EFFECT OF BIOTURBATION ON METAL-SULFIDE OXIDATION IN SURFICIAL FRESH-WATER SEDIMENTS

Recent studies have demonstrated the role of acid-volatile sulfide (AV S) in controlling the bioavailability of several cationic metals in an oxic sediments. However, metal-sulfide complexes can be relatively lab ile 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 bioturb ation. We used different densities of the burrowing oligochaete Lumbri culus variegatus in a series of laboratory experiments to evaluate the effect of bioturbation on oxidation of AVS and subsequent bioavailabi lity of cadmium and zinc spiked into freshwater sediments. Metal bioav ailability was determined directly by bioaccumulation in the test orga nisms and indirectly through analysis of interstitial (pore) water met al concentrations. In our studies, horizon-specific sediment analyses were conducted to assess spatial differences in AVS and pore-water met al 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 e levated interstitial water concentrations of cadmium but not zinc. Con centrations of cadmium in pore water from deeper horizons (below the z one of active burrowing) were consistently lower than those in the sur ficial sediments. The bioaccumulation of cadmium and zinc by L. varieg atus was reflective of pore-water concentrations of the two metals, i. e., there was significant accumulation of cadmium, but not zinc, by th e oligochaetes. Overall, our results indicate that bioturbation can en hance the bioavailability of some cationic metals in surficial sedimen ts, via oxidation of AVS, and demonstrate the importance of analyzing surficial sediments when assessing bioavailability of metals in sedime nts.