Greenhouse gas emissions from non-perennial streams

Micha Nebel

doi:10.25918/thesis.591

Non-perennial rivers make up more than half of global river networks and ~70% of Australia’s. Increasing extreme weather driven by climate change, together with human disturbances such as channelisation, damming, and irrigation, may push more rivers toward non‑perennial flow regimes. However, the major greenhouse gases (GHGs) carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) remain poorly quantified in these systems. There is a limited understanding of the mechanisms driving differences in GHG fluxes across wet and dry conditions, as well as the role of diel and land use variation. This research project quantified GHG fluxes from non-perennial streams in subtropical Australia and investigated the processes underlying these fluxes. The investigations included diel and seasonal GHG measurements in nine non-perennial streams under different land uses along the coast of north-eastern New South Wales (NSW) and were conducted bimonthly over a year. Fluxes to the atmosphere were measured across three hydrological conditions: exposed sediments, stagnant water, and flowing water. Alongside measurements of dissolved organic carbon, dissolved nutrients, and GHG concentrations in water. Additionally, channel physics, physiochemical parameters including dissolved oxygen, and weather conditions were monitored. Random Forest models and multi-factor analysis were used to detect key drivers of observed differences in diel and seasonal GHG fluxes. Further, annual estimates were calculated using reach-scale area-weighted GHG fluxes. I found that, overall, stream diel CO2 and CH4 fluxes did not increase with land use intensity in the catchment, whereas N2O fluxes revealed a land use effect, with higher fluxes from agricultural sites compared to forest sites. I found that night CO2 fluxes were elevated compared to day fluxes, whereas N2O revealed an opposite pattern. CH4 fluxes were higher at night at exposed wet sediment and stagnant water conditions, likely attributed to enhanced ebullition triggered by night time temperature drops. I found that hydrological conditions were the main control on GHG dynamics. CO2 and N2O peaks were measured during flowing conditions, and CH4 peaks were measured during exposed wet sediment conditions. Oxygen and substrate (organic carbon and nitrate) availability were strong predictors for GHG fluxes across hydrological conditions. Annual estimates revealed a disproportionate contribution of exposed streambed sediments for CO2 and CH4, whereas N2O emissions were dominated by the contribution of flowing conditions. These results underscore the importance of monitoring GHG fluxes across diverse hydrological conditions and wetting-drying dynamics to reduce uncertainties in the global GHG budget.

Greenhouse gas emissions from non-perennial streams

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Greenhouse gas emissions from non-perennial streams

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