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Trace metals readily accumulated by stream periphyton may enter aquatic food chains through grazer ingestion. Hence, experiments were conducted to determine the ecotoxicological responses of the grazing mayfly Baetis tricaudatus to dietary cadmium. Short-term feeding experiments indicated that B. tricaudatus nymphs did not initially avoid grazing on cadmium-contaminated diatom mats. During a partial life-cycle experiment, 4 and 10 µg/g of dietary cadmium significantly inhibited grazing, whereas 10 µg/g significantly inhibited growth. Feeding inhibition was the likely mechanism that inhibited growth (i.e., through reduced energy intake). Conversely, when exposed to waterborne cadmium using lethal toxicity test procedures, B. tricaudatus nymphs were relatively tolerant (96-h median lethal concentration, 1,611 µg/L). Thus, sublethal responses to dietary exposure appeared to be more sensitive than lethal responses to waterborne exposure. Because adult mayfly fecundity is a function of nymph size at emergence, dietary cadmium exposure could increase the extinction probability within mayfly populations. The present study highlights the importance of dietary exposure routes in determining the ecotoxicological responses of an organism to a contaminant. Furthermore, the findings emphasize the advantage of evaluating a combination of ecologically relevant, lethal and sublethal endpoints in laboratory methods used to generate data for ecological risk assessment and regulation. [ABSTRACT FROM AUTHOR]

Prey choice in juvenile brook trout (Salvelinus fontinalis) was investigated under sublethal Cd stress in the presence of alternative prey types. Following 30-d exposures to either 0, 0.5, or 5.0 µg/L Cd, individual fish held in artificial stream channels were presented with both motile (Baetis tricaudatus) and nonmotile (Chironomus tentans) prey items and their foraging behavior and capture success was observed. The Cd-exposed trout were found to express a significant preference for C. tentans larvae as alternative prey to B. tricaudatus nymphs. Nevertheless, these preliminary findings indicate that, irrespective of prey choice, the capture efficiency of Cd-stressed trout declined by 20 to 55% with increasing Cd concentration when compared with control fish, and the activity of individual trout increased by 25% over controls, although these responses were not statistically significant. Cadmium significantly affected fish condition factors: The condition factor of control fish increased by 34% over the 30-d period, and Cd-exposed fish declined in condition by 12 to 18%. It is hypothesized that such a switch by fish to feeding on benthic prey under natural exposure conditions and their proximity to contaminated sediment may further exacerbate the sublethal effects of Cd on these individuals by intensifying or prolonging Cd exposure through a combination of trophic transfer and altered foraging behavior. [ABSTRACT FROM AUTHOR]

The foraging and predator-avoidance behaviors of mayfly (Baetis tricaudatus) nymphs and predator--prey interactions were investigated under differing combinations of cadmium concentration and predation risk. Predators consisted of stonefly (Kogotus nonus) nymphs, juvenile brook trout (Salvelinus fontinalis), and longnose dace (Rhinichthys cataractae), either alone or in combination. Organisms were exposed to 0, 0.5, or 5.0 µg/L Cd for 7 d (invertebrates) or 30 d (vertebrates) prior to observation in artificial stream channels. Behavioral observations (20-min duration) were made both day and night over a 24-h period. Mayfly feeding rate was assessed by image analysis of grazed diatom mats cultured on ceramic tiles. Both predators and cadmium produced significant effects on the behavior of B. tricaudatus and K. nonus nymphs, with mayflies showing a hierarchical response in the presence of multiple predators. A significant increase in activity was observed in S. fontinalis exposed to 5.0 µg/L Cd, with significant decreases in capture efficiency of prey items apparent at 0.5 µg/L Cd. Conversely, the foraging behavior of R. cataractae was unaffected by cadmium, possibly as a result of species-specific cadmium tolerance. Such sublethal effects of a contaminant on the energy budgets of individuals and the interactions between species have implications for community structure in aquatic food webs [ABSTRACT FROM AUTHOR]