Increasing toxicity of enrofloxacin over four generations of Daphnia magna

• Exposure to enrofloxacin leads to an increasing toxicity over Daphnia generations.
• Multigenerational test shows population disruption unnoticeable by official ones.
• Intermittent exposure scenarios not necessarily less harmful than continuous ones.
• Perinatal exposure to enrofloxacin may act as an ‘all-or-nothing’ toxicity effect.
• Harm to crustacean populations in the real context cannot be excluded.

The effects of both continuous and alternate exposure to 2 mg L−1 of enrofloxacin (EFX) on survival, growth and reproduction were evaluated over four generations of Daphnia magna. Mortality increased, reaching 100% in most groups by the end of the third generation. Growth inhibition was detected in only one group of the fourth generation. Reproduction inhibition was >50% in all groups and, in second and third generations, groups transferred to pure medium showed a greater inhibition of reproduction than those exposed to EFX. To verify whether the effects observed in these groups could be explained by the perinatal exposure to the antibacterial, a reproduction test with daphnids obtained from in vitro exposed D. magna embryos was also carried out. Perinatal exposure to EFX seemed to act as an ‘all-or-nothing’ toxicity effect as 31.4% of embryos died, but the surviving daphnids did not show any inhibition of reproduction activity. However, the embryonic mortality may at least partially justify the inhibition of reproduction observed in exposed groups along the multigenerational test. Concluding, the multigenerational test with D. magna did show disruption to a population that cannot be evidenced by the official tests. The increasing deterioration across generations might be inferred as the consequence of heritable alterations. Whilst the concentration tested was higher than those usually detected in the natural environment, the increasing toxicity of EFX across generations and the possible additive toxicity of fluoroquinolone mixtures, prevent harm to crustacean populations by effects in the real context from being completely ruled out.