Yuichi Iwasaki

Effects of heavy metals on riverine benthic macroinvertebrate assemblages with reference to potential food availability for drift-feeding fishes.

We examined the influence of heavy metal pollution from an abandoned mine on benthic macroinvertebrates, at population and community levels, and the potential amount of food available for drift-feeding fish in northern Japanese streams. We studied multiple polluted and unpolluted sites with similar longitudinal positions to avoid problems caused by upstream-downstream comparisons. The ranges of zinc, copper, cadmium, and lead concentrations among the study sites were 5 to 812 microg/L, less than 0.12 to 5.2 microg/L, less than 0.0026 to 4.9 microg/L, and 0.1 to 18.6 microg/L, respectively. The abundance of several populations and community metrics showed a significant negative response to heavy metal pollution. Mayfly diversity and abundance was relatively sensitive to heavy metal pollution. In addition, the biomass of groups of macroinvertebrate taxa that are highly available for salmonids were significantly reduced at metal-polluted sites; this decrease in the most highly available group was noticeable (99% at the heavily polluted upper sites and 69% at the moderately polluted lower sites in spring). These results suggest that we should consider the indirect effect of pollution on food availability for the conservation of fish populations that depend on drifting macroinvertebrates.

Testing an application of a biotic ligand model to predict acute toxicity of metal mixtures to rainbow trout

The authors tested the applicability of a previously developed biotic ligand model (BLM) to predict acute toxicity of single metals and metal mixtures (cadmium, lead, and zinc) to rainbow trout fry (Oncorhynchus mykiss) from a single available dataset. The BLM used in the present study hypothesizes that metals inhibit an essential cation (calcium) and organisms die as a result of its deficiency, leading to an assumption that the proportion of metal-binding ligand (f) is responsible for the toxic effects of metals on the survival of rainbow trout. The f value is a function of free-ion concentrations of metals computed by a chemical speciation model, and the function has affinity constants as model parameters. First, the survival effects of single metals were statistically modeled separately (i.e., f-survival relationship) by using the generalized linear mixed model with binomial distribution. The modeled responses of survival rates to f overlapped reasonably irrespective of metals tested, supporting the theoretical prediction from the BLM that f-survival relationships are comparable regardless of metal species. The authors thus developed the generalized linear mixed model based on all data pooled across the single-metal tests. The best-fitted model well predicted the survival responses observed in mixture tests (r = 0.97), providing support for the applicability of the BLM to predict effects of metal mixtures.

Application of a generalized linear mixed model to analyze mixture toxicity: Survival of brown trout affected by copper and zinc

Increased concerns about the toxicity of chemical mixtures have led to greater emphasis on analyzing the interactions among the mixture components based on observed effects. The authors applied a generalized linear mixed model (GLMM) to analyze survival of brown trout (Salmo trutta) acutely exposed to metal mixtures that contained copper and zinc. Compared with dominant conventional approaches based on an assumption of concentration addition and the concentration of a chemical that causes x% effect (ECx), the GLMM approach has 2 major advantages. First, binary response variables such as survival can be modeled without any transformations, and thus sample size can be taken into consideration. Second, the importance of the chemical interaction can be tested in a simple statistical manner. Through this application, the authors investigated whether the estimated concentration of the 2 metals binding to humic acid, which is assumed to be a proxy of nonspecific biotic ligand sites, provided a better prediction of survival effects than dissolved and free-ion concentrations of metals. The results suggest that the estimated concentration of metals binding to humic acid is a better predictor of survival effects, and thus the metal competition at the ligands could be an important mechanism responsible for effects of metal mixtures. Application of the GLMM (and the generalized linear model) presents an alternative or complementary approach to analyzing mixture toxicity.

Comparison of population-level effects of heavy metals on fathead minnow (Pimephales promelas).

To evaluate the population-level effects of heavy metals (copper, zinc, cadmium, hexavalent chromium, nickel) on fathead minnow, Pimephales promelas, we first estimated the concentration-effect relationships between the metal concentrations and individual traits (juvenile survivability, hatchability, fertility) by using toxicity data collected from the literature. A Leslie matrix model of fathead minnow was used to calculate population growth rates from these relationships. The population threshold concentrations (PTCs) leading to zero net growth of the fish population were as follows: Cu, 27.4; Cd, 33.2; Zn (soft water), 81.5; Zn (hard water), 85.8; Ni, 504.8; Cr, 3251.6 (microg L(-1)). By comparing the PTCs with no observed effect concentrations (NOECs), we found that some PTCs were equivalent to or even lower than the corresponding NOECs. This result suggests that current ecological risk assessments based on the NOECs can be inadequate for protecting aquatic populations and more efforts on the population-level studies are needed.

Does the Choice of NOEC or EC10 Affect the Hazardous Concentration for 5% of the Species?

We evaluated if the choice of no observed effect concentration (NOEC) or a 10% effect concentration (EC10) affects the hazardous concentrations for 5% of the species (HC5s) estimated from species sensitivity distributions (SSDs). By reviewing available literature reporting NOECs and reanalyzing original toxicity data to estimate EC10s, we developed two SSDs for five chemicals (zinc, lead, nonylphenol, 3,4-dichlorobenzenamine, and lindane) based separately on 9-19 EC10s and NOECs. On average, point estimates of HC5s based on EC10s were 1.2 (range of 0.6-1.9) times higher than those based on NOECs. However, both EC10-based and NOEC-based HC5s estimated for five substances were on the same order of magnitude, and their 95% confidence intervals overlapped considerably. Thus, although EC10 was chosen as a representative of ECx in this study, our results suggest that the choice of ECx (e.g., EC5, EC10, or EC20) or NOEC does not largely affect the resulting HC5s. Therefore, use of NOECs would be acceptable particularly in regulatory contexts, although the NOEC has important shortcomings and should be used with caution.

Importance of antecedent environmental conditions in modeling species distributions

Although species distributions can change in an unexpectedly short period of time, most species distribution models (SDMs) use only long-term averaged environmental conditions to explain species distributions. We aimed to demonstrate the importance of incorporating antecedent environmental conditions into SDMs in comparison to long-term averaged environmental conditions. We modeled the presence/absence of 18 fish species captured across 108 sampling events along a 50-km length of the Sagami River in Japan throughout the 1990s (one to four times per site at 45 sites). We constructed and compared the two types of SDMs: 1) a conventional model that uses only long-term averaged (10-yr) environmental conditions; and 2) a proposed model that incorporates environmental conditions 2 yr prior to a sampling event (antecedent conditions) together with long-term averages linked to life-history stages. These models both included geomorphological, hydrological, and sampling conditions as predictors. A random forest algorithm was applied for modeling and quantifying the relative importance of the predictors. For seven species, antecedent hydrological conditions were more important than the long-term averaged hydrological conditions. Furthermore, the distributions of two species with low prevalence could not be predicted using long-term averaged hydrological conditions but only using antecedent hydrological conditions. In conclusion, incorporating antecedent environmental factors linked with life-history stages at appropriate time scales can better explain changes in species distribution through time.

Quantifying Differences in Responses of Aquatic Insects to Trace Metal Exposure in Field Studies and Short-Term Stream Mesocosm Experiments

Characterizing macroinvertebrate taxa as either sensitive or tolerant is of critical importance for investigating impacts of anthropogenic stressors in aquatic ecosystems and for inferring causality. However, our understanding of relative sensitivity of aquatic insects to metals in the field and under controlled conditions in the laboratory or mesocosm experiments is limited. In this study, we compared the response of 16 lotic macroinvertebrate families to metals in short-term (10-day) stream mesocosm experiments and in a spatially extensive field study of 154 Colorado streams. Comparisons of field and mesocosm-derived EC20 (effect concentration of 20%) values showed that aquatic insects were generally more sensitive to metals in the field. Although the ranked sensitivity to metals was similar for many families, we observed large differences between field and mesocosm responses for some groups (e.g., Baetidae and Heptageniidae). These differences most likely resulted from the inability of short-term experiments to account for factors such as dietary exposure to metals, rapid recolonization in the field, and effects of metals on sensitive life stages. Understanding mechanisms responsible for differences among field, mesocosm, and laboratory approaches would improve our ability to predict contaminant effects and establish ecologically meaningful water-quality criteria.

Lower sensitivity of cyprinid fishes to three acetylcholinesterase inhibitor pesticides: an evaluation based on no-effect concentrations

Researchers have suggested that cyprinid fishes are less sensitive to chemical stress than comparable fish families, yet few empirically based evaluations of this hypothesis have been conducted. In this study, we developed a generalized linear mixed model in which the no-effect concentrations (NECs; threshold concentration below which no effect on survival is predicted during prolonged exposure) of 29 fish species from 13 families exposed to an acetylcholinesterase inhibitor pesticide (carbaryl, chlorpyrifos, or malathion) were used as the response variable. The corresponding specific somatic maintenance (SSM) rates, as a size-independent proxy for fish metabolism and a categorical variable regarding whether the species is a cyprinid, were used as the predictor variables. We included SSM rates in the analysis because previous work demonstrated that they are negatively correlated with NECs. Our results indicate that the NECs for cyprinid fishes were significantly higher than those for other fishes, suggesting that cyprinids are indeed less sensitive to the three studied pesticides. Although the SSM rates were negatively related with the NECs, the actual relationship between the two was not clear, implying that the importance of SSM rates may depend on the taxonomic group tested.

Spatial and temporal variation in vertical migration of dissolved 137Cs passed through the litter layer in Fukushima forests

We examined spatial variation in vertical 137Cs flux from the litter layer using lysimeters combined with copper-substituted Prussian blue in two forests (deciduous broad-leaved and Japanese cedar (Cryptomeria japonica)), approximately 40 km northwest of the Fukushima Daiichi Nuclear power plant. The study ran from August 2016 to February 2017 in three periods; summer (10 Aug-4 Oct), autumn (5 Oct-30 Nov) and winter (1 Dec-27 Feb). Twenty-five and 15 lysimeters were installed in the deciduous broad-leaved and the Japanese cedar sites within 400 and 300 m2 areas with 3-5 m intervals, respectively. The geometric means of the flux in the deciduous broad-leaved site were 0.51, 0.085 and 0.060 kBq/m2/month in summer, autumn and winter periods, respectively. In the Japanese cedar site, the mean fluxes were 0.45, 0.036 and 0.023 kBq/m2/month. The ratio of 137Cs flux during the survey period to litter 137Cs inventory was 6% and 1% on average in the deciduous broad-leaved and Japanese cedar sites, respectively. The 137Cs flux in the summer period was much larger than those in other periods, resulting from higher precipitation in the summer. Our fine scale observation with 5 m interval showed very large spatial variation in the 137Cs flux and the differences between maximum and minimum range from 8 to 104 times, but were mostly 20-25 times. The spatial variations in the 137Cs flux were affected positively by those in the litter 137Cs inventory and negatively by canopy openness.

Comparing macroinvertebrate assemblages at organic-contaminated river sites with different zinc concentrations: Metal-sensitive taxa may already be absent

We investigated responses of macroinvertebrates to different zinc concentrations in urban rivers contaminated with organic matter in a regional-scale monitoring survey and a smaller-scale field study. The present study was designed to test our prediction that total zinc concentrations of ∼60 μg/L (twice the Japanese environmental quality standard) do not lead to significant reductions in richness or abundance of macroinvertebrates in organic-contaminated rivers (biochemical oxygen demand of >3 mg/L). At the organic-contaminated sites in both surveys, very few species were present, and metal-sensitive heptageniid and ephemerellid mayflies were generally absent. In the regional-scale study, total zinc concentrations of up to 70 μg/L resulted in little reduction in macroinvertebrate richness. In the local-scale study, macroinvertebrate richness and abundance were not greatly reduced at the polluted downstream site with a total zinc concentration of 48 μg/L. Results from both surveys support our prediction. Therefore, an important implication of this study is that macroinvertebrate taxa that are susceptible to metal pollution should be sparse or absent in organic-contaminated rivers, so the impacts of metals such as zinc may be limited owing to the species-poor communities. Further research is required to evaluate the importance of reduced zinc bioavailability associated with increased organic matter and water hardness to the species-poor communities in organic-contaminated rivers.