Dissolved natural organic matter (DNOM) is a complex mixture of heterogeneous organic macromolecules that are formed in the environment, mainly by decay of plant and animal remains. All freshwaters contain a certain amount of DNOM, though in the boreal region the levels of DNOM are usually high. During the past decades, there has been a significant increase of DNOM in surface waters in the Nordic countries, Scotland and North-East America. DNOM’s ability to absorb light results in a characteristic yellow-brown colour of the water. DNOM has numerous functional groups giving the molecules specific properties, including serving as complexing agents for soft metals such as mercury. That way, DNOM increases the mercury mobility, thereby increasing the loading of the metals from soil to surface waters. On the other hand, the complexation of metals to especially high molecular weight (HMW) DNOM is expected to reduce the toxicity of the metals by making them less bioavailable. Long-term increase in DNOM may be associated with changes in the size fractions and thus biodegradability of the DNOM, which might affect bioavailability of mercury to the nutrient web. In this thesis, it was hypothesised that mercury (totHg) is mainly bound to high molecular weight DNOM, and that this fraction is less bioavailable than the low molecular weight (LMW) DNOM. It was further hypothesised that because HMW DNOM is larger and more aromatic, it has higher sUVa (Abs254/DOC) and sVISa (Abs400/DOC) values, as well as lower SAR (Abs254/Abs400) values than LMW DNOM, which may result in lower bioavailability of mercury in the HMW fractions. In order to study these hypotheses, samples from lake Langtjern were collected in the spring, late summer and fall at three different positions of the lake (outlet, inlet and hypolimnion (6 m depth)). The samples were fractionated using Tangential Flow Filtration (TFF) with a 100 kDa cut-off. Samples were fractionated into high molecular weight DNOM (>100 kDa) and low molecular weight DNOM (<100 kDa). The results from this study showed that the main bulk of DOC was found in the high molecular fraction, and that the concentration of totHg was higher in the HMW fractions than in the LMW fraction as viewed across all seasons and collection sites. The mean value of totHg in the HMW fraction was 3.3 ngL-1, and 0.93 ngL-1 in the LMW fraction. The mercury concentration in the LMW fraction decreased from the spring to the fall, whereas no change was observed in the HMW fractions. LMW fractions were smaller and less aromatic than the HMW fractions. Biodegradation experiments showed that the HMW fractions were less easily biodegradable than the LMW fractions, which may be seen in relation to the larger size and more aromatic character. There was a significant interaction between the fraction size and the season, and between the fraction size and the sample site in the mercury density of DNOM (measured as totHg/DOC). These interactions show that the extent to which mercury was bound to the different size fractions changed over the season and over the sampling sites. Differences in the mercury density of DNOM between the fractions were not observed in the spring, whereas in the late summer and fall samples the highest mercury density was found in the HMW DNOM. In the LMW fractions, the density of mercury in the DNOM decreased strongly from the spring to late summer and fall, whereas minor changes were observed for the HMW fractions. The interaction between the sampling sites and the fraction size showed that the differences in mercury density of DNOM was observed at the hypolimnion and in the outlet, but not in the inlet. These observations suggest that changes in the DNOM occur in the lake, which may lead to uptake of mercury in the nutrient chain. This would be in accordance with the observed higher biodegradation of the LMW fraction, measured as respiration rate.