Effects of chronic copper exposure on the nutritional composition of Hyalella azteca

The toxicity of dietborne metals currently is a topic of high interest in aquatic toxicology. We hypothesized that, in addition to causing direct physiological effects when ingested by organisms, dietborne metals might also cause indirect foodweb effects. One such indirect effect might be alteration of the nutritional content of prey, leading to decreased growth and (or) survival of consumers. Because dietborne and waterborne metal exposures usually occur concurrently in aquatic ecosystems, we exposed two generations of a representative grazer species-the freshwater amphipod Hyalella azteca-to dietborne copper (Cu) concurrent with waterborne Cu in the laboratory for a total of 27 days (first generation) and 45-57 days (second generation). The aqueous Cu concentrations were 0.0099 (control), 0.054 and 0.11 microM (0.6, 3.4 and 7.0 microg Cu per l, respectively) at 24 degrees C, pH 7.5-8.3, alkalinity of approximately 3 mEq/l and hardness of approximately 4 mEq/l; whereas the dietborne Cu exposures were via biofilm growing on cotton gauze in the same exposure aquaria. In a preliminary test, we demonstrated that the highest Cu concentrations were at the threshold for adversely affecting survival and reproduction. In the two-generation test, first- and second-generation amphipods exposed to the intermediate and high aqueous Cu concentrations accumulated significantly higher amounts of Cu than control amphipods. However, none of the traditional nutritional composition categories (protein, total lipid, ash and 'other') differed among the treatments. The only significant difference in nutritional composition was an elevated concentration of linolenic acid (18:3 n-3) in the second-generation amphipods exposed to 0.054 microM Cu.