## 4 Results and Discussion
### Temporal and Regional Hg Trends  
To reach an optimal discriminating capacity for detecting temporal and regional trends, we used a normalization procedure minimizing bias due to biological factors (see above). The relationships between the Hg content of the 504 analyzed mussels and the dry mass of their soft tissue (TW) or shell length (L) were highly significant (\(p<0.001\)). The Equation used for the normalization model built with the entire sampling set is:  
\[\text{logHg}=1.39\pm 0.15\;\text{logL}+0.49\pm 0.04\;\text{logTW}-0.36\;(\text{R}^{2 }=0.87;\text{n}=504) \tag{1}\]  
compared to:  
\[\text{logHg}=1.43\pm 0.49\;\text{logL}+0.34\pm 0.16\;\text{logTW}-0.48\;(\text{R}^{ 2}=0.91;\text{n}=143) \tag{2}\]  
obtained for the St Lawrence Mussel Watch performed in 1977-1979 [1]. Regression coefficients, obtained forty years apart, are not statistically different (\(p<0.01\)). Normalized Hg content (individual of 35 mm length and 0.17 g dry weight) varied from 10.9 to 66.6 ng per individual, with a grand mean of 42.0 \(\pm\) 2.1 ng (\(\pm\)1 standard deviation), and median of 44.2 ng, for the 504 individuals. Expressed as Hg concentration in the soft tissue, the range becomes 0.06-0.39 \(\mu\)g g\({}^{-1}\) (dw). A similar normalization procedure applied to the 1977-1979 data, gave a very similar range: 12 to 64 ng per individual mussel [1] for the same stations. This finding strongly suggests that the Hg bioavailable to marine mussels in the study area has not changed over 40 years. At first glance, these observations are surprising since a decrease in Hg inputs to the St. Lawrence Estuary and Gulf waters could be expected (see below). First, riverine Hg inputs to the St. Lawrence Estuary should have decreased due to the implementation of the International regulation on the Great Lakes basin [38; 39; 40]. Few time-series observations of Hg inputs from riverine sources, but a high-frequency sampling experiment of water was performed over 18 months in 1995-96 in the St. Lawrence River. Mercury export to the LSLE was estimated at ~1.2 Mg y\({}^{-1}\), for a mean dissolved Hg concentration of 0.60 \(\pm\) 0.46 ng L\({}^{-1}\)[41]. Since that period, no systematic study has been published on temporal variations of Hg concentrations in the St. Lawrence River. According to a recent government report, the Hg flux to the St. Lawrence River has not changed between 1995-1996 and 2004-2008 [42]. A comparison of mussel mean Hg contents at stations from the head of the LSLE (Stations 53-56), where freshwater influence is maximum, fails to show a statistically significant difference (\(p<0.01\)) in Hg tissue levels between 1977-1979 (44.6 \(\pm\) 6.9 ng, n = 5) and 2016-2019 (37.4 \(\pm\) 6.6 ng, n = 5). It could be interpreted to imply that Hg bioavailability in freshwaters of the LSLE has not changed in the last 40 years. Secondly, atmospheric Hg concentrations and wet deposition in Eastern North America are thought to have declined during the 1990-2010 period [43; 44; 45]. Regulations for reducing Hg emissions were implemented in New England and Eastern Canada [46]. In an assessment of Hg sources and fate in a marine environment very close to the Gulf of St Lawrence (The Gulf of Maine), [PERSON] et al. [47] reported that: \"Temporal patterns in sentinel species (mussels and birds) have in some cases declined in response to localized point source mercury reductions but overall Hg trends do not show consistent declines\". Likewise, studies of coastal Massachusetts, New Hampshire, and Maine, reveal that no significant temporal trends in mussel Hg concentrations were found between 1990 and 2010 at 12 of the 15 monitored stations [19]. Our results are also consistent with those of Hg trends in herring gull eggs from Atlantic Canada collected between 1972 and 2008 [48]. These authors reported that, after adjusting Hg trends for dietary shifts, environmental Hg in coastal ecosystems had remained relatively constant in Eastern Canada over the previous 36 years. According to [PERSON] et al.