Depth-dependent molecular composition and photo-reactivity of dissolved organic matter in a boreal lake under winter and summer conditions

[摘要] Transformations of dissolved organic matter (DOM) in boreal lakes lead tolarge greenhouse gas emissions as well as substantial carbon storage insediments. Using novel molecular characterization approaches andphotochemical degradation experiments we studied how seasonal patterns inwater column stratification affected the DOM in a Swedish lake under earlyspring and summer conditions. Dissolved organic carbon (DOC) concentrationswere consistently higher above the sediment when compared to surface watersthroughout the sampling periods. Photobleaching alone could not explainthis difference in DOC because the lake was covered by 40 cm-thick iceduring late winter sampling and still showed the same DOC trend. Thedifferences in the molecular diversity between surface DOM in winter andsummer were consistent with ongoing photobleaching/decarboxylation and apossible bacterial consumption of photo-products. Additionalphoto-degradation experiments using simulated sunlight showed a productionof highly oxidized organic molecules and low molecular weight compounds inall late winter samples and also in the deep water sample in summer. In thesurface summer DOM sample, few such molecules were produced during thephoto-degradation experiments, confirming that DOM was already photobleachedprior to the experiments. This study suggests that photobleaching, andtherefore also the ice cover during winter, plays a central role in surfaceDOM transformation, with important differences in the molecular compositionof DOM between surface and deep boreal lake waters. The release of DOC fromboreal lake sediments also contribute to this pattern. Photochemicaldegradation of DOM may be more extensive following ice-out and water columnturnover when non-light exposed and thereby photosensitive DOM isphoto-mineralized. Hence, the yearly DOM photo-mineralization may be greaterthan inferred from studies of recently light-exposed DOM.

[学科分类] 地球化学与岩石