Uptake of dissolved Ag, Cd, and Co by the clam, Macoma balthica:: Relative importance of overlying water, oxic pore water, and burrow water

The facultative deposit-feeding clam Macoma balthica is used as a bioindicator organism for assessing coastal metal contamination, Previous work has evaluated the assimilation of metals from different possible food sources for this clam, but no studies have measured the uptake rates of metals from different dissolved sources, This study specifically compares three different dissolved sources: overlying water (SW), oxic pore water (OPW) from a depth of <1 cm (entrained during surface deposit feeding), and burrow water (BW) (a mixture of anoxic pore water and overlying water). Uptake rates of dissolved Ag, Cd, and Co in M. balthica were measured in short-term laboratory experiments using radiotracers. Clams were exposed to metals in water only for SW and surface OPW treatments, In the BW treatment, metal uptake was compared in clams placed in radiolabeled organic-poor or organic-rich sediment under conditions in which feeding was inhibited. Uptake rate constants from SW for Ag, Cd, and Cc were 0.35, 0.033, and 0,035 Lg(-1) day(-1), respectively, Lower uptake of dissolved metals from OPW was noted but was only significant for Co. Metal uptake from BW and SW were also comparable; however, the trend showed lower Ag and higher Co uptake from BW. Metal distributions and concentrations in the two radiolabeled sediments were affected by active irrigation of SW into the burrows; dissolved metal concentrations in BW were approximately 30% lower than that in the bulk pore water concentrations. In the organic-rich sediment, Cd and Ag partitioned more in the dissolved phase (<0.2 mum) and Co more in the particulate phase as compared with the organic-poor sediment. A sensitivity analysis using measured rate constants for uptake and a range of metal concentrations from field studies suggested that, under most conditions, uptake of dissolved Ag is primarily from OPW, Co is mostly from BW, and Cd uptake varies depending on its concentration in each compartment. Little Co or Ag is likely to be taken up from SW, whereas 20-50% of Cd may be accumulated from this source. Thus, SW, OPW, and BW are all potential sources of metals for M. balthica, and the relative importance of these sources differs among metals and is dependent on the dissolved metal concentrations in each compartment.