The current study aimed to determine the acute and sub-chronic toxicity of ammonia to juvenile surf clams (Mactra chinensis Philippi). Acute toxicity tests were conducted with seven concentrations of ammonium chloride using a 96 h static-renewal approach. Sub-chronic ammonia exposure tests (20 d exposures) were conducted with 6 concentrations at 20 °C. The 96 h median lethal concentration (96 h LC50) was 11.1 (10.0; 12.0) mg/L total ammonia nitrogen (TAN) and 0.56 (0.50; 0.60) mg/L unionized ammonia (NH3). The relative growth rate was significantly reduced at concentrations higher than 1.6 mg/L TAN (0.075 mg/L NH3) in the 20 d tests. The estimated maximum acceptable toxicant concentration (MATC) based on the reduced growth of juvenile M. chinensis was between 0.8 and1.6 mg/L TAN (0.038–0.075 mg/L NH3). Histopathological changes were evaluated in the surviving clams after 20 days of exposure. Exposure to 14.1 mg/L TAN (0.661 mg/L NH3) resulted in changes in the mantle, foot and digestive diverticulum. We also examined the antioxidant enzyme activities of superoxide dismutase (SOD) and catalase (CAT) in 10 d and 20 d at 6 different levels (plus a control) of ammonia from 0.8 mg/L to 14.1 mg/L TAN. Ammonia exposure at 0.8 mg/L TAN (0.038 mg/L NH3) significantly affected SOD and CAT activities. The level of enzymic activity decreased with the increasing concentration of TAN. The results improved our understanding of oxidative damage under ammonia exposure and provided data for the aquaculture of sunray surf clams.
The increasing presence of nanomaterials in consumer products has led the scientific community to study the environmental fate of these contaminants of emerging concern. Silver nanoparticles, used mainly for their antibacterial properties, are among the most common nanomaterials. Understanding their transformations and interactions with living organisms, especially under environmentally relevant conditions that can modify metal bioavailability, is a crucial step in the study of their impacts on aquatic ecosystems. In the present study, citrate-coated silver nanoparticles (20 nm; 10 µg/L) were added to the surface freshwater layer of mesocosms simulating a stratified estuary. The investigation by dialysis of the nanoparticle dissolution showed that a large amount of total silver was found in the freshwater layer (and a very low amount in the seawater layer) and that 5–15% was in the form of dissolved silver. These results indicate that the halocline, separating fresh water from seawater, acted as a strong density barrier limiting the sedimentation of the nanoparticles. A simple trophic chain, composed of the freshwater alga Chlamydomonas reinhardtii and the invertebrate Daphnia magna, was used to determine silver bioavailability. This study suggests that citrate-coated silver nanoparticles do not significantly contribute to Ag accumulation by algae but may do so for invertebrates.
An aquatic mesocosm facility consisting of thirty 15,000 L tanks was constructed in Vegreville, Alberta to support environmental research. In 2017, an experiment was conducted as an inaugural run for the facility; this study continued through the winter of 2017/18 (over-wintering is a unique capability of the facility) and concluded in the fall of 2018. Here, we report key methods used to evaluate the effects of two independent variables: (1) a soil layer covering the floor of the mesocosms, and (2) vegetation installed in the mesocosms. Although a range of response variables were measured during this study, we limit our analysis here to the physicochemical (e.g., pH, turbidity, conductivity, and dissolved oxygen) and biological/ecological response variables (e.g., macrophytes, phytoplankton/metaphyton, and macroinvertebrates) that differed due to these two variables. The presence of a soil layer covering the floor of the mesocosm was associated with increased turbidity on some days and depths in 2017. Specific conductivity was higher in the presence of soil and its associated adventitious vegetation. During this initial study, we gained a better understanding of the characteristics and mechanics of the mesocosms, which informs design and implementation of future experiments.
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