The land-ocean aquatic continuum (LOAC) is a complex system of terrestrial, freshwater, and marine environments. It is the interface where large amounts of terrestrial organic matter, carbon and nitrogen enter estuaries and wetlands through rivers and groundwater inputs. These inputs are transformed by biogeochemical processes occurring within the coastal environment before being exported to the ocean. There is intense hydrological and climatic variability along the LOAC. Biogeochemical processes occurring along the LOAC can be understood as a reactive transport pathway; with short- (diurnal) to long-term (periodic) hydrological and climatic patterns influencing respective reactive and transport fluxes within and between the riverine, estuarine, and marine components.
Approximately forty percent of the world’s population live within 100 km of the coast, attracted by the key ecosystem services to humans provided by LOAC environments. As a result, human-induced changes locally along the LOAC and also globally, are dramatically altering the intermittent hydrological regimes with changes in episodic flood, storm and drought events. Human impact along the LOAC has cascading effects on the capacity of natural systems to sequester carbon, remediate excess nutrients, and provide adequate conditions for biogeochemical processes. Furthermore, anthropogenic intervention focusing on utilizing components of the LOAC for enhanced carbon storage may be jeopardized by increasingly intermittent systems. The fluxes of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) may oscillate between effluxes and uptake depending on intermittent drivers.
Although scientific efforts are now focused on the effects of intermittent hydrology and climatic events exacerbated by anthropogenic change, it is still unclear how reactive transport within the LOAC may be influenced. Collectively, the component systems of the LOAC represent a disproportionate impact on greenhouse gas fluxes. The question remains: how can we characterize these fluxes within inherently intermittent systems?
Variation in temporal drivers influencing biogeochemical processes continues to obscure trends in long-term data. Work is being done at a global scale to rethink the meaning of average conditions and to specifically contextualize periodic, ephemeral, and intermittent drivers on biogeochemical processes on annual scales. This session welcomes studies that focus on the 1) individual LOAC components and/or 2) the LOAC as a whole; through a multidisciplinary lens, exploring topics such as spatial connectivity, and short- and long-term impacts of intermittent events and human impacts on biogeochemical processes that influence the cycling of carbon and nitrogen and associated greenhouse gas fluxes.
Lead Organizer: Jacob Yeo, Southern Cross University (jacob.yeo@scu.edu.au)
Co-organizers:
Ryan Felton, Southern Cross University (r.felton.11@student.scu.edu.au)
Jian-Jhih Chen, National Kaohsiung University of Science and Technology (jianjhihchen@nkust.edu.tw)
Bradley Eyre, Southern Cross University (bradley.eyre@scu.edu.au)
Presentations
09:00 AM
Spatial Variability of Terrestrial Carbon Inputs from the Catchment to Reach Scale in SubArctic, Sweden (9412)
Primary Presenter: Cheristy Jones, University of New Hampshire (cheristy.jones@unh.edu)
As the Arctic continues to warm, shifting hydrological flow paths due to permafrost thaw can move highly biodegradable, terrestrial-derived carbon (C) into streams, potentially increasing carbon dioxide (CO2) and methane (CH4) emissions. Most studies characterize these C fluxes from a single point within a catchment, typically at large river outlets. However, different land cover types across the landscape can serve as “control points”, disproportionately contributing dissolved organic carbon (DOC), CO2, and CH4 to streams. To investigate the spatial variability of aquatic C emissions and landscape connectivity across a subcatchment, we sampled riparian porewater and stream water. Our study subcatchment is 15 km2 and includes the permafrost peatland Stordalen Mire in northern Sweden (68°21′ N 18°49′ E). There are four stream branches with landscape transitions from alpine tundra to birch forest to a discontinuous permafrost peatland. We characterized groundwater inputs, dissolved C, CH4 and CO2 fluxes along each stream branch within each land cover type and more frequently in the permafrost peatland. Both DOC and DIC concentrations varied throughout the catchment and were highest in the stream reaches that pass through the peatland. Total dissolved nitrogen and CH4 concentrations did not vary significantly throughout the catchment, potentially indicating shorter resident times. δ13C-CO2 signatures indicate microbial sources in lakes and in stream reaches that drain the peatland. Ebullitive CH4 was highest at the stream outlet and varied across the peatland reach. Further understanding of the controls of this variability are crucial for understanding watershed-scale lateral C flux as well as C transformation as these ecosystems warm.
09:15 AM
Disentangling the impact of Hydrology and Climate on Interannual Intermittent Stream Metabolism (8883)
Primary Presenter: Ryan Felton, Southern Cross University (r.felton.11@student.scu.edu.au)
Rivers and stream networks are the means by which terrestrial carbon is transported to marine systems. Additionally, the biogeochemical processes during this transport create reactive fluxes of carbon in the form of greenhouse gasses, namely carbon dioxide and methane. This process, quantified as ecosystem metabolism, has become an important means to quantify reactive carbon fluxes at broad spatial and temporal scales. When quantifying carbon cycling and greenhouse gas emissions from stream networks average hydrologic conditions are often used to annualize estimates. However, with increased variability and intensity of climatic regimes it is unclear the degree to which these assumptions mask the impact of hydrologic extremes. Flooding, droughts, and increased global intermittency of stream networks requires a paradigm shift for reporting annual rates of carbon cycling from riparian systems. This work, based on multi-year ecosystem metabolism modeling from streams in coastal Australia seeks to partition carbon processing by hydrologic and climatic regimes. This provides a rigorous means to assess the appropriate use of average annual conditions used in upscaling reactive carbon fluxes. Furthermore, the degree to which seasonality and the El Nino Southern Oscillation (ENSO) impact interannual drivers of carbon cycling is explicitly resolved. The results have important implications for upscaling and annualizing protocols used in regional, continental, and global bookkeeping of carbon and greenhouse gas budgets.
09:30 AM
SEASONAL CHANGES IN HYDROLOGY DRIVE LARGE, RAPID FLUXES OF CO2 FROM STREAMS IN WET-DRY TROPICS (9023)
Primary Presenter: Adam Rexroade, Charles Darwin University (rexroadea@gmail.com)
Warm temperatures and high terrestrial productivity cause tropical streams to be hotspots for greenhouse gas (GHG) cycling. The wet-dry tropics of Australia display intense seasonal variability of precipitation with potentially high influence on stream GHG exports. However, our understanding of seasonal GHG dynamics in tropical streams is limited by scarce time series data, particularly in headwater streams. To better understand the magnitude and fate of GHG exports from streams in highly seasonal tropical climates, we used a combination of time series data and spatial sampling to quantify the exports of CO2 downstream and to the atmosphere at a daily timescale. Additionally, we explore the main drives for CO2 export. We find that export of CO2 is largely transport–rather than source–limited, with the majority of CO2 export occurring during the three wettest months of the year. Additionally, a majority of CO2 that enters the system is emitted to the atmosphere, rather than transported downstream. This work highlights the need to account for the observed temporal variability of GHG exports in tropical systems, which in turn require sampling designs that can capture these dynamics.
09:45 AM
CARBON DIOXIDE FLUXES FROM SUBTROPICAL INTERMITTENTLY DRY AND WET STREAMS IN AUSTRALIA (9512)
Primary Presenter: Micha Nebel, Southern Cross University, Lismore, NSW, Australia (micha_nebel@web.de)
Intermittent streams, a common feature of coastal floodplains, generally have higher CO2 emissions than perennial rivers, due to their high flow dynamic. Globally 50% of rivers and streams are intermittently dry. Despite their significance, intermittent stream studies are limited, especially in the southern hemisphere. In this study, we measured CO2 fluxes in nine intermittent streams over one year in northern New South Wales, Australia. Our study compares seasonal soil-air CO2 fluxes measured from dry areas, and water-air CO2 fluxes from when water was present in the nine streams. Mean water-air CO2 fluxes were higher from flowing streams (9.3 ± 7.4 g CO2 m-2 d-1), compared to dry fluxes from streambed sediments (4.8 ± 7 g CO2 m-2 d-1), and water-air fluxes from stagnant channels (2.5 ± 1 g CO2 m-2 d-1 ) and stagnant pools (2.5 ± 1.7 g CO2 m-2 d-1). These findings highlight the need to account for CO2 fluxes under varying hydrological conditions in intermittent streams when calculating inland water carbon budgets.
10:00 AM
TIDAL AND LUNAR VARIABILITY IN SOIL GREENHOUSE GAS FLUXES IN A BRAZILIAN MANGROVE FOREST (8971)
Primary Presenter: Gabriel Coppo, Universidade Federal do Espirito Santo (coppogabriel@gmail.com)
Mangroves are valuable coastal ecosystems known to sequester carbon at high rates compared to other tropical forests. Greenhouse gas (GHG) emissions affect the carbon balance in these forests, but there is limited information on the temporal variability in these fluxes along tidal and lunar cycles. We conducted high-frequency daily measurements in an urban mangrove forest in southeast Brazil for 14 days measuring soil GHG fluxes (CO2 and CH4) following a static closed opaque chamber technique using a LI-COR LI-7810 analyzer. Overall, we observed average flows of 65.15±30.49 mmol/m2/day for CO2 and 0.97±1.43 mmol/m2/day for CH4. Both gases showed daily variation in flux, with marginal influences of tidal regime for CO2 flux. During the ebb tide, the CO2 flux was slightly higher (69.56±26.15 mmol/m2/day) than during the flood tide (60.95±33.64 mmol/m2/day). The CH4 flux did not showed differences among ebb (1.16±1.87 mmol/m2/day) and flood tide (0.78±0.75 mmol/m2/day). Further, the CH4 flux was directly associated with the temperature inside the chamber. When extrapolating to the total radiative balance (GWP100), the CH4 fluxes potentially reduce in 43% the carbon burial in the mangrove forests. Total soil GHG emissions were equivalent to 14.70±8.62 Mg CO2e/ha/year, which represents an emission of 17645 Mg CO2e/year for the entire mangrove forest area in the study site.
10:15 AM
Linking hydrological and biogeochemical ‘hot moments’ to take the Aquatic Pulse (9430)
Primary Presenter: François Birgand, North Carolina State University (birgand@ncsu.edu)
Much of the Aquatic Pulse finds its origin in surface and subsurface hydrological pulsations in watersheds. Biogeochemical process reactivity has often been assumed to be slow relative to hydrological flashiness. Until now, there has been little research specifically looking at the linkage between biogeochemical to hydrological ‘hot moments’. More and more evidence suggest that this untapped cross-disciplinary research may unveil over-looked, yet possibly fundamental processes, at the source of the much of the aquatic pulse. For example, riparian buffers have been known for decades as natural hot spots for denitrification to remove from groundwater excess nitrogen produced by agriculture. The intermittent and pulsed nature of hydrology dictates that a majority of the water and nitrate fluxes be transported through the riparian zones in a minority of the time, leaving little exposure time for denitrification to occur. To explain the water quality benefits of riparian zones observed over decades, ‘hot moments’ of denitrification rates must specifically occur during the pulses of flow and nitrate. To test this hypothesis, we conducted experiments in a denitrification bed where we simulated pulses of flow and nitrate concentrations. Results show that in this system, denitrification rates were largely enhanced (x2 to x4) minutes after wetting and drying cycles and sustained for 1-2 days. These results, with others in the literature suggest that linking hydrological and biogeochemical ‘hot moments’ should be investigated and may explain to a significant extent the Aquatic Pulse.
SS26 - The role of (hydrologic and climatic) intermittency in the cycling of carbon and nitrogen and associated greenhouse gas fluxes across the land-ocean aquatic continuum (LOAC)
Description
Time: 9:00 AM
Date: 31/3/2025
Room: W201CD