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The influence of site-specific conditions on contaminant bioavailability and toxicity to benthic invertebrates is a key consideration in the environmental risk assessment process. This is particularly relevant for contaminants with complex speciation chemistries, such as arsenic. The present study addressed uncertainties regarding arsenic toxicity to a mayfly (Baetis tricaudatus) under low dissolved oxygen (DO) conditions characteristic of many contaminated sites. Arsenic toxicity (arsenite, As(III); arsenate, As(V)) to mayfly nymphs was assessed under two DO scenarios (68 and 84% saturation). Arsenic speciation ratios were determined during testing to confirm the nature of arsenic exposure. The present study found that As(III) was more lethal and bioaccumulated to a greater degree in B. tricaudatus compared to As(V), but the sublethal toxicities of the two arsenic species were similar. Nymph growth and development were significantly inhibited after 12 d of exposure to both 1 mg/L of As(III) and As(V). Exposure to arsenic under low DO conditions (6.5 mg/L, 68% saturation) did not significantly affect As(III) or As(V) toxicity and bioaccumulation over 12 d. The DO level of 6.5 mg/L, however, appeared to be marginally lethal to B. tricaudatus. Results indicate that the Canadian arsenic criterion for the protection of aquatic life (5 μg/L) is protective of B. tricaudatus and is low enough to accommodate differences in arsenic toxicity because of the interconversions between As(III) and As(V). These findings provide insight regarding the toxicity and speciation of arsenic under DO conditions considered to be low for this lotic mayfly species and representative of existing conditions at mine sites in northern Canada. [ABSTRACT FROM AUTHOR]

The present study examined the effects of pulse exposures of the insecticide imidacloprid on the mayfly, Epeorus longimanus Eaton (Family Heptageniidae), and on an aquatic oligochaete, Lumbriculus variegatus Müller (Family Lumbriculidae). Pulse exposures of imidacloprid are particularly relevant for examination, because this insecticide is relatively soluble (510 mg/L) and is most likely to be at effect concentrations during runoff events. Experiments examined the recovery of organisms after a 24-h pulse exposure to imidacloprid over an environmentally realistic range of concentrations (0, 0.1. 0.5, 1, 5, and 10 μg/L). Effects on feeding were measured by quantifying the algal biomass consumed by mayflies or foodstuffs egested by oligochaetes. Imidacloprid was highly toxic, with low 24-h median lethal concentrations (LC50s) in early mayfly instars (24-h LC50, 2.1 ± 0.8 μg/L) and larger, later mayfly instars (24-h LC50, 2.1 ± 0.5 μg/L; 96-h LC50, 0.65 ± 0.15 μg/L). Short (24-h) pulses of imidacloprid in excess of 1 μg/L caused feeding inhibition, whereas recovery (4 d) varied, depending on the number of days after contaminant exposure. In contrast to mayflies, oligochaetes were relatively insensitive to imidacloprid during the short (24-h) pulse; however, immobility of oligochaetes was observed during a 4-d, continuous-exposure experiment, with 96-h median effective concentrations of 6.2 ± 1.4 μg/L. Overall, imidacloprid reduced the survivorship, feeding, and egestion of mayflies and oligochaetes at concentrations greater than 0.5 but less than 10 μg/L. Inhibited feeding and egestion indicate physiological and behavioral responses to this insecticide. [ABSTRACT FROM AUTHOR]

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°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. [ABSTRACT FROM AUTHOR]

We used mesocosms to examine the impact of different concentrations of pulp mill effluent (PME) on structural and functional endpoints of a benthic assemblage in the Saint John River (NB, Canada) during 1999 and 2000. Previous studies on this effluent's effects produced conflicting results, with field surveys suggesting a pattern of mild nutrient enrichment, while laboratory toxicity tests linked effluent exposure to moderate contaminant effects. Experimental treatments included three concentrations of sulfite pulp mill effluent (0, 5, 10% v/v PME). Endpoints for the assessment included algal biomass and taxonomic composition, benthic invertebrate abundance and composition, and insect emergence. Low concentrations of PME increased periphyton biomass and caused changes in community structure within the diatom-dominated community. Pulp mill effluent addition had little effect on several structural endpoints measured for benthic invertebrates, including abundance and taxonomic richness, but significantly changed community composition. For both periphyton and benthic invertebrates, community composition endpoints were more sensitive indicators of PME exposure. Insect emergence was a highly relevant functional endpoint. When benthic and emerged insects were combined, total abundance increased with PME addition. Results from two trophic levels, which provided multiple lines of evidence, indicated that the main impact of these PME concentrations is nutrient enrichment rather than effluent toxicity. Our findings also suggest that benthic invertebrate and periphyton assemblages, algal biomass production, and insect emergence are sensitive response measures. Future studies may confirm this observation. The consideration of both functional and structural endpoints at different trophic levels can greatly improve our understanding the effects of discharges to rivers. Such an understanding could not have been obtained using standard assessment techniques and illustrates the value of mesocosms and the benthic community assemblage approach in environmental assessment. [ABSTRACT FROM AUTHOR]

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]