Boron serves as a necessary micronutrient, but elevated concentrations may exert toxic effects, which has raised concern over its increasing presence in the environment owing to anthropogenic activities. This study assessed the sublethal effects of boron on Daphnia magna, which is a commonly employed model species in freshwater ecotoxicology. D. magna neonates were subjected to boron concentrations ranging from 0.5 to 350 mg B/L over 48 h, and acute toxicity (EC₅₀), along with swimming velocity, heart rate, and oxidative stress responses, were evaluated as toxicological endpoints. Swimming velocity increased significantly at 80 mg B/L before declining, while heart rate significantly decreased at 250 mg B/L (p < 0.05). In addition, significant increases in oxidative stress responses were observed at sublethal concentrations of 40 and 80 mg B/L (p < 0.05), highlighting the sensitivity of oxidative stress responses to boron exposure. These findings demonstrate the previously underexplored sublethal effects of boron on D. magna, including alterations in swimming velocity, heart rate, and antioxidant defenses, emphasizing the need for integrated endpoints in ecotoxicological assessments.

Despite the critical role of essential nutrients in maintaining overall health in freshwater organisms, the effects of nutritional deficiency remain largely unknown. This study investigated the biological and biochemical effects of single and combined deficiencies of boron (B), magnesium (Mg), and calcium (Ca), essential nutrients for physiological functions, in Daphnia magna. Biological responses included life-history parameters (survival, growth, and reproduction), while biochemical effects comprised catalase (CAT) and glutathione S-transferase (GST) activity, as markers of oxidative stress and detoxification, respectively. B deficiency conditions significantly affected survival, growth, reproduction, and CAT activity, while Mg-deficient conditions were associated with significantly reduced survival, long-term growth, reproduction, and CAT activity. Under Ca-deficient conditions, the most severe responses were observed, including reduced survival, growth, reproduction and increased GST activity. Combined B omission with Ca or Mg also produced pronounced adverse responses compared with the corresponding single-nutrient deficiency conditions. Fluorescence imaging revealed treatment-dependent differences in B-associated fluorescence, consistent with differences in B availability. Impaired life-history traits and elevated GST activity suggest cellular stress or activation of detoxification processes associated with reduced organismal performance. The findings demonstrate that nutrient deficiencies act as chronic stressors, highlighting the ecological importance of essential nutrient scarcity in freshwater ecosystems.