Bioaccumulation and Chronic Toxicity (Critical Body Concentrations) of Metals in Potamopyrgus antipodarum: An Ecological Risk Assessment Tool

Christopher W. Hickey and Michael L. Martin

Organisms in the natural receiving environment are subjected to widely time-varying exposures to contaminants as a result of continuous and intermittent discharges. For contaminated land sites this variability may be particularly large in relation to rainfall events, groundwater level and soil moisture saturation. Measurements of total metal concentrations in the receiving water are difficult and do not provide estimates of toxicological effects because toxicity is a function of exposure concentration, metal speciation and bioavailability. Much of the metals discharged during rainfall events will be particle associated and of low bioavailability.

Biological effects are usually a function of bioaccumulation. Thus, whole body concentration should provide a better predictor of effects than concentrations in sediment or water. Bioaccumulation-toxicity response relationships for metals (As & Cu) have been determined for three native species: the hydrobiid snail (Potamopyrgus antipodarum); the common bully (Gobiomorphus cotidianus), and the inanga (Galaxias maculatus). Critical body burdens were determined in chronic (20-30 day) toxicity tests in the laboratory.

The results show marked differences in the concentration-response relationships for the invertebrate and fish species. The snail accumulated markedly arsenic higher body-burdens than the inanga for comparable water concentrations (Figure 1). Strong linear regressions indicated predictable species-specific relationships between tissue accumulation and water column exposure. Critical body-burden relationships were calculated for snails (Figure 2) and fish species (Figure 3) based on measured snail morbidity (inability to overturn) and fish survival. The indicative no observed effect concentrations (NOEC) were based on a 10% effect threshold. The NOEC differed markedly between snails, at about 300 mg/kg, and the fish species, at about 22 mg/kg. Survival declines rapidly above these critical body-burden thresholds for both snails and fish species.

Figure 1. Summary plot relating snail and bully exposure, accumulation and effects. Linear regression lines through snail and fish (inanga) datasets.

Figure 2. Tissue body-burden/ survival response relationship for arsenic exposure to snails (line is second order polynomial through 14d data). Indicative no observed effect concentration (NOEC) threshold is 300 mg/kgDW As.

Figure 3. Tissue body-burden/ survival response relationship for arsenic exposure to bullies and inanga (line is second order polynomial through all data). Indicative no observed effect concentration (NOEC) threshold is 22 mg/kgDW As.

Copper exposure body-burden relationships were only derived for inanga. Tissue concentrations showed an increasing non-linear response with water exposure (Figure 3). The critical body-burden response relationship indicated a relatively wide tolerance range for copper with a critical body-burden value of about 15 mg/kg Cu. Above this value inanga survival markedly declines.

These bioaccumulation-toxicity relationships can be used to predict chronic toxicity based on total body concentration for biomonitoring using indigenous field populations and for metal accumulated in short-term in situ transplanted organisms. The biomonitoring suitability is greater for snails for a number of reasons, including: (i) their lower natural mobility; (ii) suitability for long-term housing in simple cage systems; and (iii) ability to assess both sediment and water column associated contaminant bioavailability. Comparison of metal bioaccumulation with critical body burden concentrations can help to identify cause-effect relationships and related exposure conditions with adverse effect thresholds for individual species. Thus metal bioaccumulation can provide valuable quantitative tools for use in risk or environmental impact assessment if critical body burdens have previously been determined.