Stream Mesocosm Experiments Show Significant Differences in Sensitivity of Larval and Emerging Adults to Metals

Abstract

Evaluations of aquatic insect responses to contaminants typically use larval life stages to characterize taxa sensitivity, but the effects of contaminants to emerging terrestrial adults have received less attention. We present the results of two stream mesocosm experiments that exposed aquatic insects to mixtures of Cu and Zn. We compared responses of larvae and emerging adults in a single-species experiment with the mayfly Rhithrogena robusta and a benthic community experiment. Results showed that R. robusta larvae and emerging adults were highly tolerant of metals. In the benthic community experiment, larval and emerging adult life stages of the mayfly Baetidae were highly sensitive to metals exposure, with significant alterations in adult sex ratios. In contrast, the emergence of Chironomidae (midge) was unaffected, but larval abundance strongly decreased. Timing of adult emergence was significantly different among treatments and varied among taxa, with emergence stimulation in Chironomidae and delays in emergence in R. robusta and Simuliidae (black fly). Our results demonstrate that metal tolerance in aquatic insects is life stage dependent and that taxa sensitivity is influenced by a combination of physiology and phylogeny. Regulatory frameworks would benefit by including test results that account for effects of contaminants on metamorphosis and adult insect emergence for the development of aquatic life standards.

Bioaccumulation and Toxicity of Cadmium, Copper, Nickel, and Zinc and Their Mixtures to Aquatic Insect Communities

“…There were size differences between the tests, and more sensitive results were obtained in years with smaller average‐sized organisms. Smaller insects are generally more sensitive to metals than are larger/older ones (Kiffney and Clements 1996; Kotalik and Clements 2019; Cadmus et al 2020). However, average sizes were not remarkably different between experiments 2 and 3, and the ranges were the same.…”

“…Analysts accustomed to working with data from highly controlled laboratory toxicity tests with cultured, "standard" species may be put off by the variability inherent to naturally colonized mesocosm tests. Acknowledging concerns that our 30-d larval exposures may not be long enough to capture truly "chronic" or critical aquatic insect life-cycle stages (Schmidt et al 2013;Kotalik and Clements 2019), we think that the comparisons with chronic tests with other species (Figure 8) and with field surveys ) support the realism of the 30-d mesocosm exposures.…”

“…For instance, Brix et al (2011) found only 9 laboratory toxicity studies testing aquatic insects from 2000 to 2010 compared to hundreds of papers published with other aquatic taxa over the same time frame. Further, much of the published aquatic toxicity data with metals in aquatic insects may be of little environmental relevance if the tests were short term (acute); if larvae were exposed in unfed, water‐only exposures (Poteat and Buchwalter 2014); or if field‐collected, larger, late‐instar insects were tested (Kotalik and Clements 2019). In the wild, this imbalance is reversed.…”

Response of stream ecosystem structure to heavy metal pollution: context-dependency of top-down control by fish

“…In total, we collected a minimum of 50 individuals per family from each reach. Samples included a mixture of taxa within each family to avoid interspecific variability in metal accumulation and to obtain sufficient biomass for metal analysis (Kotalik and Clements 2019). Macroinvertebrates were stored in plastic vials and frozen at − 80 °C until metal analysis (see metal analysis section).…”

Size-Dependent Sensitivity of Aquatic Insects to Metals

“…Links between metal pollution and degradation of aquatic communities in streams are well established in the literature. − Laboratory experiments have routinely demonstrated that aquatic insects are tolerant to aqueous metals; ,− however, biomonitoring studies often indicate that aquatic insects are sensitive to metals at relatively low concentrations. ,− This discrepancy in reported metal tolerance may be the result of invertebrate assemblage size structure. − Natural benthic communities contain a diversity of taxa that can widely differ in their rates of development due mainly to phenology (i.e., seasonal environmental cues for development) and voltinism (i.e., number of life cycles per year). These life history traits are spatially and temporally variable, resulting in a diversity of invertebrate developmental sizes within and among different species that differ in response to metal exposure.…”