## 4 Results and Discussion
### Temporal and Regional Hg Trends
[43],reductions in atmospheric Hg deposition from North American sources could have been offset by increased deposition from global Hg sources. Figure 3 illustrates the station-to-station variability in mussel tissue concentration. Strikingly, the continuum on the South shore of the LSLE- Gaspe Peninsula (Stations 56 to 14) exhibits a gradual Hg decrease from the brackish estuarine water originating from the St Lawrence Estuary to the Gaspesian coast as they are diluted by the Gulf waters originating from the Anticosti Gyre. This pattern strongly suggests that St. Lawrence River waters are a significant source of bioavailable Hg. Seaward, along the Gaspe Current, mussels exhibit very low Hg contents and variability (Stations 48, 49, 16 to 24, Figures 1 and 3). This would imply that Gaspe Current waters are impoverished in bioavailable Hg compared to waters of the LSLE. Increasing Hg reduction and evasion in the atmosphere in the productive waters (Anticosti Gyre and Gaspe Current, see Section 3) may favor this Hg depletion. It should be noted that the Gaspesian coastline is also an area where riverine inputs are small compared to other LSLE and Gulf shores. Another Hg dilution structure is visible along the Baie des Chaleurs (Stations 6 to 3, Figures 1 and 3) where riverine inputs are also important (see Section 3). Interesting to note is that, during the 1077-79 Mussel Watch, high Hg contents in mussel soft tissue were also observed at stations along the southern shores of the LSLE and of the Baie des Chaleurs where brackish waters are present (see Figure 4 in Ref. [1]). The North Shore of the LSLE and the Moyenne Cote Nord coastlines display inter-station large variations in mussel tissue Hg content, even within the same oceanographic entity (e.g., Moyenne Cote Nord, Figures 1 and 3). These fluctuating distributions are difficult to interpret since no steady geographical trend can be observed in Hg content. Nevertheless, some hypotheses can be put forward to explain these. Mercury content in the coastal waters of the North Shore of the St. Lawrence Estuary and Gulf are most likely influenced by upwellings and organic-rich river waters that drain through the Canadian Shield soils [49; 50]. The organic matter of these circumneutral pH waters contains strong Hg-binding functional groups (e.g., thiol) that favor Hg transport in solution [51; 52; 41; 53]. On the other hand, upwellings bring saline low Hg waters of Atlantic origin to the surface, especially in the Minganie region (Stations 44-47, Figures 1 and 3) [54]. These Atlantic waters enter the Gulf through  
Figure 3: Mercury content (ng) of individual mussels after normalization to 35 mm-long and 0.17 g virtual specimens. Mean contents are indicated by a horizontal bar and the median by a cross. Each station combines 10 to 12 individual mussels. Stations are eastward distributed on the x-axis. Cabot Strait and are known to contain low picomolar Hg levels [55]. Conversely, waters entering the Northeastern Gulf through the Strait of Belle Isle originate from the Labrador Sea Current, known for its relative high Hg concentrations due to the inputs of the organic-rich rivers outflowing the Arctic Canadian Archipelago [56]. These waters, brought by the Labrador Sea Current, are transported westward along the J. Cartier Passage, a remote region without any known trace-metal sources but with relatively high Hg content in sampled mussels (Figures 1 and 3). It is interesting to note that low Hg content mussels are located along the Gaspe Peninsula (Stations 48, 49, 16-24) and Minganie (Stations 44-47) coasts, where commercial or experimental mussel farms are installed (mapaq.gouv.qc.ca/fr/Peche/aquaculture) [57; 58]. This latter area is known for its low-temperature upwellings in summer [54].