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| Titel |
Identifying hot-spot methane emission sites in an impounded river |
| VerfasserIn |
A. Maeck, S. Flury, T. DelSontro, M. Schmidt, D. F. McGinnis, H. Fischer, P. Fietzek, A. Lorke |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061623
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| Zusammenfassung |
| Greenhouse gas (GHG) emissions on a landscape level are difficult to assess due to large
spatial and temporal variations in fluxes. Freshwater ecosystems in particular, despite their
limited spatial extent, can often have a disproportionately high impact on the GHG balance
within the terrestrial landscape. Compounding the difficulties in assessing GHG emissions
within these aquatic systems is that local production and emission rates show large variations
due to small-scale physical and biogeochemical heterogeneity within the waterbody and
sediment. Therefore, to ultimately characterize the total methane emissions from a limnic
system using a bottom-up approach, the primary emission pathways and locations must be
identified and quantified.
To achieve this goal, we recorded continuous data with ship-mounted instruments
over a 93 km longitudinal transect of the heavily impounded River Saar which
consists of 7 dams and locks located every 15 km on average (Germany, France). A
Contros HydroC Methane sensor measured the dissolved methane concentration to
locate hot-spots and to estimate the diffusive flux across the water-air interface.
Gas bubbles were detected and mapped using a Simrad Scientific echosounder
and were processed to quantify ebullition-flux rates. Concentration differences
directly up- and downstream of the dams were used to quantify the atmospheric
transport component due to outgassing at the dams. Additional water and sediment
samples along the 93 km transect completed the data set and allowed for sensor
calibration.
Methane concentrations ranged from 60Â nM up to 1800 nM during the survey. Sharp and
significant increases in dissolved methane concentrations were observed towards the forebay
of three dams, with lower concentrations observed in the immediate tailwater of
these dams. The areas of increasing concentrations coincided well with acoustically
detected bubbles and with the distribution of cohesive muddy sediments. Ebullition
flux rates varied between 20 to as much as 1300 mg CH4 m-2 d-1. While the
areas where ebullition was detected covered only 15% of the total water surface
area, they contributed 43% of the total emissions, while outgassing at the dams was
quantified to yield about 53% of the total emissions (the remaining 4% by surface
diffusion).
Both pathways combined account for over 90% of the total methane emissions to the
atmosphere and were identified using our high-resolution longitudinal methane
concentration and acoustic bubble-detection survey. Our approach and results illustrate
the importance of performing spatially-fine surveys on accurately resolving the
spatial extent of outgassing “hot-spots” and pathways within freshwater systems.
Temporal extrapolation of the quantified emissions would lead to the emission of 117 t
CH4 yr-1 or 87 t CH4-C yr-1 from the Saar river system, but this estimate is
limited because it represents only a snap-shot of the emission pattern and seasonal
variations are not included. Despite this limitation, we show the importance of
rivers, which are under-represented in the literature, as significant GHG contributors. |
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