Importance of perennial and non-perennial streams for carbon dioxide and methane emissions during rain events and drying-rewetting cycles (StreamFlux)
Final Report Abstract
Inland waters contribute substantially to the global cycling of organic carbon and emissions of carbon dioxide (CO2) and methane (CH4) to the atmosphere. While perennial streams and rivers are widely recognized as key contributors to global and large-scale emissions of CO2 and CH4, these assessments are often based on extrapolation from stable baseflow conditions without including the effect of rain events or period of reduce flow regimes, e.g., under mild droughts, or non perennial (intermittent and ephemeral) streams in general. In addition, while anthropogenic impacted lotic systems are considered in large-scale carbon budgets, artificial lotic system, such as channels, have been largely overlooked and only very recently indicated as relevant sources CO2 and CH4 to the atmosphere. During the first part of the project, we investigated seasonal dynamics of CO2 and CH4 in temperate riverine networks aiming at assessing the relevance of these lotic system for emissions, under both baseflow and during rain events. This was achieved by combining a wide spectrum of in-situ measurements, to quantify CO2 and CH4 concentrations and fluxes, hydraulic and hydrological characteristics and key physical and biogeochemical parameters (i.e., temperature, nutrients). Overall, the main drivers of CO2 dynamics within the catchment were found be less relevant for CH4 dynamics, indicating that the dynamics of these two carbon-relevant gases in streams are modulated by contrasting biophysical controls. Though rain events appeared to superimpose an additional modulation on CO2 and CH4, whose relevance in regard to baseflow was found to be season depended, both gases still showed contrasting responses to the same overall drivers. This further confirms the results of our investigations of CH4 and CH4 emissions dynamics from lowland headwaters in the southern UK (River Avon catchment), where we showed, e.g., that CO2 is largely imported from the catchment in proportion to discharge, with CO2 emissions offset by in-stream metabolism. For CH4, emissions and flow-driven dilution occur faster than biological methane oxidation in the water column, resulting in changes in streambed release of CH4 been dampened, thus minimizing diel variability. As both River Queich and Rive Avon catchments occur on different geologies and are characterized by a wide range of hydrological conditions, our observations are extending above idiosyncratic assessments of local watersheds and hints at a more widely applicable framework. During the second half of the project, we have monitored CO2 and CH4 dynamics and emissions from newly-established artificial channels (RSM - Riparian Stream Mesocosms facility). There, we followed the development of the channels from dry soil to established lotic system via riverine wetting, over the course of one summer. We have reported disproportionally high sediment accumulation, compare to the donor river and linked that to enhanced CH4 production and onset of widespread CH4 ebullition. As ebullition was found to become the dominant emission pathway, at least during the summertime, more emphasis should be given to its quantification and assessment on seasonal to annual time scales. As the accumulation sediment accumulation, regarded as emission-relevant shortcoming of artificial channels, is mostly linked to their design and operation, it very likely the dynamics observed at the RSM are representative of a large range of artificial channels and drainage ditches worldwide, regardless of whether they occur on peat, mineral soil or fully artificial substrates, such as concrete.
Publications
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Contrasting Biophysical Controls on Carbon Dioxide and Methane Outgassing From Streams. Journal of Geophysical Research: Biogeosciences, 127(1).
Rovelli, L.; Olde, L. A.; Heppell, C. M.; Binley, A.; Yvon‐Durocher, G.; Glud, R. N. & Trimmer, M.
