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 microg/g of dietary cadmium significantly inhibited grazing, whereas 10 microg/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 microg/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.

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Low dissolved oxygen (DO) conditions occur frequently during sediment toxicity testing, with potentially adverse effects on test organisms. The present study addressed the current lack of good information regarding low DO thresholds for toxicity tests using two common test species, juvenile Hyalella azteca and Chironomus tentans larvae. Results indicated that H. azteca was less tolerant of hypoxia than C. tentans. The 10-d highest- and no-observed-effect concentrations (HOEC and NOEC, respectively) for H. azteca were 1.2 +/- 0.1 and 2.9 +/- 0.1 mg/L DO, respectively. The 10-d NOEC for C. tentans was 1.2 +/- 0.1 mg/L DO, the lowest test concentration. Mortality was the predominant response of H. azteca to low DO exposure, with changes in growth and positioning behavior only evident at lethal DO concentrations. Although exposure to 1.2 +/- 0.1 mg/L DO for 10 d did not affect C. tentans survival or growth, significant behavioral changes were evident at 2.0 +/- 0.1 mg/L DO or less. Overall, the present results indicate that the North American guidelines for low DO thresholds during 10-d toxicity tests seem reasonable for juvenile H. azteca. However, the Environment Canada Guideline (3.4 mg/L DO at 23 degrees C) may be considered to be conservative for 10-d toxicity testing with C. tentans if only short-term effects on survival and growth are considered.

Contaminant uptake by algae, and its subsequent toxicity, has important ramifications for aquatic biomonitoring and environmental risk assessment. To study the effects of cadmium on diatom mats, a series of experiments was undertaken. These investigated the sensitivity of Navicula pelliculosa mats to cadmium, uptake of cadmium across a range of exposure concentrations, influence of mat biomass and thickness on uptake, and cadmium uptake by mats over time. Diatom mat formation proved to be sensitive to cadmium exposure, with a 96-h EC(50) of 31 microg/L. The rapid uptake of cadmium over 15 min was a linear function of exposure concentration and was not significantly affected by mat thickness. Cadmium uptake over time was also a linear function of exposure concentration for time periods up to 5 h. Linear uptake was likely due to the availability of algal binding sites as cadmium ions diffused through the diatom mats. Internal high-pH microenvironments may also have influenced uptake, through cadmium precipitation or enhanced adsorption within the mats. The lack of a significant relationship between mat biomass and uptake could be explained by the static water exposure conditions. Other studies have shown that cadmium uptake by algal mats was only significantly affected by biomass under flowing water conditions. Flowing water appeared to facilitate the diffusion of cadmium ions through the algal mats. Our research demonstrates the propensity of diatom mats to adsorb cadmium to achieve concentrations that could inhibit macroinvertebrate grazing. Overall, these findings contribute to a greater understanding of cadmium bioavailability in aquatic ecosystems and to the further development of benthic algae as an effective biomonitoring tool.

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 microg/g of dietary cadmium significantly inhibited grazing, whereas 10 microg/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 microg/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.