Uptake of 109Cd from sediments by the bivalves Macoma balthica and Protothaca staminea

Abstract

This study (1) compared the relative importance of suspended particulate matter (SPM) and deposited sediment (DS) as a source of cadmium exposure to two sediment ingesting bivalves, and (2) determined the importance of feeding behaviour on cadmium uptake from SPM and DS by comparing metal uptake from ingested sediments by a facultative and an obligate filter-feeder. Two types of sediment, deposited (DS) and suspended (SPM) sediments were sampled from the Fraser River Estuary and geochemically characterized with respect to amounts of easily reducible manganese and reducible iron (manganese and iron oxides respectively), organic matter, and amounts of cadmium associated with each of the three components. Sediments were radiolabeled with 109Cd, with the labelled sediments then fed to Macoma balthica and Protothaca staminea, a facultative filter-feeder and an obligate filter-feeder respectively. Amounts of radiotracer accumulated by the bivalves from the two types of sediment over an 8-day period were then assessed. The geochemistry of DS was distinct from that of SPM with DS containing almost half the concentrations of oxides of manganese and iron as compared to SPM. Natural cadmium was recovered primarily from the easily reducible (associated with oxides of manganese) followed by the reducible (associated with oxides of iron) sediment component, for both SPM and DS. In contrast, 109Cd was recovered from the reducible fraction, followed by the easily reducible fraction from the two types of sediment. No natural cadmium or 109Cd was recovered from the organic component of either sediment. For both bivalves, uptake of 109Cd from sediment as compared to water accounted for ca. 80% of accumulated radiotracer. Uptake of 109Cd from DS was significantly greater than from SPM, most notably for P. staminea. For M. balthica, the amount of accumulated 109Cd was best explained by the degree of isotope desorption from DS. In contrast, 109Cd accumulation by P. staminea was best described by its feeding behaviour. Metal accumulation by sediment-ingesting organisms is not simply related to sediment metal concentrations but will also depend on the strength of association of the metal within the sediment (for facultative feeders), and the feeding behaviour of the bivalve. 109Cd partitioning in DS and SPM did not parallel the field partitioning of cadmium which precluded our objective of assessing the relative importance of SPM and DS as a source of cadmium to facultative filter-feeders. To elucidate the role of complex sediment geochemistry in influencing metal availability to sediment ingesting organisms, further study is needed on ways to either label or spike natural complex sediments such that the speciation achieved under laboratory conditions is that which is observed in the field.