Relating metal exposure and chemical speciation to trace metal accumulation in aquatic insects under natural field conditions

A modelling framework to assess multiple metals impacts on marine food webs: Relevance for assessing the ecological implications of deep-sea mining based on a systematic review

Industrial deep-sea mining will release plumes containing metals that may disperse over long distances; however, there is no general understanding of metal effects on marine ecosystems. Thus, we conducted a systematic review in search of models of metal effects on aquatic biota with the future perspective to support Environmental Risk Assessment (ERA) of deep-sea mining. According to results, the use of models to study metal effects is strongly biased towards freshwater species (83% freshwater versus 14% marine); Cu, Hg, Al, Ni, Pb, Cd and Zn are the best-studied metals, and most studies target few species rather than entire food webs. We argue that these limitations restrain ERA on marine ecosystems. To overcome this gap of knowledge, we suggest future research directions and propose a modelling framework to predict the effects of metals on marine food webs, which in our view is relevant for ERA of deep-sea mining.

Effect of long-term exposure to copper on survival and development of two successive generations of Culex pipiens (Diptera, Culicidae)

Aquatic invertebrates can be exposed to copper from various sources, including agricultural applications. For example, concentrations up to 1000 µg L^-1 are found within rice fields, where copper-containing formulations are used as fungicides and algaecides. We conducted toxicity tests to study lethal and sublethal effects of copper sulfate pentahydrate on all immature stages across two generations of Culex pipiens mosquitoes as our model organism. Mortality was dose-dependent at concentrations of 500 µg L^-1 and above in the first generation, and 125 µg L^-1 and above in the second generation. The median lethal concentrations (LC₅₀) of copper sulfate pentahydrate for larval Cx. pipiens were 476 ± 30.60 µg L^-1 and 348.67 ± 23.20 µg L^-1 for the first and second generations, respectively. Generation one pupation decreased from 96% in controls to 48% at 500 µg L^-1, while the second-generation pupation decreased from 96% in controls to 17.5% at 500 µg L^-1. Mortality during the pupal stage varied from 2 to 10% at 500 µg L^-1 of first and second generations, respectively. Higher levels also delayed development to adulthood in both generations. The duration of the immature period was 14.8 days in controls in both generations, but when exposed at 500 µg L^-1 it increased to 18.8 days in the first generation and to 20.5 days in the second generation. The chronic, multi-generation exposures in this study showed greater toxicity than reported for shorter exposures of Cx. pipiens, and confamilial taxa like Culex hortensis and Anopheles hispaniola.

The use of WHAM-FTOX, parameterized with laboratory data, to simulate zooplankton species richness in acid- and metal- contaminated lakes

The WHAM-F_TOX model quantifies cation toxicity towards freshwater organisms, assuming an additive toxic response to the amounts of protons and metals accumulated by an organism. We combined a parameterization of the model, using data from multi-species laboratory toxicity tests, with a fitted field species sensitivity distribution, to simulate the species richness (n_sp) of crustacean zooplankton in acid- and metal-contaminated lakes near Sudbury, Ontario over several decades, and also in reference (uncontaminated) lakes. A good description of variation in toxic response among the zooplankton species was achieved with a log-normal distribution of a new parameter, β, which characterizes an organism's intrinsic sensitivity towards toxic cations; the greater is β, the more sensitive is the species. The use of β assumes that while species vary in their sensitivity, the relative toxicities of different metals are the same for each species (common relative sensitivity). Unbiased agreements between simulated and observed n_sp were obtained with a high correlation (r² = 0.81, p < 0.0001, n = 217). Variations in zooplankton species richness in the Sudbury lakes are calculated to be dominated by toxic responses to H, Al, Cu and Ni, with a small contribution from Zn, and negligible effects of Cd, Hg and Pb. According to the model, some of the Sudbury lakes were affected predominantly by acidification (H and Al), while others were most influenced by toxic heavy metals (Ni, Cu, Zn); for lakes in the latter category, the relative importance of heavy metals, compared to H and Al, has increased over time. The results suggest that, if common relative sensitivity operates, n_sp can be modelled on the basis of a single set of parameters characterizing the average toxic effects of different cations, together with a species sensitivity distribution.

Watershed-scale distributions of heavy metals in the hyporheic zones of a heavily polluted Maozhou River watershed, southern China

Hyporheic zone (HZ) sediments in river systems are often contaminated with heavy metals as a legacy of natural processes and anthropogenic activities. The geochemical behaviors of heavy metals in the HZ sediments at the laboratory scale have been extensively studied. However, the watershed-scale distributions of heavy metals in the HZ sediments and the processes controlling their distributions have not been well studied. Here, we report a watershed-scale study of heavy metals (i.e., Cr, Ni, Cu, Zn, Cd, and Pb) distributions in the HZ of the Maozhou River watershed, a heavily polluted area within the Pearl River Delta, southern China. Statistical analysis revealed that the spatial distribution of studied heavy metal concentrations was highly correlated with that of the sediment-associated sulfide at the watershed-scale. Metal extraction analysis and double-spherical aberration-corrected scanning transmission electron microscope imaging (Cs-STEM) further confirmed the strong association of heavy metals with sulfur. These observations demonstrated that the formation of metals-sulfide precipitates was the key process controlling the watershed-scale distributions of heavy metals (especially for Cr, Ni and Zn) in the HZ sediments. Additionally, high permeability of the HZ sediments may prevent Ni, Zn, Cd and Pb accumulation in sediments. Specially, Cu distribution was mainly affected by organic-Cu complexation. In the estuary area, salinity input likely affected the distributions of Ni, Zn and Cd through cation exchange processes. The findings improved our understanding of the distributions of heavy metals and the processes controlling their distributions at the watershed-scale, and have implications for remediating and managing contaminated HZ sediments.

Competitive interactions among H, Cu, and Zn ions moderate aqueous uptake of Cu and Zn by an aquatic insect

The absorption of aqueous copper (Cu) and zinc (Zn) by aquatic insects, a group widely used to assess water quality, is unresolved. This study examined interactions among Cu, Zn, and protons that potentially moderate Cu and Zn uptake by the acid-tolerant stonefly Zapada sp. Saturation uptake kinetics were imposed to identify competitive mechanisms. Decreasing pH reduced the maximum transport capacity, J_max, in both metals, had little effect on the Cu dissociation constant, K_D, and increased the Zn K_D. Partial noncompetitive (Cu) and partial mixed competitive (Zn) inhibitor models most closely tracked the observed Cu and Zn influx rates across pH treatments. The estimated values for acid dissociation constants for the binary (proton-receptor) and ternary (proton-metal-receptor) complexes indicated the strong inhibitory effect of protons on Cu and Zn. In neutral pH water, Cu inhibited Zn influx, but Zn had little effect on Cu influx. The mechanism of Cu-Zn interaction was not identified. Results from separate Zn experiments suggested that the insect's developmental stage may affect the apparent J_max. The study underscores some of the challenges of modeling metal bioaccumulation and informs future research directions.

Encaged Chironomus riparius larvae in assessment of trace metal bioavailability and transfer in a landfill leachate collection pond

Household wastes may constitute a vector of environmental contamination when buried, in particular through degradation and production of leachates containing significant trace metal (TM) concentrations that may constitute a serious risk to biota. The objectives of this study were to assess the bioavailability and transfer potential of various TMs present in water and sediments in a reservoir receiving landfill leachates. An active biomonitoring approach was adopted consisting of exposing naive laboratory organisms in cages deployed in the field. Aquatic insects such as Chironomus riparius larvae are good candidates since they represent key organisms in the trophic functioning of aquatic ecosystems. The results show that water, suspended particles, and sediments were significantly contaminated by various TMs (As, Cd, Cu, Ni, Pb, and Zn). Their contribution to the transfer of TMs depends, however, on the specific element considered, e.g., Cd in sediments or Pb in both suspended particles and sediments. The internal fate of TMs was investigated according to their fractionation between an insoluble and a cytosolic fraction. This approach revealed different detoxification strategies capable of preventing the induction of deleterious effects at the individual scale. However, the accumulation of several TMs in C. riparius larvae tissues may also represent a significant load potentially transferable to higher trophic levels.

Modeling whole body trace metal concentrations in aquatic invertebrate communities: A trait-based approach

Trace metal risk assessment and environmental quality standard definition require realistic models that quantify metal exposure and accumulation by biota. In the present study we propose a novel trait-based approach to predict whole body concentrations of metals in aquatic invertebrates from concentrations measured in different environmental compartments. Field data from a large riverine floodplain was used to calibrate and test the model. The prediction performance of the trait-based model was unbiased and uncertainty was below the twofold of measured concentrations for the four studied metals (Ni, Cu, Cd, Pb). The relative contribution of feeding, respiration and locomotion patterns as well as metal concentrations in three environmental compartments provided insights into the importance of different uptake pathways. The relation with the sediment (i.e., to what degree taxa live in or directly on the sediment) was shown to be the most important trait to predict metal accumulation. Overall, this study demonstrated the potential use of bioecological traits for the modeling of whole body metal concentrations of entire aquatic invertebrate communities.

Site-specific water quality criteria for lethal/sublethal protection of freshwater fish exposed to zinc in southern Taiwan

There were considerable concerns about the zinc (Zn) pollution caused by electroplating, chemical, and computer-related high-tech industrial discharges in Kaohsiung Rivers situated at south Taiwan. There is, however, a lack of site-specific water chemistry based toxicity assessment and little is known about the sublethal toxicity on freshwater fish. This study proposes an integrated framework to link experimental and mechanistic model-based data analysis of lethal and sublethal Zn toxicities for grass carp (Ctenopharyn odon idellus) populations for providing the site-specific Zn water quality threshold in Kaohsiung Rivers. A biotic ligand model (BLM) that relates toxicity impairment of physiological active sites impacted by Zn species was used to elucidate the site-specific water chemistry affecting the bioavailability and biological response of grass carp exposed to Zn. Results indicated that 96-h LC50 for mortality and 28-d EC50 for growth inhibition were 474.7 ± 1.3 (mean ± SE) and 149 ± 23.5 μg L(-1), respectively. Here the BLM-based predicted steady-state LC50s for mortality were 2110.7, 818.2, 1303.6, 563.3, and 497.1 μg L(-1), whereas measured steady-state EC50s for growth inhibition were 726.8, 326.2, 373.4, 193.9, and 170.5 μg L(-1) for the Agongdian, Houling, Love, Fengshan, and Gaoping Rivers, respectively. A positive correlation between Mg(2+) and EC50 values were found in both acute (r = 0.94, p < 0.01) and chronic (r = 0.97, p < 0.01) Zn exposures. This study suggests that the use of site-specific water chemistry data and ecophysiological traits would enhance the predictive capacities to assess the potential effect of metal toxicity posed to aquatic organisms.

Biotic ligand modeling approach: Synthesis of the effect of major cations on the toxicity of metals to soil and aquatic organisms

The biotic ligand model (BLM) approach is used to assess metal toxicity, taking into account the competition of other cations with the free metal ions for binding to the biotic ligand sites of aquatic and soil organisms. The bioavailable fraction of metals, represented by the free metal ion, is a better measure than the total concentration for assessing their potential risk to the environment. Because BLMs are relating toxicity to the fraction of biotic ligands occupied by the metal, they can be useful for investigating factors affecting metal bioaccumulation and toxicity. In the present review, the effects of major cations on the toxicity of metals to soil and aquatic organisms were comprehensively studied by performing a meta-analysis of BLM literature data. Interactions at the binding sites were shown to be species- and metal-specific. The main factors affecting the relationships between toxicity and conditional binding constants for metal binding at the biotic ligand appeared to be Ca(2+) , Mg(2+) , and protons. Other important characteristics of the exposure medium, such as levels of dissolved organic carbon and concentrations of other cations, should also be considered to obtain a proper assessment of metal toxicity to soil and aquatic organisms.