## 1 Introduction
The first study on Hg in the MGL was conducted by [PERSON] [32], who reported a variable degree of contamination in sediment (from 1 to 10 mg kg\({}^{-1}\)) and biota. [PERSON] et al. [30] made the first effort to investigate the potential behaviour of Hg trapped in sediment by applying the pyrolytic speciation technique proposed by [PERSON] et al. [33]. The authors found cinnabar compounds (mostly stable and insoluble), but only a partial area of the MGL was considered. A more comprehensive work was conducted in the framework of the MIRACLE (Mercury Interdisciplinary Research project for Appropriate Clam farming in Lagoon Environment) Project (2008-2009). The project aimed to understand Hg biogeochemical cycling by testing the coexistence of clam farming with Hg contamination in the sediments. Hg was measured in water, sediment, and biota, and its mobility and speciation were also considered together for bioaccumulation in clams [34]. The Hg distribution showed a positive gradient moving eastward (from 0.68 to 9.95 mg kg\({}^{-1}\)), with MeHg accounting for 0.08% of the total pool on average [22] and a sediment thickness involved in the contamination of more than 1 m [35]. The potential mobility of Hg at the sediment-water interface was assessed by calculation of diffusive fluxes, in situ deployment of benthic chambers (natural conditions) [36; 37], and mesocosm experiments (perturbed conditions) [9]. The authors stated that sediments could be a primary source of MeHg for the water column, especially in cases of anoxic events, and that this transfer is more active in the Grado sector, whereas the effects of a resuspension are limited in time and trivial if compared to lagoon-wide processes. Subsequent research supports the hypothesis that unusual habitats, such as salt marshes, actively participate in Hg recycling [38; 39]. It is well-known that the determination of total Hg is not useful to understanding the fate of this element and its potential toxicity for the aquatic environment. Thus, numerous speciation techniques are currently applied to define its behaviour [40]. In the MGL, both chemical and thermo-desorption techniques were applied and revealed the presence of both cinnabar and non-cinnabar compounds in variable percentages [26; 30]. Molluscs and fish play an important role in the MGL's economy [41] and seafood consumption is extremely beneficial for the population as a source of proteins, fatty acids, vitamins, and minerals. However, in a contaminated environment such as the MGL, the risks associated with the exposure of Hg/MeHg cannot be neglected and should be carefully considered [42]. In the work conducted by [PERSON], the degree of Hg bioaccumulation was quite variable, with the highest values (up to 5 mg kg\({}^{-1}\) wet weight, ww) commonly found for carnivorous fish [32], whereas in the soft tissues of Manila clams (_Ruditapes philipinarum_), Hg contents were significantly lower (0.051-0.9 mg kg\({}^{-1}\) ww) [43; 44]. Recently, [PERSON] and [PERSON] [45] investigated the relationship between total Hg content and some specific factors (i.e., sampling sites and season, size, and sex) for the grass goby _Zosterisessor ophiocphalus_. Total Hg was 0.61 \(\pm\) 0.28 mg kg\({}^{-1}\) (ww), on average, with a maximum of 1.67 mg kg\({}^{-1}\) ww. Thus, the strict legal limits established for marketed sea foods [46] and for water quality classification [47] (0.5 and 0.02 mg kg\({}^{-1}\) ww, respectively) are often overcome. In this paper, we report the results obtained from a complete survey on sediments, water, and biota conducted during the implementation of the WFD/2000/60/CE (from 2010 to 2021) [47]. The sediments were analysed for total Hg contents and speciation to determine potential bio-accessibility for biota [48]. Moreover, the quality of sediment was checked through the application of common indices of contamination [49; 50; 51]. Data on total Hg in the water column were scarce and referred to a limited area of the MGL [39], thus the results reported in this paper represent a novelty.