| The geochemical and biological mechanisms that control the accumulation of sediment-bound metals by benthic organisms are poorly understood and there are conflicting assumptions regarding the relative importance of uptake from dietary or dissolved sources. A series of laboratory experiments was performed using gamma-emitting radiotracers to evaluate the extent to which Cd, Co, Ag, Se, Cr, and Zn bound to geochemically diverse particles are assimilated by the mussel Mytilus edulis and the clam Macoma balthica. The assimilation efficiencies (AE) of oxidized and reduced radiolabeled sediments ranged from 1% for Cr to 42% for Zn. Sediment-bound metal AEs ranged from 12 to 22% for Ag, 2 to 13% for Cd and 8 to 20% for Co. AEs from phytoplankton (Thalassiosira pseudonana) were higher than from sediment AEs and ranged from 36 to 42% for Ag, 47 to 55% for Cd and 27 to 33% for Co. Both animals assimilated metals from anoxic sediment and AEs were two-fold lower then AEs from oxic sediment in M. balthica but in M. edulis AEs were about two times greater from anoxic sediment for all metals except Ag. Similarly, Ag, Cd, and Co AEs from radiolabeled iron mono-sulfide (AVS) and iron oxide particles, two important binding phases in sediment, followed the same trend as the oxidized and reduced sediments. The amount of time metals were exposed to sediment had a greater effect on M. edulis in which Cd, Co, Zn, and Se AEs decreased 2 to 4-fold over 35 d whereas metal AEs for M. balthica only decreased after 6 mo. Metal AEs were compared with radiolabeled particles extracted by, (1) sequential extraction, (2) pH 5 or pH 8 seawater for 3 h, or (3) gut juices from the two bivalves over the time period digestion. Metals extracted and desorbed in pH 5 corresponded best with metal AEs in M. edulis and Ag AEs corresponded closely with the amount of Ag dissolved in the gut juice for both clams (pH 5.0) and mussels (pH 5.5), but for other metals and extractants, correlations were inconsistent. In M. edulis, AEs of a single metal varied as much as 5-fold among different particle types compared with 2-fold variation in M. balthica. The more distinct effect of particle geochemistry on metal AEs in M. edulis may be explained partly by their less intensive and more rapid digestive processing compared with M. balthica which have an 4-fold longer initial gut passage time, and gut chemistry that has a higher concentration of surfactants and an overall lower gut pH. (Abstract shortened by UMI.)... |
