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| Titel |
Thermal and hydrodynamic variability within a gravel bar of an Alpine stream and its link to hyporheic carbon cycling |
| VerfasserIn |
Kyle Boodoo, Jakob Schelker, Christina Fasching, Amber Ulseth, Tom Battin |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250108614
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| Publikation (Nr.) |
 EGU/EGU2015-8378.pdf |
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| Zusammenfassung |
| In-stream bodies of fluvial sediment such as gravel bars (GB), form an active interface
between streamwater and the adjacent groundwater body. The hydrodynamic exchange, that
is, the varying contributions of different water sources to this mixing zone, control the GB
physical and biogeochemical conditions, including water temperature, as well as nutrient and
carbon availability, likely impacting carbon turnover. We present high frequency data for
hydraulic head and water temperature in addition to event based measurements of
electric conductivity, nutrients and dissolved organic carbon (DOC) concentration and
composition within a GB of an Alpine cold water stream (Oberer Seebach, Austria) for a
range of different flow conditions. The highest vertical temperature differences
and hydraulic head variability occurred at the head and shoulder - largest raised
area perpendicular to surface water flow (downwelling) and tail (upwelling) of the
gravel bar. At baseflow, high spatial variability of temperature (up to 4Ë C difference
among sites within the same horizontal plane) and hydraulic head was observed
within the GB. In contrast, floods resulted in markedly lower overall hyporheic
zone temperatures (average 2Ë C difference among sites within the same horizontal
plane) and spatial hydraulic head variability, compared to baseflow conditions.
Similarly, the relative difference between surface water and GB nutrient and DOC
concentrations and the overall spatial variability within the GB decreased with increasing
surface water discharge. For example, at baseflow surface water average DOC and
nitrate (NO3) concentrations were 1.40 mgL-1and 810 μgL-1respectively, and
1.97 mgL-1 and 779 μgL-1 respectively at intermediate flow. Meanwhile, DOC
and NO3 concentrations in the GB ranged from 1.40 - 3.60 mgL-1 and 150 – 950
μgL-1respectively during baseflow and 1.48 -2.25 mgL-1 and 560 –840 μgL-1
respectively during moderate flows. Furthermore, DOC and NH4 concentrations decreased
and NO3concentrations increased with depth in the GB. These results indicate a
combination of shallow surface water downwelling and groundwater upwelling through the
heterogeneous porous medium of the GB during lower flows and, increased downwelling
of surface water and less groundwater contributions during higher surface water
discharges. These flow dependent shifts in water sources are then likely to support
high levels of biogeochemical activity and/or dilution of nutrient concentrations by
increased downwelling of surface water during and after high flows. Overall our results
demonstrate that the main drivers of biogeochemical turnover (flow regime, residence
time, water temperature and DOC availability) within GBs vary for different flow
conditions, suggesting GB to act as dynamic in-stream hotspots for carbon cycling. |
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