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Greenhouse gases in arctic and alpine streams: patterns, drivers, and responses to disturbance




Dunn, Samuel T., author
von Fischer, Joseph, advisor
Baron, Jill, committee member
Gooseff, Michael, committee member
Wallenstein, Matthew, committee member

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Streams have recently received attention as previously unaccounted for sources of greenhouse gases (GHG; CH4,CO2, and N2O) to the atmosphere. While progress has been made at incorporating streams into global estimates of GHG flux, many spatial gaps remain, especially in remote regions of the Siberian Arctic and high elevation ecosystems worldwide. To address a critical gap in regional estimates of emissions and better understand the sources of variability of those emissions, we quantified the vertical flux of CH4, N2O, and CO2 and examined the sources of variability and spatial-temporal patterns of those fluxes in Siberian streams and high elevation streams. Emissions to the atmosphere from Siberian streams were smaller than expected with mean fluxes of CH4 (12.4 µmol CH4 m-2 d-1) and CO2 (2.6 mmol m-2 d-1). In contrast, downstream export of dissolved gas is three orders of magnitude larger than emissions to the atmosphere and the fate of this dissolved gas is ultimately unknown. Water column transit time, dissolved oxygen concentration, and specific conductivity explained the majority of variability in the emissions of both gases, but variability in CO2 emission was equally influenced by biological and physical processes whereas variability in CH4 emission is mainly influenced by biological variability. High elevation streams were, on average, net sources of CH4, CO2, and N2O to the atmosphere over the course of the observations period. However, instances of net uptake of these gases from the atmosphere by streams were also recorded during this time. Variability in mountainous gas emissions is strongly influenced by variability in the concentration gradient and less so by the reaeration coefficient. However, some site characteristics, namely elevation and silt fraction of sediments, were also contributing factors to overall emission variability. We observed a concurrent increase in N2O emission and stream dissolved organic carbon (DOC) during an algae bloom in an upstream lake which explained a large part of the seasonal variability and average emission rate. Stream sediments from these contrasting sites, some of which were adjacent to other aquatic systems, showed a range of responses to alterations of their chemical environment not unlike what occurred during the algal bloom. From these data we were able to observe that enhanced N2O production was only possible under aerobic conditions, suggesting that inefficient nitrification, as opposed to enhanced denitrification, was the source of the increase in N2O emissions.


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