Study On Effects Of Multiple Environmental Stresses On The Community Structure Of Crustacean Zooplankton

Warming,eutrophication,and the discharge of agricultural effluents are significant processes that impact the functioning of ecosystems.These problems,coupled with the consequences of industrial and agricultural development,contribute to increased environmental stress in freshwater lakes.This stress manifests as environmental pollution,degradation and loss of habitats,and overexploitation of natural resources.Zooplankton,being vital organisms in freshwater ecosystems,hold a central position in the food chain.They possess a high sensitivity to environmental changes and can quickly respond to disturbances.Therefore,zooplankton have the potential to serve as biological indicators for monitoring lake ecosystems.While studies have demonstrated the effects of environmental changes on various organisms in different aquatic ecosystems,the impact of environmental changes on zooplanktonic crustaceans in freshwater lakes has received less attention.In nature,environmental stress is the result of multiple factors and exhibits dynamic changes over time.Understanding how biodiversity responds to global environmental changes becomes challenging if studies solely focus on individual environmental factors or solely analyze their direct effects.In this context,controlled experiments were conducted to investigate the physiological responses of cladocera(a type of zooplanktonic crustacean)to multiple stressors,namely imidacloprid(a pesticide)and fish kairomones(chemical cues from predators).Additionally,a mesocosm ecosystem was established using zooplankton crustaceans as the study subjects to explore how different life history strategies of zooplankton respond to integrated environmental stressors in terms of community structure and growth dynamics.The experiments controlled the levels of water nutrients,temperature,and imidacloprid concentrations.This study aims to provide insights into how global change reshapes the structure and function of aquatic ecosystems.The main findings are as follows:1.Fish kairomones reduced the lifespan,time to maturity,and reproductive cycle of Daphnia similoides sinensis.They also increased the body length,tail spine length,and eye size at maturity.Although the total number of offspring was not significantly affected throughout the experiment,kairomones significantly increased the number of offspring per brood during the 21-day period(P<0.05).Imidacloprid had no effect on the length of maturity but significantly reduced the size of the eyes(P<0.001).Furthermore,imidacloprid increased the egg production rate during the later stage of the experiment.However,when the effects of both fish kairomones and imidacloprid were present simultaneously,all Daphnia individuals died by day 31,indicating a synergistic effect.While the combined effects of the fish kairomones and imidacloprid treatment group did not significantly impact the reproductive cycle and phenotypic traits of Daphnia,the results indicated antagonistic effects when the two stressors were simultaneously applied.2.None of the individual treatments had a significant effect(P>0.05)on the total density of cladocera.However,the analysis revealed a strong positive effect on cladocera density across all treatment groups.The average density of cladocera in the groups exposed to temperature,nutrients,and imidacloprid(WEP)reached a yearly maximum peak of 482.3 ind/L in September.Overall,the addition of imidacloprid inhibited the growth of copepods.On the other hand,eutrophication had a positive effect on copepods,with the mean density reaching a yearly peak of 122.6 ind/L in November.The nutrient treatment group(E),imidacloprid treatment group(P),nutrient and imidacloprid treatment group(EP),warming treatment group(W),and the WEP group all had a significant effect on copepod densities(P<0.05).The combination of temperature and imidacloprid treatment(WP)had a highly significant negative effect on copepods.The impact of environmental stress on biomass followed a similar trend to that of density changes.The WEP treatment group had a significant effect(P<0.05)on zooplankton crustaceans,reaching a maximum annual peak of 5.7 mg/L in September.Additionally,the addition of imidacloprid promoted the density of most dominant species of cladocera,while the absence of imidacloprid resulted in Nauplius and Cyclopoidea Copepodite being the dominant species,with their density almost zero after the addition of imidacloprid.3.The addition of imidacloprid increased the density of cladocera and enhanced the peak recruitment rate in each treatment group.The peak recruitment time was earlier in the WEP treatment group.The total recruitment rate of copepods reached its peak at 6.8×10~4ind/m~2/d in July.The biomass of the WP treatment group reached its maximum annual peak at 2.9×10~3mg/m~2 in September.Imidacloprid significantly reduced copepod recruitment rate(P<0.001).In treatment groups without imidacloprid,copepods reached their peak recruitment rate in September.Temperature and nutrient(WE)synergistically promoted copepod recruitment rate,reaching its maximum annual peak at 2.9×10~4ind/m~2/d in September.Imidacloprid had a significant effect on the cladocera-to-copepods ratio(P=0.001).The varying life history strategies of cladocera and copepods resulted in a decreasing trend in their ratios during warming,with the cladocera ratio approaching 100%under multiple environmental stresses.