Effect of bioturbation on metal-sulfide oxidation in surficial freshwater sediments

Recent studies have demonstrated the role of acid-volatile sulfide (AVS) in controlling the bioavailability of several cationic metals in anoxic sediments. However, metal-sulfide complexes can be relatively labile with respect to oxidation associated with factors such as seasonal changes in rates of oxidation/production of AVS. Another potentially important mechanism of AVS oxidation in surficial sediments is bioturbation. We used different densities of the burrowing oligochaete Lumbriculus variegatus in a series of laboratory experiments to evaluate the effect of bioturbation on oxidation of AVS and subsequent bioavailability of cadmium and zinc spiked into freshwater sediments. Metal bioavailability was determined directly by bioaccumulation in the test organisms and indirectly through analysis of interstitial (pore) water metal concentrations. In our studies, horizon-specific sediment analyses were conducted to assess spatial differences in AVS and pore-water metal concentrations specifically related to organism activity. Burrowing activity of the oligochaete significantly reduced AVS concentrations in surficial sediments in a density-dependent manner and resulted in elevated interstitial water concentrations of cadmium but not zinc. Concentrations of cadmium in pore water from deeper horizons (below the zone of active burrowing) were consistently lower than those in the surficial sediments. The bioaccumulation of cadmium and zinc by L. variegatus was reflective of pore-water concentrations of the two metals, i.e., there was significant accumulation of cadmium, but not zinc, by the oligochaetes. Overall, our results indicate that bioturbation can enhance the bioavailability of some cationic metals in surficial sediments, via oxidation of AVS, and demonstrate the importance of analyzing surficial sediments when assessing bioavailability of metals in sediments.