| 1. |
- Attermeyer, Katrin, et al.
(författare)
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Organic Carbon Processing During Transport Through Boreal Inland Waters : Particles as Important Sites
- 2018
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Ingår i: Journal of Geophysical Research - Biogeosciences. - 2169-8953 .- 2169-8961. ; 123:8, s. 2412-2428
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Tidskriftsartikel (refereegranskat)abstract
- The degradation and transformation of organic carbon (C) in inland waters result in significant CO2 emissions from inland waters. Even though most of the C in inland waters occurs as dissolved organic carbon (DOC), studies on particulate organic carbon (POC) and how it influences the overall reactivity of organic C in transport are still scarce. We sampled 30 aquatic ecosystems following an aquatic continuum including peat surface waters, streams, rivers, and lakes. We report DOC and POC degradation rates, relate degradation patterns to environmental data across these systems, and present qualitative changes in dissolved organic matter and particulate organic matter during degradation. Microbial degradation rates of POC were approximately 15 times higher compared to degradation of DOC, with POC half-lives of only 17 +/- 3 (mean +/- SE) days across all sampled aquatic ecosystems. Rapid POC decay was accompanied by a shift in particulate C: N ratios, whereas dissolved organic matter composition did not change at the time scale of incubations. The faster degradation of the POC implies a constant replenishment to sustain natural POC concentrations. We suggest that degradation of organic matter transported through the inland water continuum might occur to a large extent via transition of DOC into more rapidly cycling POC in nature, for example, triggered by light. In this way, particles would be a dominant pool of organic C processing across the boreal aquatic continuum, partially sustained by replenishment via flocculation of DOC.
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| 2. |
- Barbosa, Pedro M., et al.
(författare)
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Large Seasonal and Habitat Differences in Methane Ebullition on the Amazon Floodplain
- 2021
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Ingår i: Journal of Geophysical Research - Biogeosciences. - : American Geophysical Union (AGU). - 2169-8953 .- 2169-8961. ; 126:7
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Tidskriftsartikel (refereegranskat)abstract
- Tropical floodplains are an important source of methane (CH4) to the atmosphere, and ebullitive fluxes are likely to be important. We report direct measurements of CH4 ebullition in common habitats on the Amazon floodplain over two years based on floating chambers that allowed detection of bubbles, and submerged bubble traps. Ebullition was highly variable in space and time. Of the 840 floating chamber measurements (equivalent to 8,690 min of 10-min deployments), 22% captured bubbles. Ebullitive CH4 fluxes, measured using bubble traps deployed for a total of approximately 230 days, ranged from 0 to 109 mmol CH4 m−2 d−1, with a mean of 4.4 mmol CH4 m−2 d−1. During falling water, a hydroacoustic echosounder detected bubbles in 24% of the 70-m segments over 34 km. Ebullitive flux increased as the water level fell faster during falling water periods. In flooded forests, highest ebullitive fluxes occurred during falling water, while in open water and herbaceous plant habitats, higher ebullitive fluxes were measured during low water periods. The contribution of diffusive plus ebullitive CH4 flux represented by ebullition varied from 1% (high and rising water in open water of the lake) to 93% (falling water in flooded forests) based on bubble traps. Combining ebullitive and diffusive fluxes among habitats in relation to variations in water depth and areal coverage of aquatic habitats provides the basis for improved floodplain-wide estimates of CH4 evasion.Plain Language SummaryMethane is a trace gas that contributes to global warming, and wetlands are major natural sources. High concentrations of methane in sediments can lead to large releases to the atmosphere via bubbling (called ebullition). The Amazon basin is known to be an important source of CH4 to the atmosphere. We measured CH4 ebullition over two years in flooded forest, macrophytes and open water habitats in an Amazon floodplain using floating chambers that allowed detection of bubbles, and bubble traps; we also used hydroacoustics to detect bubbles in the water column. We found high spatial and temporal variability in all habitats, with ebullitive fluxes tending to be higher when water level was low or falling. While ebullition was often the major route of evasion of methane to the atmosphere, it varied from only about 1% to 93% of the diffusive plus ebullitive flux. The episodic nature and spatial variations of ebullition introduce considerable uncertainty in estimates of ebullitive CH4 flux. Our results have important implications for the regionalization of CH4 fluxes for Amazon floodplains and inland waters elsewhere, and emphasize the inter-related temporal and spatial variations in habitats and fluxes especially in aquatic systems with large seasonal variations in extent.
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| 3. |
- Bayer, Tina K., et al.
(författare)
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Future Carbon Emission From Boreal and Permafrost Lakes Are Sensitive to Catchment Organic Carbon Loads
- 2019
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Ingår i: Journal of Geophysical Research - Biogeosciences. - 2169-8953 .- 2169-8961. ; 124:7, s. 1827-1848
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Tidskriftsartikel (refereegranskat)abstract
- Carbon storage, processing, and transport in freshwater systems are important components of the global carbon cycle and sensitive to global change. However, in large-scale modeling this part of the boundless carbon cycle is often lacking or represented in a very simplified way. A new process-oriented lake biogeochemical model is used for investigating impacts of changes in atmospheric CO2 concentrations and organic carbon loading from the catchment on future greenhouse gas emissions from lakes across two boreal to subarctic regions (Northern Sweden and Alaska). Aquatic processes represented include carbon, oxygen, phytoplankton, and nutrient dynamics leading to CO2 and CH4 exchanges with the atmosphere. The model is running inside a macroscale hydrological model and may be easily implemented into a land surface scheme. Model evaluation demonstrates the validity in terms of average concentration of nutrients, algal biomass, and organic and inorganic carbon. Cumulative annual emissions of CH4 and CO2, as well as pathways of CH4 emissions, also compare well to observations. Model calculations imply that lake emissions of CH4 may increase by up to 45% under the Representative Concentration Pathway 8.5 scenario until 2100, and CO2 emissions may increase by up to 80% in Alaska. Increasing organic carbon loading to the lakes resulted in a linear response in CO2 and CH4 emissions across both regions, but increases in CO2 emissions from subarctic lakes in Sweden were lower than for southern boreal lakes, probably due to the higher importance of imported vegetation-generated inorganic carbon for CO2 emission from subarctic lakes.
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| 4. |
- Bishop, Kevin, et al.
(författare)
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Boundary-Crossing Field Research Marks the Way to Evidence-Based Management of Mercury in Forest Landscapes
- 2022
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Ingår i: Journal of Geophysical Research: Biogeosciences. - 2169-8953 .- 2169-8961. ; 127
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Tidskriftsartikel (refereegranskat)abstract
- The atmospheric deposition of long-range atmospheric mercury pollution presents forest managers with a "wicked" problem-forestry operations run the risk of mobilizing this pollution legacy. Management of that risk would benefit from a process-based understanding of how forest management influences the mercury cycle. This commentary highlights the value for building such an understanding of a comprehensive Before-After-Control-Impact study reported by McCarter et al. (2022), on the Marcel Experimental Forest in the north-central continental US. That study looked at how different types of forest harvest influenced the movement of mercury through the landscape. The results of this study place it at the minimal end of the range of impacts on Hg mobilization resulting from forest harvest. What makes this paper, together with the companion papers resulting from this study, particularly valuable for improving the understanding of forestry influences on mercury is the number of system boundaries that the study crossed: between land and atmosphere, from a forested hillslope down into a wetland, and finally up into the biota on that wetland.Plain Language Summary Forest harvest can mobilize toxic mercury from forest soils and move it into living organisms. This mercury originated in air pollution created far away from the forest, but forest managers still need to deal with the risks of this "pollution legacy" to people, fish and wildlife. A recent study in the north-central US took a detailed look at how two different types of forest harvest mobilized mercury in the soil. This study showed a relatively small impact of the forest harvest on mercury relative to some other studies. Since previous studies have found a wide range of mercury responses to forest harvest, this carefully designed and executed study has value in adding to the evidence base about forest management impacts on mercury in the environment. What is particularly valuable about this study is its comprehensiveness, since it crosses a number of environmental system boundaries: between the forest and the atmosphere, from upslope mineral soils into a downslope peatland, and from the wetland environment into the biota.
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| 5. |
- Björnerås, Caroline, et al.
(författare)
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Sediment Records Shed Light on Drivers of Decadal Iron Concentration Increase in a Boreal Lake
- 2022
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Ingår i: Journal of Geophysical Research - Biogeosciences. - : American Geophysical Union (AGU). - 2169-8953 .- 2169-8961. ; 127:3
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Tidskriftsartikel (refereegranskat)abstract
- Increasing iron (Fe) concentrations are found in lakes on a wide geographical scale but exact causes are still debated. The observed trends might result from increased Fe loading from the terrestrial catchment, but also from changes in how Fe distributes between the water column and the sediments. To get a better understanding of the causes we investigated whether there has been any change in the sediment formation of Fe sulfides (FeS) as an Fe sink in response to declining atmospheric sulfur (S) deposition during recent decades. For our study, we chose Lake Bolmen in southern Sweden, a lake for which we confirmed that Fe concentrations in the water column have strongly increased along with water color during 1966-2018. Our investigations showed that Fe accumulation and speciation varied independently of S accumulation patterns in the Lake Bolmen sediment record. Thus, we were not able to relate the positive trend in Fe concentrations to reduced FeS binding in the sediments. Furthermore, we found that Fe accumulation rates increased along with lake water Fe concentrations, indicating that increased catchment loading rather than a change in the distribution between the sediments and the water column has driven the increase in Fe concentrations. The increased loading may be due to land-use change in the form of an extensive expansion of coniferous forest during the past century. Altered forest management practices and increased precipitation may have led to enhanced weathering and erosion of organic soil layers under aging coniferous forest.
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| 6. |
- Blackburn, Meredith, et al.
(författare)
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Evaluating hillslope and riparian contributions to dissolved nitrogen (N) export from a boreal forest catchment
- 2017
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Ingår i: Journal of Geophysical Research: Biogeosciences. - : AMER GEOPHYSICAL UNION. - 2169-8953 .- 2169-8961. ; 122, s. 324-339
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Tidskriftsartikel (refereegranskat)abstract
- Catchment science has long held that the chemistry of small streams reflects the landscapes they drain. However, understanding the contribution of different landscape units to stream chemistry remains a challenge which frequently limits our understanding of export dynamics. For limiting nutrients such as nitrogen (N), an implicit assumption is that the most spatially extensive landscape units (e.g., uplands) act as the primary sources to surface waters, while near-stream zones function more often as sinks. These assumptions, based largely on studies in high-gradient systems or in regions with elevated inputs of anthropogenic N, may not apply to low-gradient, nutrient-poor, and peat-rich catchments characteristic of many northern ecosystems. We quantified patterns of N mobilization along a hillslope transect in a northern boreal catchment to assess the extent to which organic matter-rich riparian soils regulate the flux of N to streams. Contrary to the prevailing view of riparian functioning, we found that near-stream, organic soils supported concentrations and fluxes of ammonium (NH4+) and dissolved organic nitrogen that were much higher than the contributing upslope forest soils. These results suggest that stream N chemistry is connected to N mobilization and mineralization within the riparian zone rather than the wider landscape. Results further suggest that water table fluctuation in near-surface riparian soils may promote elevated rates of net N mineralization in these landscapes.
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| 7. |
- Bring, Arvid, et al.
(författare)
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Arctic terrestrial hydrology : A synthesis of processes, regional effects, and research challenges
- 2016
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Ingår i: Journal of Geophysical Research - Biogeosciences. - 2169-8953 .- 2169-8961. ; 121:3, s. 621-649
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Forskningsöversikt (refereegranskat)abstract
- Terrestrial hydrology is central to the Arctic system and its freshwater circulation. Water transport and water constituents vary, however, across a very diverse geography. In this paper, which is a component of the Arctic Freshwater Synthesis, we review the central freshwater processes in the terrestrial Arctic drainage and how they function and change across seven hydrophysiographical regions (Arctic tundra, boreal plains, shield, mountains, grasslands, glaciers/ice caps, and wetlands). We also highlight links between terrestrial hydrology and other components of the Arctic freshwater system. In terms of key processes, snow cover extent and duration is generally decreasing on a pan-Arctic scale, but snow depth is likely to increase in the Arctic tundra. Evapotranspiration will likely increase overall, but as it is coupled to shifts in landscape characteristics, regional changes are uncertain and may vary over time. Streamflow will generally increase with increasing precipitation, but high and low flows may decrease in some regions. Continued permafrost thaw will trigger hydrological change in multiple ways, particularly through increasing connectivity between groundwater and surface water and changing water storage in lakes and soils, which will influence exchange of moisture with the atmosphere. Other effects of hydrological change include increased risks to infrastructure and water resource planning, ecosystem shifts, and growing flows of water, nutrients, sediment, and carbon to the ocean. Coordinated efforts in monitoring, modeling, and processing studies at various scales are required to improve the understanding of change, in particular at the interfaces between hydrology, atmosphere, ecology, resources, and oceans.
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| 8. |
- Burke, S. A., et al.
(författare)
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Long-Term Measurements of Methane Ebullition From Thaw Ponds
- 2019
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Ingår i: Journal of Geophysical Research - Biogeosciences. - 2169-8953 .- 2169-8961. ; 124:7, s. 2208-2221
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Tidskriftsartikel (refereegranskat)abstract
- Arctic regions are experiencing rapid warming, leading to permafrost thaw and formation of numerous water bodies. Although small ponds in particular are considered hot spots for methane (CH4) release, long-term studies of CH4 efflux from these surfaces are rare. We have collected an extensive data set of CH4 ebullition (bubbling) measurements from eight small thaw ponds (<0.001 km(2)) with different physical and hydrological characteristics over four summer seasons, the longest set of observations from thaw ponds to date. The measured fluxes were highly variable with an average of 20.0 mg CH4 . m(-2) . day(-1) (median: 4.1 mg CH4 . m(-2) . day(-1), n = 2,063) which is higher than that of most nearby lakes. The ponds were categorized into four types based on clear and significant differences in bubble flux. We found that the amount of CH4 released as bubbles from ponds was very weakly correlated with environmental variables, like air temperature and atmospheric pressure, and was potentially more related to differences in physical characteristics of the ponds. Using our measured average daily bubble flux plus the available literature, we estimate circumpolar thaw ponds <0.001 km(2) in size to emit between 0.2 and 1.0 Tg of CH4 through ebullition. Our findings exemplify the importance of high-frequency measurements over long study periods in order to adequately capture the variability of these water bodies. Through the expansion of current spatial and temporal monitoring efforts, we can increase our ability to estimate CH4 emissions from permafrost pond ecosystems now and in the future.
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| 9. |
- Campeau, A., et al.
(författare)
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Stable Carbon Isotopes Reveal Soil-Stream DIC Linkages in Contrasting Headwater Catchments
- 2018
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Ingår i: Journal of Geophysical Research-Biogeosciences. - : American Geophysical Union (AGU). - 2169-8953 .- 2169-8961. ; 123:1, s. 149-167
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Tidskriftsartikel (refereegranskat)abstract
- Large CO2 evasion to the atmosphere occurs as dissolved inorganic carbon (DIC) is transported from soils to streams. While this physical process has been the focus of multiple studies, less is known about the underlying biogeochemical transformations that accompany this transfer of C from soils to streams. Here we used patterns in stream water and groundwater C-13-DIC values within three headwater catchments with contrasting land cover to identify the sources and processes regulating DIC during its transport. We found that although considerable CO2 evasion occurs as DIC is transported from soils to streams, there were also other processes affecting the DIC pool. Methane production and mixing of C sources, associated with different types and spatial distribution of peat-rich areas within each catchment, had a significant influence on the C-13-DIC values in both soils and streams. These processes represent an additional control on C-13-DIC values and the catchment-scale cycling of DIC across different northern landscape types. The results from this study demonstrate that the transport of DIC from soils to streams results in more than just rapid CO2 evasion to the atmosphere but also represents a channel of C transformation, which questions some of our current conceptualizations of C cycling at the landscape scale. Plain Language Summary Large carbon dioxide emission to the atmosphere occurs as rainwater percolates through soils and into streams. This physical process is important for the global carbon cycle and has been the focus of multiple studies. However, less is known about the underlying processes that accompanies this transfer of carbon dioxide from soils to streams. Here we analyze the stable isotope composition of soil and stream carbon dioxide and demonstrate that methane production and mixing of carbon sources also occur in soils and streams. These processes were linked to different types and configurations of peat-rich areas, for example, bogs, fens, and riparian zones, found within each of the three studied catchments. Our results therefore demonstrate that the export of carbon dioxide from soils to streams not only results in emissions to the atmosphere but also represents a channel of transformation. This questions some of our current conceptualization of the catchment-scale cycling of carbon dioxide.
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| 10. |
- Campeau, Audrey, et al.
(författare)
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Stable carbon isotopes reveal soil - stream DIC linkages in contrasting headwater catchments
- 2018
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Ingår i: Journal of Geophysical Research - Biogeosciences. - : American Geophysical Union (AGU). - 2169-8953 .- 2169-8961. ; 123:1, s. 149-167
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Tidskriftsartikel (refereegranskat)abstract
- Large CO2 evasion to the atmosphere occurs as dissolved inorganic carbon (DIC) is transported from soils to streams. While this physical process has been the focus of multiple studies, less is known about the underlying biogeochemical transformations that accompany this transfer of C from soils to streams. Here we used patterns in stream water and groundwater C-13-DIC values within three headwater catchments with contrasting land cover to identify the sources and processes regulating DIC during its transport. We found that although considerable CO2 evasion occurs as DIC is transported from soils to streams, there were also other processes affecting the DIC pool. Methane production and mixing of C sources, associated with different types and spatial distribution of peat-rich areas within each catchment, had a significant influence on the C-13-DIC values in both soils and streams. These processes represent an additional control on C-13-DIC values and the catchment-scale cycling of DIC across different northern landscape types. The results from this study demonstrate that the transport of DIC from soils to streams results in more than just rapid CO2 evasion to the atmosphere but also represents a channel of C transformation, which questions some of our current conceptualizations of C cycling at the landscape scale. Plain Language Summary Large carbon dioxide emission to the atmosphere occurs as rainwater percolates through soils and into streams. This physical process is important for the global carbon cycle and has been the focus of multiple studies. However, less is known about the underlying processes that accompanies this transfer of carbon dioxide from soils to streams. Here we analyze the stable isotope composition of soil and stream carbon dioxide and demonstrate that methane production and mixing of carbon sources also occur in soils and streams. These processes were linked to different types and configurations of peat-rich areas, for example, bogs, fens, and riparian zones, found within each of the three studied catchments. Our results therefore demonstrate that the export of carbon dioxide from soils to streams not only results in emissions to the atmosphere but also represents a channel of transformation. This questions some of our current conceptualization of the catchment-scale cycling of carbon dioxide.
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