Population Dynamics And Response Mechanism Of Daphnia Under Elevated Temperature In Winter And Spring

Most researchers believed that the global warming process would affect biological growth and development in many aspects,especially the temperature rise in winter and spring.Field survey results indicated that the zooplankton biomass showed a certain interannual variation with seasonal changes in the Nanwan Reservoir.Daphnia,as a key factor in the water ecosystem,had a significant effect on the control of phytoplankton in the water.It also maintained the long-term clear-water-state in spring.As an important factor controlling the water-clear-state of water bodies,Daphnia's seasonal variation mechanism suggested a significant response to water temperature fluctuations.However,there were few studies on the temperature control of Daphnia,and most of them focused on the research of suitable growth temperature.The growth of Daphnia in the winter and spring is urgent to pay attention to in water quality protection.On the one hand,Daphnia's own population dynamics are unpredictable as the water temperature increases in winter and spring.On the other hand,the quality of algae as a food of Daphnia is also affected by temperature,which may indirectly affect the population dynamics of Daphnia.Based on these,we studied the response mechanism of Daphnia growth and nutrition status under different temperature conditions and different food conditions.The research conclusions were as follows:(1)From September 2014 to June 2016,a field observation experiment was carried out in the Nanwan Reservoir around the Daphnia population for two years(one sampling per season,a total of 8 samples).The results showed that from the time scale,Daphnia reached the peak of biomass in the winter and spring of 2015 and 2016(December to March),when the water temperature was 9.16?~14.1?.The water body remained more transparent than other seasons during this period.From the space scale,the biomass of Daphnia in the lacustrine was significantly higher than that in the littoral zone,while the lacustrine maintains greater transparency than in the littoral zone.Combining these two aspects,the results showed that the large-scale development of the Daphnia population in spring can effectively maintain the clear water state of the water body and controled the growth of phytoplankton.However,with the advancement of global warming,the increase of water temperature in winter and spring may affect the variation of Daphnia population and further affect the maintenance of clearwater state of the water body.In the end,we further designed control experiments to study the effects of Daphnia growth on different temperature and food quality conditions.(2)In order to verify the response of the Daphnia population to different ambient temperatures,we launched a study that the growth and reproductive status and fatty acid analysis of D.hyalina under different temperatures.The results showed that the lifetime of D.hyalina increased with the decrease of temperature when the ambient temperature was in the range of 12-16?.However,when the temperature was 10?,the life of D.hyalina was shortened,which was presumed to be affected by the low temperature.When 12?,it got longest life span.At the same time,the size of the D.hyalina became smaller as the temperature decreased within a certain range.When the ambient temperature was in the range of 10-16?,the reproduction of the D.hyalina was also affected by the temperature change.The D.hyalina had the maximum number of eggs at 14?.The number of larvae was also the highest at 14? and 16?,indicating that 14? could stimulate D.hyalina for reproduction.Below this temperature,it was not conducive to the growth and reproduction of D.hyalina.While above this temperature,it is not conducive to reproduction.But was conducive to the survival of larvae.When the ambient temperature is in the range of 10-16?,the increase in temperature can increase the rate of molting of the D.hyalina.According to the our analysis,12-14? may be the best survival temperature range for D.hyalina in terms of reproduction and longevity.When the ambient temperature was in the range of 10-16?,the total fatty acid content of the D.hyalina had a negative correlation with the temperature change.But the content of unsaturated fatty acids varies significantly with temperature.The monounsaturated fatty acids and polyunsaturated fatty acids of D.hyalina individual reached a maximum at 14?.It was observed that the content was decreased below this temperature and above this temperature,although no significant difference in biostatistics was found.(3)In order to verify whether the algae quality was affected by different temperatures,the control experiments of Chlorella at different culture temperatures were carried out.The experimental results showed that when culture temperature was in the range of 10-16?,the growth abundance of Chlorella increased with the increase of temperature and the growth at 10? was the slowest.As the temperature increased,the Chlorella growed actively and its agglomeration rate also decreased significantly.On the other hand,the low temperature promoted the total fatty acid content and the polyunsaturated fatty acid content of Chlorella.At 12?,the content of polyunsaturated fatty acid C18:3N3 accumulated by Chlorella was significantly higher than that of other treatment groups.Therefore,from the perspective of the demand and intake of unsaturated fatty acids by Daphnia,the culture conditions of Chlorella at 12?had a higher food quality.(4)To further verify whether the growth of Daphnia was regulated by food quality,we carried out a control experiment on the growth and reproductive differences and fatty acid analysis of D.hyalina under different food quality.The fecundity ofD.hyalina were the best which was fed by Chlorella at 14?.The food between 12? and 14? significant promoted the reproduction of Daphnia.High quality foods promoted the molting rate of D.hyalina's.Daphnia's fatty acid content was mainly controlled by food sources.Food which grows on the 12? has much higher unsaturated fatty acids content,which increases unsaturated fatty acids enrichment of Daphnia.Among all,C18:3n3 content changes are most significant.the total fatty acids content of D.hyalina change is consist with the total fatty acids content of food which grow in different temperature environment.