We present high‐resolution vertical profiles of methylated mercury (MeHgT = monomethylmercury + dimethylmercury) concentrations in the water column of the open and marginal areas of the Mediterranean Sea. MeHgT concentrations ranged from <0.015 pmol L⁻¹ to 0.430 pmol L⁻¹ (n = 130), with maximal concentrations occurring within the maxima of oxygen consumption. Within the most biologically active zone (0‐600 m), the MeHgT vertical profiles exhibited a nutrient‐like pattern. The MeHgT vs. phosphate relationships were highly significant (p < 0.001), whichever the station. We argue that most of the MeHgT in the water column originates from in situ methylation of inorganic HgII associated with the regeneration of the organic matter. The demethylation of MeHgT as water ages seems responsible for the MeHgT distribution in deep waters. The influences of the methylmercury inputs from the atmosphere, rivers, and coastal or abyssal sediments appear to be locally restricted.

We report here the results of total mercury (HgT) determinations along the 2014 Geotraces Geovide cruise (GA01 transect) in the North Atlantic Ocean (NA) from Lisbon (Portugal) to the coast of Labrador (Canada). HgT concentrations in unfiltered samples (HgT_UNF) were log-normally distributed and ranged between 0.16 and 1.54 pmol L⁻¹, with a geometric mean of 0.51 pmol L⁻¹ for the 535 samples analysed. The dissolved fraction (< 0.45 µm) of HgT (HgT_F), determined on 141 samples, averaged 78 % of the HgT_UNF for the entire data set, 84 % for open seawaters (below 100 m) and 91 % if the Labrador Sea data are excluded, where the primary production was high (with a winter convection down to 1400 m). HgT_UNF concentrations increased eastwards and with depth from Greenland to Europe and from subsurface to bottom waters. The HgT_UNF concentrations were similarly low in the subpolar gyre waters ( ∼ 0.45 pmol L⁻¹), whereas they exceeded 0.60 pmol L⁻¹ in the subtropical gyre waters. The HgT_UNF distribution mirrored that of dissolved oxygen concentration, with highest concentration levels associated with oxygen-depleted zones. The relationship between HgT_F and the apparent oxygen utilization confirms the nutrient-like behaviour of Hg in the NA. An extended optimum multiparameter analysis allowed us to characterize HgT_UNF concentrations in the different source water types (SWTs) present along the transect. The distribution pattern of HgT_UNF, modelled by the mixing of SWTs, show Hg enrichment in Mediterranean waters and North East Atlantic Deep Water and low concentrations in young waters formed in the subpolar gyre and Nordic seas. The change in anthropogenic Hg concentrations in the Labrador Sea Water during its eastward journey suggests a continuous decrease in Hg content in this water mass over the last decades. Calculation of the water transport driven by the Atlantic Meridional Overturning Circulation across the Portugal–Greenland transect indicates northward Hg transport within the upper limb and southward Hg transport within the lower limb, with resulting net northward transport of about 97.2 kmol yr⁻¹.

Various species of marine mussels have been used, in the last 50 years, as sentinel organisms for monitoring metal contamination along marine coasts. There are two main reasons for this: these mollusks concentrate metals in their soft tissue and they are geographically widespread. In practice, trace metal concentrations in mussel soft tissue reveal (after some correction for biotic effects) the contamination level of their surrounding environment. We present the results of a mercury (Hg) survey in Mytilus spp. collected in the summers of 2016, 2018, and 2019 at 51 stations distributed along the coasts of the Estuary and Gulf of St. Lawrence. Mercury concentrations ranged from 0.063 to 0.507 µg g⁻¹ (dry weight, dw), with a grand mean of 0.173 ± 0.076 µg g⁻¹ dw (±1 standard deviation), and a median of 0.156 µg g⁻¹ dw for the 504 individuals analyzed. Mercury contents per individual mussel were significantly (p < 0.01) related to shell length and dry tissue weight, with the smaller individuals having the highest Hg concentrations. To take into account these biotic effects, we normalized Hg concentrations of the mussel soft tissue for constant shell length (L) and soft tissue weight (TW) based on the log-log relationships between Hg content and L or TW. The normalized Hg contents of mussels varied from 10.9 to 66.6 ng per virtual individual of 35 mm length and 0.17 g dry weight. A similar normalization procedure applied to 1977–1979 data, yielded a very similar range: 12 to 64 ng. This observation suggests that the Hg bioavailable to marine mussels in the study area did not change over a span of 40 years. Regional Hg distribution patterns indicate a gradual decrease of Hg content in mussels downstream from freshwater discharges to the St. Lawrence Estuary and the Baie des Chaleurs, suggesting that rivers constitute a significant Hg source in these estuarine systems. Atmospheric Hg deposition and concentration in marine waters of the Atlantic Ocean are known to have decreased in the last decades. However, in coastal environments, the response to these changes does not seem to be rapid, probably because of the long residence time of Hg in soils before being exported to coastal areas.