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The biogeochemical cycling and anthropogenic inputs of cadmium and silver in the Strait of Georgia, British Columbia Kuang, Cheng


We present the temporal and spatial variations of two toxic trace metals, cadmium (Cd) and silver (Ag) in the Salish Sea, with a focus on the southern Strait of Georgia (SoG). Our results from field measurements and a box model application show that conservative mixing between riverine discharge and oceanic input largely governs the distribution of dissolved Cd, and anthropogenic influence is negligible. Conservative mixing between low-Cd river water and high-Cd Pacific water largely governs the spatial distribution and temporal variability of dissolved Cd in the Salish Sea. Spatially, concentrations decrease as the Pacific source water flows from the deep basin of Juan de Fuca Strait (818 ± 6 pmol/kg) towards the SoG basin (maximum of 717 pmol/kg) due to riverine dilution. Time-series sampling in the southern SoG between 2017 and 2018 reveals a small but significant decrease of dissolved Cd concentrations (17 ± 7 pmol/kg) in deep waters during the winter. We attributed the decline to a downward flux of dissolved Cd across the sediment-water interface due to Cd sulfide precipitation in sediment porewaters. In addition, enrichment of particulate Cd at 50 m in the water column of the southern SoG may be reflective of sulfide formation within anaerobic microenvironments associated with sinking particles, potentially released from municipal effluent. There is a scarcity of data on dissolved Ag in the open ocean and coastal waters due to analytical challenges in the quantitative determination of its extremely low concentrations in pristine environments. This study provides the first water column measurements of dissolved Ag in the SoG, using a commercially available seawater preconcentration system (seaFAST, Elemental Scientific) combined with analysis on an inductively coupled plasma mass spectrometer (ICP-MS). Dissolved Ag concentrations are variable with depth and season, ranging from 3.4 pmol/kg to 12.8 pmol/kg in the southern SoG. Results suggest that incorporation of dissolved Ag into sinking particles and subsequent desorption in deep waters play an important role in its biogeochemical cycling. Anthropogenic Ag loadings from three major sewage outfalls located in the Metro Vancouver and Capital Regional District are low compared to riverine and oceanic inputs.

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