Increasing levels of toxic mecury (Hg) is found in surface waters in boreal parts of Fennoscandia. This is of great concern, as the methylmercury (MeHg) form, can be introduced and accumulate in the aquatic food chain. One of the substances that has a great influence on transport of Hg and the formation of MeHg is the dissolved natural organic matter (DNOM) present in surface waters. DNOM binds the Hg and thereby enhances its transport from the pool of Hg in the forest floor. It is also an necessary source of energy and nutrients for the sulphur reducing bacteria that methylate Hg. The size and structure of DNOM, however, will affect how it influences the transport and methylation of Hg. This study aims to generate new insight into how DNOM influences Hg transport in the environment, and specifically, how its size may have consequences for the concentration and bioavailability of Hg. Samples from the Inlet, Hypolimnion and Outlet of the dystrophic Lake Langtjern in Norway was collected at three different seasons and size fractionated. Tangential flow filtration was used to separate the DNOM in the samples at a 100 kDa size cut-off. The water samples and their two size fractions were analysed for water chemistry, DOC concentration, monomeric organic and labile inorganic-aluminium and -iron fractions, and total Hg (TotHg) and MeHg concentrations. UV-Vis spectra of the samples were also taken to determine DNOM structure, and the bioavailability of the DNOM fractions was investigated by monitoring the oxygen consumption of bacteria added to the samples. Seasonal and spatial trends as well as the effect of explanatory factors on response parameters have been assessed. It was found that 87 to 92 % of TotHg and 87 to 94 % of MeHg are found in the HMW fractions. Moreover, the low molecular weight (LMW) fraction of DNOM is more bioavailable to bacteria: The consumption of LMW DNOM was found to be around six times faster than for the HMW fraction. This raises the question regarding how bioavailable the MeHg in the HMW fraction is as DNOM may also serve to adsorb the toxic forms of Hg , rendering them less bioavailable and thus toxic.