## 1 Introduction  
Mercury (Hg) is a naturally occurring element (from 0.021 to 0.056 mg kg\({}^{-1}\) in the Earth's crust) [1]. It is ubiquitous in the environment and presents in three main forms: elemental, inorganic, and organic, all characterised by variable toxic effects for mammals [2,3]. Hg is released into the environment from both natural and anthropogenic sources, but the latter prevails, and consequently, Hg is nowadays widespread at levels that exceed the thresholds set by environmental laws in the atmosphere, soils, riverine, and ocean systems [4, 5, 6, 7]. Hg enters the aquatic environment through wet and dry atmospheric depositions, riverine runoff, erosion of contaminated soils, and direct discharge [8]. Once in the water column, Hg settles and sinks in sediment where it accumulates but can also be re-mobilised to the upper water column (both in dissolved and particulate form), especially in case of resuspension due to natural (i.e., tidal currents, wind waves, storm events, and wave-current interaction) and anthropogenic (i.e., dredging, traveling, and fisheries activity) factors [9]. Under favourable conditions, Hg can be methylated by specific microorganisms (e.g., sulfate-reducing bacteria, SRB, Fe(III)-reducing bacteria) to form methylmercury (MeHg), an organic species that, unlike the inorganic form, tends to bioaccumulate through the trophic chain [10] and be biomagnified up to final consumers [11]. The occurrence of neurotoxic and genotoxic effects in cases of both acute and chronic exposure to Hg and its compounds, especially MeHg, is well known [12; 13]. To protect the environment and human health, the Minamata Convention on Hg was implemented, and it is actually signed by 140 countries worldwide [14]. Extraction of Hg from mining, chlor-alkali plants (CAP), coal mining, industrial wastewaters, gold mining, the chemical industry, munition manufacturing, artisanal small-scale gold mining, agriculture runoff, and oil refineries are the main anthropogenic activities responsible for the worldwide contamination of sediments [8]. In detail, mining activity is recognised as the main source of Hg for downstream sediments, with a continuous increase in extraction activity from 1500 to 1970, with peaks corresponding to silver mining in Spanish America, gold mining in North America, World War II, and massive industrial uses, followed by a steep decline until today [15]. The Mediterranean Sea can be considered a Hg hotspot for contamination of sediments, water, and biota [16]. In this area, the extraction of cinnabar (\(\alpha\)-HgS) and native Hg (Hg\({}^{0}\)) was conducted starting from the Roman Age until the last decades of the 20 th century [17; 18; 19], and a variable degree of contamination has been reported for air, water, and sediment [20]. The historical mining activity (about 500 years) conducted at Idrija (NW Slovenia) [21] caused diffuse Hg contamination in the sediments of the Isonzo/Soca River and of the alluvial plain [22], the Gulf of Trieste, and Marano and Grado Lagoon (hereafter referred to as MGL) [23; 24; 25; 26]. Even though the mining district closed in 1995, it is clear that the 35,000 T of mercury that has been released into the environment [27] is still a problem. The MGL, which is the area investigated in this paper, is still receiving periodic inputs of particulate material enriched in Hg under favourable meteorological conditions (precipitation and ENE wind direction), which enter the lagoon through the easternmost tidal inlets (Primero; about 1.4 kg of Hg in a tidal semi-cycle) [28], whereas a partial loss of enriched Hg particulate material was calculated for the Grado inlet [29]. The water from the Aussa River also flows into the central part of the MGL, which is called the Buso basin. This latter drains an industrial site where a CAP operated from 1949 and discharged uncontrolled effluents until 1984. About 186 T of Hg were deposited in the LGM [30], and the inputs of contaminated particles and water are still considerable [31].