholas D. Ward, Pacific Northwest National Laboratory (nicholas.ward@pnnl.gov)
Presentations
02:30 PM
Re-evaluating the photochemical sink for terrestrial and marine dissolved organic carbon (9542)
Primary Presenter: Leanne Powers, State University of New York College of Environmental Science and Forestry (lcpowers@esf.edu)
Miller and Zepp (1995)* provided the first experimental evidence that the direct abiotic photooxidation of dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) represents a significant sink for terrestrial DOC in the oceans. Since then, it has largely been assumed that DIC is the primary carbon photoproduct of DOC photochemistry in marine waters. However, direct measurements to support this presumption are lacking. Moreover, the magnitude of the photochemical sink for both terrestrial and marine DOC remains highly uncertain and therefore the importance of sunlight in DOC cycling remains an open question. Here, we present our current understanding of the photochemical sink for DOC from inshore to offshore waters. For the open ocean, it is unclear whether DIC is the main carbon photoproduct of DOC photochemistry. Our results suggest that the photoproduction of low molecular weight organic compounds may dominate in blue waters. Finally, as these products are all biological substrates, we attempt to evaluate whether the photoproduction of these species represents a substantial sink for marine DOC. *Miller, WL & RG Zepp. 1995. Geophys. Res. Let. 22: 417-420.
02:45 PM
TIDAL SYSTEMS WITH EXTENSIVE WETLANDS DOMINATE GREENHOUSE GAS EMISSIONS FROM AUSTRALIAN ESTUARIES (9539)
Primary Presenter: Jacob Yeo, Southern Cross University (jacob.yeo@scu.edu.au)
Estuaries play an important role in connecting the global carbon and nitrogen cycles of the land-to-ocean continuum. The amount of emitted estuarine greenhouse gases (GHG; CO2 and CH4) can be influenced by the geomorphology and magnitude of disturbance in the estuaries. Australia has over 1,000 estuaries along 38,000 km of coastline, of which 75% are classified as low or moderately disturbed. This makes Australia a good analogue for the ~34% of coastal regions classified globally as less than moderately disturbed (>40% intact). To assess Australia’s contribution to global estuary GHG emissions, water-air GHG fluxes from 36 measured and 11 published Australian estuaries were upscaled to 974 assessed estuaries in Australia, classified into three estuary types (lagoons, small deltas, and tidal systems) and four disturbance groups (low to very high). We found that the effect of disturbance on estuarine water-air GHG flux were strongly influenced by the geomorphic type of the estuaries. Although increasing disturbance correlated to increased emissions, its effect was less pronounced in tidal systems as compared to lagoons. However, annual GHG emissions from Australian estuaries were dominated by low and moderately disturbed tidal systems with extensive wetlands even though they had lower water-air CO2 and CH4 flux rates. This was due to their extensive distribution across Northern Australia. As such, estuarine geomorphology, which captures drivers like wetland extent, in combination with disturbance, should be considered in GHG emission estimates.
03:00 PM
Transformation of riverine nutrients and dissolved organic matter from source to sea (8836)
Primary Presenter: Norbert Kamjunke, Helmholtz Centre for Environmental Research (norbert.kamjunke@ufz.de)
Transformations of nutrients and organic matter are of considerable importance in large rivers on their way to the sea, but studies investigating a river system from source to sea are scarce. We tested the hypotheses: (1) dissolved nutrients are incorporated by phytoplankton and transferred into particles with increasing river length, (2) lignin- and polyphenol-like terrestrial components will decrease from upstream to downstream regions, (3) oxygen and aromatic content and molecular mass of DOM will decrease towards the tidal and coastal parts, and (4) chlorophyll a concentration and salinity are important environmental drivers of nutrient dynamics and DOM composition. We investigated the Elbe River in Central Europe from the headwaters via the lowland river and the tidal region towards the coastal waters in the North Sea. Chlorophyll a concentration and oxygen saturation increased longitudinally in the river but showed distinct minima in the estuary upstream of the salinity gradient. Dissolved nutrients were taken up by algae in the freshwater part but were released again at algal die-off, and a part of the nitrate was used for denitrification. Organic carbon was dominated by lignins and polyphenols of terrestrial origin in the upstream region. The O:C ratio, the number of double bounds, aromaticity, and molecular mass decreased with increasing river stretch. The proportion of organic nitrogen components increased in the estuary and coastal regions. Overall, our unique data set helps understanding the dynamic transfer of nutrients and a distinct variability of DOM composition along the land-ocean gradient.
03:15 PM
On the downswing of the C-SAW: dissolved organic matter losses from the landscape after extreme weather in coastal North Carolina (9565)
Primary Presenter: Christopher Osburn, North Carolina State University (closburn@ncsu.edu)
Estuaries are prominent biogeochemical reactors in the land-ocean-aquatic continuum (LOAC) that provide essential ecosystem services. The increasing frequency of extreme events (e.g., tropical cyclones) are altering the sources and quality of organic matter exported to estuaries from the landscape. We posit that one effect of this symptom of climate change is a saturation of carbon in coastal waters after such events – but the source and fate of the organic matter driving the saturation remain poorly known. Here we present the changes in dissolved organic matter (DOM) quality in the Neuse River Estuary, a major tributary to the nation’s second largest estuarine complex, the Albemarle-Pamlico Sound, in eastern North Carolina, caused by extreme precipitation events. We used a new fluorescence-based approach for estimating DOM sources across the LOAC based on ordinary least squares regression. We found the majority of DOM in the lower estuary and sound was dominated by upstream river, soil leachate, and wetland sources. Further, we estimated that >60% of DOM in the sound originated from riparian wetlands above head-of-tide of the estuary and persisted for months during a period in which we estimated CO2 supersaturation in this otherwise productive estuary. Our results suggest a new paradigm of Carbon Saturation And Weather (C-SAW) such that extreme rainfall events from tropical cyclones deplete DOM stored in wetlands, saturating estuaries with long-residence times, and leading to dramatic changes to the estuarine carbon sink.
03:30 PM
Changing water flow regimes regulate nutrient retention and habitat quality in a complex coastal landscape (9537)
Primary Presenter: Matthew Hipsey, The University of Western Australia (matt.hipsey@uwa.edu.au)
The nature of nutrient transformations and redistribution in coastal aquatic landscapes varies in response to short- and long-term changes in hydrologic connectivity, which are mediated by river flow regimes and ocean dynamics. Coastal lagoons with topographically restricted connection to the ocean and regulated river flows are particularly sensitive since poor flushing and evaporation promotes nutrient retention and degraded water quality. Resolving nutrient dynamics is important for their sustainable management, yet being able to accurately resolve nutrient budgets remains challenging due to complex hydrological regimes and habitat heterogeneity. In this study, we undertake a systematic nutrient budget and habitat assessment of a large shallow lagoon (Coorong, South Australia), using a high-resolution coupled hydrodynamic-biogeochemical-habitat model. The interplay between hydrological drivers and biogeochemical processes was quantified using an adjusted Damköhler number, comparing the timescales of nutrient flushing versus processing. This showed the general transition from hydrologic to biogeochemical control with increasing distance from the main ocean connection, modified by episodes of increased flows and external loads. Whilst water age was a useful indicator of the factors controlling the overall rate of net nutrient retention, interannual variability in retention between areas of the lagoon was explained based on the river inflow regime and changes in mean sea level. We demonstrate that high rates of evapo-concentration and limited water connectivity have led to a persistent accumulation of nutrients, and poor water and sediment quality within the lagoon, which has also been associated with a decline in habitat availability for key species. Scenarios are used to provide evidence that increasing environmental water allocations through the river to offset recent declines in flows reaching the river mouth would reduce the nutrient retention, and we discuss the potential for net nutrient export to the ocean and habitat recovery under sustained high flows.
SS14B - Biogeochemical Connections and Ecosystem Adaptation Across the Land-Ocean Continuum
Description
Time: 2:30 PM
Date: 30/3/2025
Room: W207CD