Photo-reactivity of dissolved organic carbon in the freshwater continuum

The patterns in dissolved organic carbon (DOC) photo-mineralization along the freshwater continuum from land to sea are poorly known. Specifically, it has not been resolved how the photo-degradation ofDOC into CO2 (PD)depends on the combination of intrinsic properties of DOC and extrinsic variables that affect the photo-reactions. We measured PD per unit of absorbed ultraviolet light energy (PD-E-w) in headwater streams, lakes, intermediate rivers and river mouths in Sweden. Surprisingly, no trend of decreasing PD-E-w was found with decreases in colored DOC. However, there was a relationship between PD-E-w and pH, best described by a quadratic (U-shaped) curve, indicating environmental control of photo-reactivity. Interestingly, the highest values for both of these variables were recorded for river mouths. Moreover, PD-E-w increased with proxy variables for the amount of autochthonous DOC in the water. Thus, changes in pH and autochthonous DOC input along the continuum may sustain high DOC photo-mineralization throughout continental aquatic networks.
Funding Agencies|Lund University; Helge Ax:son Johnsons Stiftelse [130622]; Kungliga Fysiografiska Sallskapet i Lund [32953]; FORMAS [239-2014-698, 210-2012-1461]; Swedish Research Council VR [2012-00048]