The adverse effect of chemical additives leaching from microplastics (MPs) on Daphnia magna populations is not fully understood. In this study, D. magna populations were exposed to polyethylene (PE) MP fragments (5.0 mg/L), PE MP fragments containing the ultraviolet stabilizer benzophenone-3 (MP/BP-3 fragments, 5.0 mg/L), and BP-3 leachate (79 ± 10 µg/L) from PE MP/BP-3 fragments. The test duration was 42 days to observe the population dynamics of D. magna. BP-3 leachate delayed the development and somatic growth of D. magna, resulting in a significantly lower (p <
0.05) population size (number of organisms) compared with the control group. For instance, the population size was 7.7 ± 8.1 and 115 ± 3.5 (n = 3), respectively, at 18 d. However, both MP and MP/BP-3 fragments did not significantly decrease (p >
0.05) the population size of D. magna until day 18. These findings suggest that chemical additive leachates from MPs may have a distinct adverse effect on aquatic organisms, requiring further comprehensive investigation.