CO2 dynamics along Danish lowland streams: water–air gradients, piston velocities and evasion rates

Abstract

We measured CO2 concentration and deter- mined evasion rate and piston velocity across the water- air interface in flow-through chambers at eight stations along two 20 km long streams in agricultural landscapes in Zealand, Denmark. Both streams were 9-18-fold supersaturated in CO2 with daily means of 240 and 340 lM in January-March and 130 and 180 l Mi n June-August. Annual CO2 medians were 212 l Mi n six other streams and 460 lM in four groundwater wells, while seven lakes were weakly supersaturated (29 lM). Air concentrations immediately above stream surfaces were close to mean atmospheric conditions except during calm summer nights. Piston velocity from 0.4 to 21.6 cm h -1 was closely related to current velocity permitting calculation of evasion rates for entire streams. CO2 evasion rates were highest in midstream reaches (170-1,200 mmol m -2 day -1 ) where CO2-rich soil water entered fast stream flow, while rates were tenfold lower (25-100 mmol m -2 day -1 ) in slow- flowing lower reaches. CO2 evasion mainly derived from the input of CO2 in soil water. The variability of CO2 evasion along the two lowland streams covered much of the range in sub-Arctic and temperate streams reported previously. In budgets for the two stream catchments, loss of carbon from soils via the hydrolog- ical cycle was substantial (3.2-5.7 mmol m -2 day -1 ) and dominated by CO2 consumed to form HCO3 - by mineral dissolution (69-76%) and export of organic carbon (15-23%) relative to dissolved CO2 export (7-9%).