Importance of CH4 and CO2 fluxes from stream and river networks: Constraints from agricultural watersheds in Southern Minnesota

Rivers play an important role in regional and global carbon (C) budgets as they transport large quantities of C from upstream and terrestrial sources to coastal and aquatic receptors. Rivers also act as biogeochemical reactors, partitioning carbon into aqueous and gaseous species, which then undergo exchange with the atmosphere. These fluxes, and their implications for greenhouse gas budgets, are poorly understood due to a sparsity of measurements, and because of the complex suite of geomorphological, biological, geochemical and meteorological factors driving the exchange. Here we present the seasonal and spatial distribution of CO₂ and CH₄ dissolved concentrations and fluxes from a stream and river network in the Cannon River Catchment (~1500 mi²) in Southern Minnesota. Approximately 65 rivers and streams (spanning stream order 1-9) were sampled, including coupled measurement of dissolved concentrations and fluxes to quantify gas transfer velocities (k). The majority of sampled streams were saturated with aqueous CH₄ (>150-2000 nmol l^-1 or higher) and CO₂ (100-400 μmol l^-1). Fluxes of CH₄ and CO₂ across different stream orders ranged from 10-150 nmol m^-2 s^-1 and 2-40 μmol m^-2 s^-1, respectively. In this presentation we will examine how water chemistry and flow regimes drive CO₂ and CH₄ exchange rates across different seasons, and present an up-scaled C-balance for the catchment and for the agricultural Corn Belt of the US Midwest.