A major problem with risk assessment of chemicals is the extrapolation of laboratory single-species toxicity tests, which oversimplify the actual field situation by ignoring species interactions, to natural communities. The authors tested if the bioavailability-normalized 5% hazardous concentration (HC5) estimated from chronic planktonic single-species toxicity data (HC5 ) for zinc (Zn) is protective for a plankton community and investigated the direct and indirect effects of Zn (at HC5 and HC50) on a freshwater community's structure and function. Microcosms were exposed to 3 different Zn concentrations (background, HC5 = 75 μg Zn/L and HC50 = 300 μg Zn/L) for 5 wk. The planktonic groups revealed a consistent no-observed-effect concentration for the community of 75 μg Zn/L, similar to or higher than the HC5 , thus suggesting its protectiveness in the present study. At 300 μg Zn/L a significant reduction in cladocerans resulted in increases of rotifer, ciliate, and phytoplankton abundance. In addition, the phytoplankton community shifted in dominance from grazing-resistant to edible species. Contrary to the species sensitivity distribution (SSD) prediction, which identified phytoplankton as the most sensitive group, only the total chlorophyll and the abundance of 2 phytoplankton species were adversely affected at 300 μg Zn/L. Thus, although the HC5 estimated from the bioavailability-normalized SSD was overall protective for the plankton community, the SSD was not able to correctly predict the species sensitivity ranking within their community context at the HC50. Environ Toxicol Chem 2016;35:2698-2712. © 2016 SETAC.

It is generally assumed that as long as the majority of species experiences no direct adverse effects attributable to a single substance (i.e., potentially affected fraction [PAF] <5%), no significant structural or functional effects at the community level are expected to occur. Whether this assumption holds for mixed metal contamination is not known. In the present study, we tested this by performing a microcosm experiment in which a naturally occurring freshwater planktonic community was exposed to a copper-nickel-zinc (Cu-Ni-Zn) mixture for 8 wk and various structural and functional community-level traits were assessed. In the low mixture concentration treatments (i.e., Ni-Zn mixtures, because there was no difference in Cu concentrations in these treatments with the control), community-level effects were relatively simple, only involving phytoplankton species groups. In the high mixture concentration treatments (Cu-Ni-Zn mixtures), community-level effects were more complex, involving several phytoplankton and zooplankton species groups. Multisubstance PAF (msPAF) values for all mixture treatments were calculated by applying the concentration addition model to bioavailability-normalized single-metal species sensitivity distributions (SSDs). Consistent effects on the structural traits community composition, abundance of zooplankton species groups, species diversity, and species richness and on the functional trait dissolved organic carbon (DOC) concentration (as a proxy for the microbial loop and pelagic food web interactions) were only observed at msPAF values >0.05 (i.e., in the Cu-Ni-Zn mixture). However, consistent effects on the abundance of various phytoplankton species groups (structural traits) and on 2 measures of community respiration, overnight Δ dissolved oxygen (ΔDO) and ΔpH (functional traits), were already observed at msPAF values of ≤0.05 (i.e., in the Ni-Zn mixture). This indicates that the threshold msPAF value of 0.05 was not protective against metal mixture exposure for all community-level structural and functional endpoints in the present study. A possible explanation for this result is the mismatch between the species in the SSD and those in our microcosm community. Indeed, our data suggest that the presence of one single dominant and very Zn- and/or Ni-sensitive species in the investigated community (i.e., a cyanobacteria of the genus Oscillatoria), which is not represented in the SSD of these metals, was probably the driver of all observed effects at or below an msPAF of 0.05. Overall, the present results show that SSDs are not necessarily a good predictor of community-level effects for all types of communities and that the presence of dominant sensitive species may result in significant, consistent effects on certain structural and functional community-level endpoints at msPAF values ≤0.05, which is generally considered protective in many regulatory frameworks. Environ Toxicol Chem 2018;37:2380-2400. © 2018 SETAC.

Ecotoxicological studies mainly consist of single-species experiments evaluating the effects of a single stressor. However, under natural conditions aquatic communities are exposed to a mixture of stressors. The present study aimed to identify how the toxicity of zinc (Zn) is affected by increased temperature and increased phosphorus (P) supply and how these interactions vary among species, functional groups, and community structure and function. Aquatic microcosms were subjected to 3 Zn concentrations (background, no Zn added, and 75 and 300 μg Zn/L), 2 temperatures (16-19 and 21-24 °C), and 2 different P additions (low, 0.02, and high, 0.4 mg P L wk ) for 5 wk using a full factorial design. During the study, consistent interactions between Zn and temperature were only rarely found at the species level (4%), but were frequently found at the functional group level (36%), for community structure (100%) and for community function (100%; such as dissolved organic carbon concentrations and total chlorophyll). The majority of the Zn × temperature interactions were observed at 300 μg Zn/L and generally indicated a smaller effect of Zn at higher temperature. Furthermore, no clear indication was found that high P addition by itself significantly affected the overall effects of Zn on the community at any level of organization. Interestingly, though, 90% of all the Zn × temperature interactions observed at the species, group, and community composition level were found under high P addition. Collectively, the results of our study with the model chemical Zn suggest that temperature and phosphorus loading to freshwater systems should be accounted for in risk assessment, because these factors may modify the effects of chemicals on the structure and functioning of aquatic communities, especially at higher levels of biological organization. Environ Toxicol Chem 2018;37:2413-2427. © 2018 SETAC.

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Under natural conditions, organisms can experience a variety of abiotic (e.g., temperature, pH) and biotic (e.g., species interactions) factors, which can interact with toxicant effects. By ignoring species interactions conventional ecotoxicological studies (i.e., single-species tests) oversimplify the actual field situation. We investigated whether temperature and interspecific competition affected the effects of zinc (Zn) on a Daphnia longispina population. The D. longispina populations were exposed in a full factorial design to 3 different Zn treatments (background, 29, and 110 μg Zn/L), 2 different temperature regimes (cold, 17-18 °C; warm, 21-22 °C), and 2 interspecific competition levels (no interspecific Brachionus competition = no Brachionus calyciflorus added; interspecific Brachionus competition = B. calyciflorus added). Interspecific Brachionus competition and temperature by itself had a limited effect on the Daphnia abundance but significantly interacted with the highest Zn concentration. Without Brachionus competition the D. longispina juvenile and adult abundances under warm conditions were up to 5.5 and 21 times lower, respectively, in the high Zn treatment in comparison with the Zn control, whereas under cold conditions no significant Zn effect was observed. However, with Brachionus competition the highest Zn treatment was on average 2.2 times less toxic to the D. longispina juvenile abundance at higher temperatures. Under cold conditions the highest Zn treatment affected the juvenile abundance sooner and up to 9 times more negatively when simultaneously faced with Brachionus competition. It is possible that the competition for food reduced the amount of energy that could be used by D. longispina for reproduction, and the metabolic costs increased as a result of Zn stress. The present study clearly illustrated the influence of temperature and competition on the effects of a chemical stressor. Thus, not considering such factors in ecological risk assessment may underestimate or overestimate risks in aquatic ecosystems when extrapolating data from standard single-species tests to the field. Environ Toxicol Chem 2018;37:1668-1678. © 2018 SETAC.