Origin and partitioning of mercury in the polluted Scheldt Estuary and adjacent coastal zone

Estuaries and coastal zones are areas with complex biogeochemical and hydrological cycles and are generally facing intense pollution due to anthropogenic activities. An emblematic example is the Scheldt Estuary which ends up in the North Sea and has been historically heavily contaminated by multiple pollutants, including mercury (Hg). We report here Hg species and their levels in surface waters of the Scheldt Estuary and the Belgian Part of North Sea (BPNS) from different sampling campaigns in February-April 2020 and 2021. Along the estuary, Hg concentration on suspended particles ([HgSPM]) pro-gressively decreased with increasing salinity and was strongly correlated with organic matter content (%Corg) and origin (identified with delta 13Corg). While [HgSPM] drives total Hg concentration in the estuary (total dissolved Hg, HgTD is only 7 +/- 6 %), annual and daily variations of total Hg levels were mostly attributed to changes in SPM loads depending on river discharge and tidal regime. In the BPNS, a significant fraction of total Hg occurs as HgTD (40 +/- 21 %) and the majority of this HgTD was reducible (i.e. labile Hg), meaning potentially available for microorganisms. Compared to the '90s, a sig-nificant decrease of [HgSPM] was observed in the estuary, but this was not the case for [HgTD], which can be due to (1) still significant discrete discharges from Antwerp industrial area, and (2) higher Hg partitioning towards the dissolved phase in the water column relative to the '90s. Our results highlight the important contribution of the Scheldt estuary for the Hg bud-get in North Sea coastal waters, as well as the need for seasonal monitoring of all Hg species.

Automated summary

This study reports on the species and levels of mercury in the surface waters of the Scheldt Estuary and the Belgian Part of North Sea. The concentration of mercury on suspended particles gradually decreased along the estuary with increasing salinity and was strongly correlated with organic matter content and origin. In the North Sea, a significant fraction of total mercury occurs in a soluble form, potentially available for microorganisms. The results highlight the important contribution of the Scheldt estuary to the mercury budget in North Sea coastal waters and emphasize the need for seasonal monitoring of all mercury species.