Effect Of Norfloxacin On Behavior And Population Dynamics Of Daphnia Magna

Antibiotics are efficiency in treating bacterial infection,as a result, they are not only extensively used for human medicine, but also widely used in animal farming for disease therapy and as feed additives for disease preventing and growth promotion. For example, in USA, the annual usage of antibiotics is approximately 13067 tons, with 30% to 70% of this total being used for livestock. In China, the annual production of antibiotics is 210000 tons, with 46% being used for livestock. Along with increasing the production of antibiotics, there has an increased possibility of their release into the aquatic environments, which may be harmful to the health of mankind and give big threat to the environment. Among the different classes of antibiotics, norfloxacin (NOR) is the most frequency used veterinary medicines since it plays an important role in treating diseases caused by both gram-negative and gram-positive bacteria. It is reported that NOR has been detected in many different types of waters, including wastewaters collected from hospital, animal production and aquaculture, and even in natural waters (e.g. rivers and lakes). However, most present studies have been focused on the acute toxicity effects of NOR on aquatic organisms, while, few studies have explicitly examined the roles of NOR on ecosystem functions, especially for phytoplankton-zooplankton dynamics.This study was designed to test whether NOR affects the survival, behaviors and grazing rate of zooplankton individuals, and thus the corresponding phytoplankton-zooplankton dynamics. Thus, we conducted several microcosm experiments to examine the effects of NOR on toxicity of Daphnia magna in the absence of algal food Chlorella pyrenoidosa, on behavior and grazing rate of Daphnia magna in the presence of algal food Chlorella pyrenoidosa, and on phytoplankton-zooplankton dyancmis. Our results showed that:(1) Norfloxacin is highly toxic to D. magna.With increasing NOR concentration and exposure time, mortality rate of D. magna significantly increased. The 48h and 96h LCsowere 175.8mg/L and 107.6mg/L, respectively. Besides, for concentrations of 300 and 400mg/L, Daphnia individuals died only after 12h of exposure, then this value could reach more than 90% after 96h of exposure. Even for relatively low concentrations of NOR (100 and 200 mg/L), the mortality rate of D. magna will significantly increase with increasing exposure time.(2)In sublethal experiment,average swimming ability of Daphnia magna decreased significantly with increasing NOR concentrations at both the 4h and 12h hours in the experiment(P<0.05),mainly featured in the decreasing of vertical movement times,while the ratio from horizontal to vertical movement times and total static times duration increasing.Compared to CK,respectively,the dosage of 25,50and 100mg/L reduced by 7.3%,44.5% and 68.1% at 4h,surprisingly,the reduced value 22.1%,44.3%and 61.5% of swimming velocity has nothing to do with the time variations(4h,12h,P>0.05).(3)The grazing rate of Daphnia magna significantly reduced when both phytoplankton and zooplankton exposed to NOR, there was without influence of NOR concentrations on Chlorella pyrenoidosa at 4h when the aquatic microcosm was absent of feeders,but the density of phytoplankton significantly decreased with increasing of NOR concentrations(P<0.05).Regardless of the time-scaling varing,grazing rate always sustaining down along with increasing of NOR concentrations(P<0.01,P<0.05).(4)Additionally, the feeding rate was significantly and positively correlated with average swimming ability regardless of exposure time. Our data indicate that NOR may affect animal’s palpitation frequency and copecod feets swimming frequency, then lower the cost of energy of predation behavior so as to gain organism physical and ecological countermeasures.Another probability analysis was detoxification by means of decreasing of grazing and accelerating excretion of bromatoxism, which had a positive correlation between them at any scale of test time (4,r2=0.990,P<0.01;12,r2=0.921,P<0.05).(5)At last, NOR changed the population growth model,decreased instantaneous growth rate,population scales to some degrees in the aquatic microcosm system.compared to CK,the index of contamination groups was lower,but comparison within 25,50 and 100mg/L groups showed a reversed relationship with NOR dosage.so it played a key role of ecological balance in water environment.Above of all,antibiotic pollution in water can changed zooplankton’s survival, activity and foraging success,population dynamics.what endangering the ecological balance between phytoplankton and zooplankton in aquatic ecosystem.we therefore suggest that NOR deserves much more attention in future studies.