T7 AM Metals in the Environment: Dietary Concerns in Aquatic Systems
Tuesday, 15 November 2005: 8:00 AM - 11:40 AM in 327-329

(CRO-1117-736437) Cadmium and copper bioaccumulation for the freshwater snail Lymnea stagnalis: Using stable metal isotope methodology to merge biodynamics and the BLM approach.

Croteau, M.-N.¹, Luoma, S.N.¹, ¹ U.S. Geological Survey, Menlo Park, CA, USA

ABSTRACT- Understanding the processes that govern metal uptake and loss dynamics in aquatic organisms can help to explain species-specific as well as metal-specific differences in bioaccumulation. Dynamics are especially helpful in addressing questions such as why mollusks accumulate high levels of metals in nature. Here, we used metal stable isotopes (as an alternative to radioisotopes) to trace Cd and Cu bioaccumulation dynamics for the freshwater snail Lymnea stagnalis. We manipulated metal isotopic ratios in both water and food to determine unidirectional metal influx from solution, metal assimilation efficiency from ingested food and efflux rate constants. Metal binding characteristics were also quantified upon short-term exposures to waterborne ¹⁰⁶Cd and ⁶⁵Cu at different water hardness levels. Both Cd and Cu uptake in L. stagnalis exhibited saturation kinetics. Maximum transport capacity for Cd was twice that for Cu, whereas the transport site affinity was 2 to 4-times lower for Cd compared to that for Cu. For example, snails exposed to Cd concentrations varying from 0.01nM to 2.5uM in deionised, soft and moderately hard water had J_max and K_m values ranging from 211 to 238 nmol g^-1 d^-1 and from 102 to 198 nmol L^-1, respectively. Binding sites for Cd are of lower-affinity/higher-capacity than for Cu. The higher affinity/lower-capacity of the Cu sites suggests Cu would be more toxic than Cd (higher the log K, greater the toxicity). Moreover, our results suggest that low loss rate constants combined with high metal assimilation efficiencies from food likely explain the elevated metal levels found in mollusks in nature. The importance of diet as a source of metals for these gastropods was considered by incorporation of our laboratory-derived parameters for dissolved and dietary uptake into a biodynamic model. Forecast of bioaccumulation, prediction of chronic toxicity and generation of quality criteria are all possible outcomes of the stable isotope approach.

Key words: bioaccumulation dynamics, snail metal binding, dietary uptake, metal stable isotopes