Investigating the dynamics controlling dissolved organic matter bioreactivity in agricultural stream water and its link to carbon evasion : A statistical analysis

Abstract: While streams and rivers emit large quantities of CO2 to the atmosphere, both the mechanisms controlling CO2 evasion as well as the sources of CO2 are still uncertain. One potential source of stream CO2 is in-stream degradation of dissolved organic matter (DOM). Streams and rivers transport, stores and transform large quantities of DOM, which can be more or less biologically reactive (bioreactive) meaning how easily accessible the organic material is for biological degradation. Little is known about the dynamics controlling DOM bioreactvity in agricultural stream water. This project investigated potential physico-chemical and land-use controls of DOM bioreactivity and its link to CO2 evasion in ten Swedish agricultural streams by correlating estimated values of DOM bioreactivity as well as CO2 evasion rates with different parameters for physico-chemical and land-use controls measured during eleven field excursions. A direct positive correlation was found between DOM bioreactivity and CO2 evasion rates during spatial correlation but not when all measurements were used. DOM bioreactivity was found to correlate to parameters for DOM quality and quantity, concentrations of dissolved oxygen (D.O.) and total dissolved solids (TDS) as well as stream width, water depth, specific discharge, temperature and electric conductivity. Nutrients, fraction land-use, catchment size, vegetation cover and turbulence were not found to be potential controls of either DOM bioreactivity or CO2 evasion. CO2 evasion was found to have fewer significant correlations with parameters for DOM quality compared to DOM bioreactivity, indicating a weaker link between CO2 emissions and DOM quality in the stream water, and was not tied to variables affected by increased discharge. Spatial and temporal analysis indicated that while CO2 evasion rates were more site specific, bioreactivity was more influenced by temporal variations where periods of increased bioreactivity and increased variability between sites occurred during periods of increased specific discharge, supporting existence of a hydrological influence on DOM bioreactivity.