Factors controlling the temporal variability and spatial distribution of dissolved cadmium in the coastal Salish Sea

Abstract

Cadmium (Cd) is a transition metal that can be toxic to aquatic biota even at low levels. The adverse effects of Cd are of particular concern in waterbodies along the eastern margin of the Pacific Ocean, where anthropogenic inputs are superimposed on the naturally elevated oceanic concentrations. While the accumulation of Cd in marine bivalves is well documented, few studies have investigated the cycling of dissolved Cd in estuarine and coastal waters that are susceptible to Cd enrichment. Here we examine the temporal and spatial variations of dissolved Cd in the coastal waters of Salish Sea in the northeast Pacific, with a focus on the Strait of Georgia (SoG). We find that the distribution of dissolved Cd in the Salish Sea is largely governed by conservative mixing between the upwelled Pacific water and freshwater discharge from the Fraser River, with a biological overprint transferring some of Cd from surface to deep water. Anthropogenic loadings of Cd from municipal effluent are negligible compared to natural inputs. Spatially, dissolved Cd and salinity gradually decrease as the Pacific source water flows landward from Juan de Fuca Strait into the SoG. Time-series sampling in the southern SoG reveals a small but significant decrease of dissolved Cd concentrations in the SoG deep layer over a winter stagnation period. We attribute this decline to diffusive removal, as driven by Cd sulfide precipitation in suboxic porewaters. The intermittent stagnation of the SoG deep layer provides a natural “benthic chamber” to estimate elemental exchange between deep water and sediment on a basin-scale, which could be valuable for studying the biogeochemical cycling of other trace metals.