Sediments have commonly been thought of as the final repositories of contaminants in marine systems, particularly of particle-reactive metals. In recent years, however, it has been recognized that some metals may be mobilized by benthic animals and bioaccumulated. These animals can acquire metals dissolved in pore waters or overlying water and also can assimilate metals associated with ingested sediments. Recent data suggest that metal bioaccumulation is related to the geochemical phases with which the metals are associated in the sediments and to the particular biological characteristics of the species in question. To date, no systematic assessment across different types of sediments, metals, and benthic animals has been made.
The objective of this project was to evaluate the bioavailability of arsenic, cadmium and chromium from sediments with divergent characteristics and relate this information to the phase speciation of these metals in the sediments. Through an understanding of the geochemical and biological processes governing uptake, models can be established to make site-specific predictions of how metals will bioaccumulate from different sediment regimes.
We studied the influence of sediment geochemistry on bioavailability of arsenic, cadmium and chromium in deposit-feeding polychaetes. Metal phase speciation in sediments was determined with a sequential extraction scheme, and assimilation efficiencies (AEs) of ingested metals were determined by pulse-chase feeding experiments using γ-emitting isotopes. Worms were fed sediments collected from geochemically diverse estuaries that were labeled by sorbing dissolved radiotracers or mixing with radiolabeled algae. Uptake of sediment-bound metals was compared to that from labeled algae or goethite.
Metal AEs showed a positive relationship with the exchangeable and carbonate sedimentary fractions, while metals in iron and manganese oxides and acid volatile sulfides, or in pyrite and other refractory material were inversely correlated with AEs. Arsenic was most bioavailable from algae (72%), less from sediments mixed with algae (24 -70%) and least from sediments labeled directly (1 – 12%). Arsenic AEs in sediments labeled directly showed a positive correlation with sedimentary Mn and Al and negative correlation with Fe. Cadmium AEs were positively correlated with salinity and negatively correlated with sedimentary organic carbon. AEs of Cr from sediments or algae were < 5%, but 34% from pure goethite.
By quantifying the relationship of metal speciation in sediments with their bioavailability for deposit-feeding polychaetes, the present study provides new insight into understanding metal bioaccumulation in benthic invertebrates.