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A biophysical model for calculating the effective parameters of low-frequency magnetic fields was developed by Lednev based on summarized empirical data. According to this model, calcium ions as enzyme cofactors can be the primary target of low-frequency magnetic fields with different parameters tuned to calcium resonance. However, the effects of calcium-resonant combinations of static and alternating magnetic fields that correspond to Lednev's model and differ by order in frequency and intensity were not studied. It does not allow for confidently discussing the primary targets of low-frequency magnetic fields in terms of the magnetic influence on ions-enzyme cofactors. To clarify this issue, we examined the response of freshwater crustaceans Daphnia magna to the impact of combinations of magnetic fields targeted to calcium ions in enzymes according to Lednev's model that differ in order of magnitude. Life-history traits and biochemical parameters were evaluated. Exposure of daphnids to both combinations of magnetic fields led to a long-term delay of the first brood release, an increase in the brood size, a decrease in the number of broods, and the period between broods. The amylolytic activity, proteolytic activity, and sucrase activity significantly decreased in whole-body homogenates of crustaceans in response to both combinations of magnetic fields. The similarity in the sets of revealed effects assumes that different magnetic fields tuned to calcium ions in biomolecules can affect the same primary molecular target. The results suggest that the low-frequency magnetic fields with parameters corresponding to Lednev's model of interaction between biological molecules and ions can remain effective with a significant decrease in the static magnetic background. [ABSTRACT FROM AUTHOR]