Metal concentrations of resident and transplanted freshwater mussels Hyridella menziesi (Unionacea: Hyriidae) and sediments in the Waikato River, New Zealand

Heavy metals and metalloids were measured in freshwater mussels (Hyridella menziesi) and sediments in the upper Waikato River, New Zealand. Mercury (Hg) concentrations ranged 35-fold in sediments (0.025-0.87 mg kg−1 dry wt) and 144-fold in mussels (0.091-13.1 mg kg−1 dry wt). Arsenic (As) concentrations ranged 190-fold in sediments (7.9–1520 mg kg−1 dry wt) and only 10-fold in mussels (15.0–153 mg kg−1 dry wt). There was no significant correlation between sediment total concentrations of either Hg or As and mussel tissue levels. Sediment quality guidelines were exceeded for Hg and As at most sites and for copper, chromium, iron and manganese at a few sites. Tissue concentrations exceeded levels considered to be safe for human consumption for Hg at Lake Aratiatia and As at several sites. Transplant experiments with caged mussels were used to measure uptake and depuration rates of Hg and As. Half-lives for mussel uptake and depuration of Hg were 6–12 months. Arsenic uptake rates could not be determined but the depuration showed a half-life of about 2.5 months. Physiological measurements were consistent with increased food availability at some sites rather than contaminant effects. These results highlight the need to understand biochemical changes induced by factors that are independent of environmental concentrations of contaminants, such as the influence of food levels and the potential for metabolic adaptation in biomonitoring organisms.