## 4 Discussion  
A complete investigation of Hg contamination in the MGL is an issue that cannot be neglected because of the important ecological and socio-economic values of this coastal ecosystem [41]. As previously stated, the contamination of surface sediment involves the whole lagoon area and up to 1 m in depth [22; 35]. Thus, it represents a permanent issue, and _in toto_ reclamation is unfeasible for both economic and ecological concerns. The effects of re-suspended sediments on Hg biogeochemical cycling are well known and still a matter of debate [9; 132; 133; 134; 135]. The situation is comparable to that found inland through the Isonzo River plain, where specific risk management methods are currently under investigation [22]. The MGL is also an important habitat for fish fauna and a nursery ground for juvenile and migratory species, and it has been a suitable location for fish farm activities since the 16 th century. Nowadays, about forty fish farms of variable dimensions and potential productivity are still active [112], and the restoration of some abandoned areas is in progress. Today the main source of Hg and its more toxic form, MeHg, for humans is fish consumption [135] and exposure can cause progressive damage to the nervous system, particularly in foetuses and children (i.e., decreasing fine motor-adaptive function, reduction in IQ, attention deficit disorder) [136]. It is well known that the degree of bioaccumulation/biomagnification along the trophic chain depends on several factors. At first glance, total Hg content appears to be the most important factor; however, MeHg production via specific bacterial activity and, to a lesser extent, abiotic processes are strongly dependent on key factors such as physico-chemical conditions (i.e., oxygen content, pH, solar radiation/temperature, labile natural organic matter, water residence time), and almost entirely on Hg speciation [137; 100; 138]. Thus, it surprises that heavily contaminated sites (i.e., Pialassa Baiona) do not show a correspondent bioaccumulation in selected organisms [81]. In the MGL, the not-mobile fraction of Hg (Hg-S) prevails, and this is a factor that normally limits the bioavailability of Hg for methylation. On the other hand, the most contaminated areas of the Grado basin are characterised by a higher degree of bioaccumulation. In addition, relative production/fluxes of MeHg seem to be site-specific, being quite different among sites characterised by comparable Hg contamination [26; 36]. As a result, it is possible to hypothesise that a better understanding of organic matter composition and its behavior, whether labile or not, could be useful in determining the potential for methylation [137]. It should be noted, however, that the LMG shows comparable Hg recycling  
Figure 6: THQ in the water bodies of the Grado (yellow) and Marano (green) Lagoons. coupled to MeHg production in the adjacent Gulf of Trieste and that this is more intense during the summer period [36]. It can be noticed that, despite the Hg contamination, the relative amount of Hg in the water column does not exceed the limit set by the WFD, and it was significantly lower than those observed in the Gulf of Trieste, probably as a consequence of the solid-phase porewater distribution coefficient (\(K_{\rm D}\), L kg\({}^{-1}\)) [139] that is about one order of magnitude (\(10^{6}\) vs. \(10^{5}\)) higher than that of the gulf. Exposure to Hg (MeHg) through fish consumption is a great concern, especially during the pre-natal period [140], but the studies conducted in different contexts showed no clear evidence [141]. Due to the potential adverse effect on residents, epidemiological studies were also conducted on residents of the Friuli-Venezia Giulia region. Hg analysis in human hair from residents of the Gulf of Trieste revealed that a risk alert seems inappropriate, although it appeared prudent to limit the ingestion of local fish to <4 servings/month for pregnant women and children [142].