###### Abstract  
At the Nalon River estuary (Astruias, Northern Spain), the occurrence of Hg is due to historical mining activity which has resulted in environmental issues of great concern. Although several studies have investigated the sediment compartment regarding Hg contamination, no information is currently available on the fate of Hg and MeHg in the water column. Considering different hydrodynamic/seasonal conditions, water samples were collected along the estuary to evaluate Hg and MeHg distribution and partitioning behaviour between solid and aqueous phases. The complementary effect of the river discharge and tidal currents contributed to the prevalence of the dissolved (\(4.02\pm 1.33\) ng L\({}^{-1}\)) or particulate (\(8.37\pm 4.20\) ng L\({}^{-1}\)) Hg under different conditions of discharge in summer and autumn, respectively. Conversely, particulate MeHg prevailed when the river flow was low, especially at the estuary mouth (\(25.8\pm 19.1\) pg L\({}^{-1}\)) and most likely due to the resuspension of fine particles promoted by a stronger tidal current. In comparison with the total Hg concentration, extremely low amounts of dissolved and particulate MeHg were observed, and strong interactions between MeHg and organic carbon highlighted a negligible risk of increased mobility and potential bioaccumulation of MeHg. eurm; methylmercury; water column; suspended particulate matter +
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may also be affected by resuspension events induced by natural (e.g., tidal currents, riverine flow conditions, wave action and bioturbation) [15; 16; 17] and anthropogenic factors (e.g., dredging, shipping and trawling) [18; 19; 20; 21]. This is of relevant concern, especially in the case of shallow aquatic systems where sediment resuspension can be considered the main factor governing the redistribution of particles as well as potentially toxic trace elements adsorbed on their reactive surface [22]. The coastal area of Asturias (northern Spain) is one of the most economically productive regions due to historical extraction activity of coal and sulphide ores (mainly _cinnubar_, HgS). Indeed, Asturias is known as one of the largest Hg producers worldwide and the main Hg deposits are in Mieres and Pola de Lena (the La Pena-Terronal and La Sotterana mining districts, respectively) [23; 24]. Although this long-term mining activity has certainly promoted the economic growth of the region, it can also be identified as the primary source of several environmental issues. Indeed, several studies focused on different environmental matrices such as soils, mine tailings, vegetation, surface water and sediments in areas affected by Hg mining [25; 26; 27; 28; 29; 30] highlighted the occurrence of notable inputs of Hg and other potentially toxic trace elements to the Nalon River drainage basin, which is the main hydrological system in Asturias, and its estuary [25; 29; 31; 32]. At the Nalon River estuary, the behaviour of Hg has been investigated with respect to Hg contamination in estuarine sediments and salt marshes [33; 34; 35], Hg bioaccumulation in halophytes [36] and Hg mobility in resuspended contaminated estuarine sediments [37]. Conversely, there is a lack of information pertaining to the water column. Indeed, the primary aim of this research is to assess the occurrence of Hg and MeHg and their water column distribution along the Nalon River estuary focusing on the partitioning behaviour between solid (suspended particulate matter, SPM) and dissolved phases as well as on the effects of different seasonal conditions during low-normal river discharge. Investigation into the mobility and fate of dissolved and particulate Hg and MeHg, as well as their potential mobility in the estuarine environment of the Nalon River, is of great importance with reference to the goals of the European Union's Water Framework Directive (WFD, European Parliament, Council of the European Union, 2000) and the European Union's Marine Strategy Framework Directive (MSFD, European Parliament, Council of the European Union, 2008) [38; 39].