Linking allochthonous dissolved organic matter and boreal lake sediment carbon sequestration: The role of light-mediated flocculation

We measured flocculation of dissolved organic carbon (DOC) in the water from a humic lake (DOC 5 14.9 mg C L21) and from an adjacent mire (DOC 5 25.7 mg C L21), in in situ enclosure experiments with different light regimes. Light stimulated the formation of organic particles in both waters, and organic particle formation was observed at all incubation depths, even in the dark controls. Production of phytoplankton biomass was negligible, and allochthonous DOC was the most important precursor of the sinking particles. 8–22% and 25– 60% of the loss of DOC in lake and mire water, respectively, could be accounted for by flocculation. Depthintegrated flocculation based on the enclosure experiments was 14.7 mg C m22 d21. Lake-water DOC concentration and water color has been increasing during the last decade, and sediment trap studies show that gross sedimentation of organic carbon also increased. Thus flocculation of allochthonous DOC, stimulated by light, constitutes a pathway for the sequestration of carbon in lake sediments. Inland waters receive large amounts of organic carbon from their watersheds, but only about half of the organic carbon reaches the sea (Algesten et al. 2003; Cole et al. 2007). Most of the loss occurs in lakes (Algesten et al. 2003; Cole et al. 2007), primarily because of mineralization into CO2 followed by evasion to the atmosphere (Kling et al. 1991; Cole et al. 1994) and retention in sediments (Molot and Dillon 1997). Lake sediments are a considerable sink of organic carbon in boreal ecosystems. A study in boreal Finland suggests that carbon sequestration at the landscape level is greater in lake sediments than in forests and terrestrial soils (Kortelainen et al. 2004). Dissolved organic carbon (DOC) is the dominating fraction of organic carbon in most lake waters (Wetzel 2001), and in the unproductive forest lakes with high loads of allochthonous organic carbon typical of the boreal zone, particulate organic carbon (POC) generally constitutes less than 3–10% of the total carbon (Wetzel 2001; Kortelainen et al. 2006). Because most of the organic carbon is dissolved, transformation of DOC into a gravitoidal state, i.e., coagulation and flocculation into sinking POC, appears to be an important prerequisite for allochthonous carbon sequestration in lake sediments. However, the extent and mechanisms of such transformation processes are currently not well understood. Solar radiation, especially in the ultraviolet (UV) range, has a multitude of effects on the organic matter in aquatic systems, such as changes in structure, molecular weight, and optical properties (Bertilsson and Tranvik 2000). Solar radiation induces cleavage of high-molecular-weight DOC into a variety of photoproducts and inorganic carbon (Mopper et al. 1991; Moran and Zepp 1997; Bertilsson and Tranvik 2000), and increasing bioavailability of dissolved organic matter (DOM) to bacteria (Lindell et al. 1995; Wetzel et al. 1995). It has also been indicated that DOC can be transformed to particles by photochemical reactions. Irradiation of DOC has been reported to stimulate particle formation, involving iron as catalyst (Gao and Zepp 1998). Similarly, intense irradiation of humic water cleaved humic molecules into fragments, but also within hours resulted in a precipitate of brownish particles (Backlund 1992). Based on these observations we hypothesize that light-mediated flocculation of DOC into POC relocates organic carbon from the water column to the sediments. We conducted enclosure experiments and sediment trap campaigns, demonstrating flocculation of DOC into POC under natural conditions, and showing that sunlight plays an important role in the transformation from a dissolved or 1 Corresponding author ([email protected]).