Kelly S. Johnson

The potential effects of microplastics (MPs, < 5 mm) in aquatic systems, especially through interactions with other environmental stressors, are a growing concern. In a prior study, we found that ingested polyethylene MPs altered the toxicity of Bacillus thuringiensis israelensis (Bti) to the midge Chironomus riparius, with larger particles (125 µm) increasing Bti toxicity and smaller particles (34–50 µm) reducing it. The current study describes the effects of the same MP particles on the toxicity of spinosad, an insecticide with a neurotoxic mode of action. Large and small plastic particles were fed to larvae that were simultaneously exposed to spinosad (0, 10, 20, 40, 53, and 67 mg/L) for 21 days. Natural kaolin particles (1–10 μm) were used as a nonplastic control particle. Spinosad alone reduced larval survival and adult emergence as expected, while ingested kaolin and MPs had no effect by themselves. In contrast to our findings with Bti, none of the particles altered spinosad toxicity to larvae at 7 days. However, the smaller-sized MP reduced adult emergence compared to spinosad alone. This pattern suggests that MPs can alter spinosad efficacy, but the effect depends on particle size, and the physiological mechanism appears to be different from those observed with Bti. These findings reinforce the importance of using a variety of sublethal toxicological endpoints and accounting for interactions with co-occurring contaminants for assessing risks associated with MPs in freshwater systems.

Chironomid nonbiting midges are common in many waterbodies, occurring at high densities in sediment and also when flying adults emerge. Although important for food webs and ecosystem processes, the mass emergences of chironomid adults make some species nuisance pests. As part of an effort to develop “push–pull” strategies for managing midge populations, we investigated the importance of visual and chemical cues used by gravid females when selecting sites for oviposition. Field and laboratory oviposition choice tests with Chironomus riparius (Meigen) were used to assess the attractiveness of dark container colors and polarized light for females seeking water for egg laying. Females were not sensitive to increased intensity of polarized light, but they laid fewer eggs in containers with white color above the water’s edge but black below the surface. A disruptive pattern of vertical black and white stripes at the water’s edge reduced egg laying to a similar degree as white. To investigate the importance of olfactory or gustatory cues in oviposition decisions, we tested 3 potential chemical repellents (damaged larvae simulating predator presence, azadiractin, and picaridin) and 5 potential attractants (a tannin–molasses mixture, leaf detritus, live Hydrilla plants, periphyton, and fermented alfalfa infusions used to bait mosquito oviposition traps). Chemical cues appeared to play a weak role, if any, in oviposition choices. Onlyazadiractin (0.02 and 0.11 ppm) reduced the number of egg ropes deposited, but the mechanism may have been from mechanical trapping and drowning, not deterrence.

Effects of season and waterbody on transparency tube estimates of suspended sediment in large rivers

Mayflies (Order Ephemeroptera) require high quality water and habitat in streams to thrive, so their appearance after restoration is an indicator of ecological recovery. To better understand the importance of restoring in-stream habitat versus water chemistry for macroinvertebrate communities, we developed taxon-specific models of occurrence for five mayfly genera (Caenis, Isonychia, Stenonema, Stenacron, and Baetis) inhabiting streams in the Appalachian Mountains, USA. Presence/absence records from past decades were used to develop single and multiple logistic predictive models based on catchment characteristics (drainage area, gradient), in-stream habitat variables (e.g., substrate, channel morphology, pool and riffle quality), and water chemistry. Model performance was evaluated using (a) classification rates and Hosmer-Lemeshow values for test sets of data withheld from the original model-building dataset and (b) a field comparison of predicted versus observed mayfly occurrences at 53 sites in acid mine drainage-impaired watersheds in 2012. The classification accuracies of final models for Caenis, Stenacron, and Baetis ranged from 50 to 75%. In-stream habitat features were not significant predictor variables for these three taxa, only water chemistry. Models for Isonychia and Stenonema had higher classification rates (81%) and included both habitat and chemical variables. However, actual occurrences of Isonychia and Stenonema at study sites in 2012 were low, consistent with the calculated probability of occurrence (P_o) < 0.60. Caenis occurred at test sites 35% of the time when the model predicted a P_o > 0.40. Stenacron showed the greatest consistency of actual versus predicted occurrences, occurring at 56% of sites when the P_o (based on pH and conductivity) was > 0.50 and only at 1 site when P_o < 0.5. The results demonstrate how predictive models of individual indicator taxa could be valuable for evaluating the relative impacts of restoring physical habitat versus water chemistry during stream remediation.

The spatial congruence of chemical and biological recovery along an 18-km acid mine impaired stream was examined to evaluate the efficacy of treatment with an alkaline doser. Two methods were used to evaluate biological recovery: the biological structure of the benthic macroinvertebrate community and several ecosystem processing measures (leaf litter breakdown, microbial respiration rates) along the gradient of improved water chemistry. We found that the doser successfully reduced the acidity and lowered dissolved metals (Al, Fe, and Mn), but downstream improvements were not linear. Water chemistry was more variable, and precipitated metals were elevated in a 3–5-km “mixing zone” immediately downstream of the doser, then stabilized into a “recovery zone” 10–18 km below the doser. Macroinvertebrate communities exhibited a longitudinal pattern of recovery, but it did not exactly match the water chemistry gradient Taxonomic richness (number of families) recovered about 6.5 km downstream of the doser, while total abundance and % EPT taxa recovery were incomplete except at the most downstream site, 18 km away. The functional measures of ecosystem processes (leaf litter breakdown, microbial respiration of conditioned leaves, and shredder biomass) closely matched the measures of community structure and also showed a more modest longitudinal trend of biological recovery than expected based on pH and alkalinity. The measures of microbial respiration had added diagnostic value and indicated that biological recovery downstream of the doser is limited by factors other than habitat and acidity/alkalinity, perhaps episodes of AMD and/or impaired energy/nutrient inputs. A better understanding of the factors that govern spatial and temporal variations in acid mine contaminants, especially episodic events, will improve our ability to predict biological recovery after remediation.