Sedimentation in Boreal Lakes—The Role of Flocculation of Allochthonous Dissolved Organic Matter in the Water Column

We quantified sedimentation of organic carbon in 12 Swedish small boreal lakes (<0.48 km2), which ranged in dissolved organic carbon (DOC) from 4.4 to 21.4 mg C l−1. Stable isotope analysis suggests that most of the settling organic matter is of allochthonous origin. Annual sedimentation of allochthonous matter per m2 lake area was correlated to DOC concentration in the water (R2 = 0.41), and the relationship was improved when sedimentation data were normalized to water depth (R2 = 0.58). The net efflux of C as CO2 from the water to the atmosphere was likewise correlated to DOC concentration (R2 = 0.52). The losses of organic carbon from the water column via mineralization to CO2 and via sedimentation were approximately of equal importance throughout the year. Our results imply that DOC is a precursor of the settling matter, resulting in an important pathway in the carbon cycle of boreal lakes. Thus, flocculation of DOC of terrestrial origin and subsequent sedimentation could lead to carbon sequestration by burial in lake sediments.