Effects of Zinc in an Outdoor Freshwater Microcosm System

Nutrient potentiate the responses of plankton community structure and metabolites to cadmium: A microcosm study

Metal pollution is a worldwide concern and may pose risks to aquatic organisms, communities, and ecosystems. The toxic effects of metals at the organism level are relatively clear. However, their impacts at the community level are still poorly understood, especially with concurred eutrophication in surface water. In the present study, the effects of Cd on the plankton community structure and function under varying nutrient conditions were evaluated using a microcosm study. The employed concentrations of Cd and nutrient were based on the values currently measured in the freshwater ecosystem. For the plankton structure, our results showed that the Chl a concentration, the abundances of total phytoplankton, Cyanophyta, and Chlorophyta, and the abundance of Copepoda decreased by Cd consistently. The Cyanophyta Oscillatoria tenuis and Copepoda nauplius were the most sensitive species to Cd in the phytoplankton and zooplankton community, respectively. For the community effects, we found the inhibitory effects of Cd on the photosystem II (PSII) activity of phytoplankton community because of the consistent decrease in the chlorophyll fluorescence parameters (F_v/F_m, Y(Ⅱ), and ETR). Furthermore, the reductions of DOC and pH by Cd were only found in the high nutrient condition, which indicated that the toxic effects of Cd on the community structure and community metabolites were aggravated by the increased nutrient. This study emphasizes the importance of considering nutrient conditions when assessing the metal ecotoxicological effects at environmentally relevant concentrations.

Influence of water hardness on zinc toxicity in Daphnia magna

Zinc is an essential trace metal required for the maintenance of multiple physiological functions. Due to this, organisms can experience both zinc deficiency and toxicity. Hardness is recognized as one of the main modifying physiochemical factors regulating zinc bioavailability. Therefore, the present study analyzed the effect of hardness on zinc toxicity using Daphnia magna. Endpoint parameters were acute‐toxicity, development, reproduction, and expression data for genes involved in metal regulation and oxidative stress. In addition, the temporal expression profiles of genes during the initiation of reproduction and molting were investigated. Water hardness influenced the survival in response to exposures to zinc. A zinc concentration of 50 μg/l in soft water (50 mg CaCO₃/L) caused 73% mortality after 96 h exposure, whereas the same zinc concentration in the hardest water did not cause any significant mortality. Moreover, increasing water hardness from 100 to 200 mg CaCO₃/L resulted in a reduced number of offspring. Fecundity was higher at first brood for groups exposed to higher Zn concentrations. The survival data were used to assess the precision of the bioavailability models (Bio‐met) and the geochemical model (Visual MINTEQ). As the Bio‐met risk predictions overestimated the Zn toxicity, a competition‐based model to describe the effects of hardness on zinc toxicity is proposed. This approach can be used to minimize differences in setting environmental quality standards. Moreover, gene expression data showed that using the toxicogenomic approach was more sensitive than the physiological endpoints. Therefore, data presented in the study can be used to improve risk assessment for zinc toxicity.

The present study analyzed the effect of hardness on zinc toxicity using Daphnia magna for acute‐toxicity, development, reproduction, and gene expression. Results showed that hardness plays an important role for Zn toxicity by effectively changing the bioavailability of Zn. The competition between Zn, Ca, and Mg can be used to normalize hardness effect on mortality. Bioavailability models used in risk assessment could be improved by considering water hardness.