Considerable variation in lithium effects on <i>Daphnia magna</i> reproduction: insights from three independent guideline-compliant tests

With global lithium (Li) production rising from 6,000 to 100,000 metric tons annually over the past three decades, environmental concerns about elevated anthropogenic Li releases have grown. Whereas marine waters show relatively stable Li concentrations (∼ 180 µg/L Li), freshwater levels vary substantially due to local geological and climatic factors. Using the sensitive freshwater crustacean Daphnia magna, we conducted three independent Daphnia magna reproduction tests according to Organisation for Economic Co-operation and Development 211 guidelines and Good Laboratory Practices on Li compounds, minimizing experimental variability. Reproduction consistently exhibited a steep concentration-response curve, with no observed effect concentration values ranging from 661 to 1,700 µg/L Li. In contrast, recent chronic studies on D. magna reported lower toxicity values by one or two orders of magnitude under apparently similar protocols. We identified four potential confounders, the exact influence of which needs to be disentangled in future studies: Li acclimation prior to exposure, water hardness and alkalinity, body size in controls, and light intensity. We argue that the current dataset on chronic Li toxicity in freshwaters is not robust enough to support the derivation of environmental exposure limits that are both protective and representative of the wide local variability in natural background levels and bioavailability conditions.