Importance of different exposure routes on the toxicity of chromium to planktonic organisms

Metal pollution of aquatic ecosystems continues to be a worldwide concern, and many studies have previously been conducted evaluating metal toxicity to aquatic organisms. However, these studies have mainly been focused on temperate species and only evaluated aqueous exposure, whereas aquatic organisms under real-world conditions are also exposed to metals through their diet. Toxicity tests were conducted evaluating the effects of Cr on cell density and biochemical composition of the microalga Chlorella vulgaris. Additionally, the toxic effects of Cr on the temperate Ceriodaphnia dubia and the tropical Ceriodaphnia silvestrii cladocerans were investigated through three contamination routes: contaminated medium (CM), contaminated food (CF; C. vulgaris), and the combination of both situations (CMF; contaminated medium and food). The toxicity tests with the cladocerans evaluated the metal effects on survival, number of newborns, and feeding activity. Exposure to 48 µg L⁻¹ total dissolved Cr reduced cell density and increased protein, carbohydrate, and lipid content in C. vulgaris. The increase in the biochemical composition and hence food quality of the microalgae after Cr exposure have contributed to the absence of toxic effects to the cladocerans in the CF treatment. No toxic effects were observed to the temperate cladoceran at any treatment. The tropical cladoceran C. silvestrii showed reduced filtration, ingestion and reproduction rates in the CM and CMF treatments at a Cr concentration below national and international standards. Implications for considering different exposure pathways in ecotoxicological studies, (tropical) risk assessments and indications for future research are discussed.