Ecological Responses Of Pelagic Crustaceans To The Vertical Distribution Of Water Temperature In Large Deepwater Reservoirs

In this study, two subtropical, lager, deepwater?reservoirs (Xin'anjiang Rerservior, Three Gorges Reservoir) were selectesd, investigated the relationship between the seasonal changes of vertical distribution and diel migration of pelagic crustacean in the water column and the seasonal vertical heterogeneity of water temperature in these two reserviors, to clarify the ecological responses of pelagic crustaceans to the vertical distribution of water temperature in large deepwater reservoirs. The main result are as follows:(1) In the investigation period, Xin'anjiang Reservior was represent to be a typical stratified reservior. In spring there is a week epilimnion between 5-25m, the temperature descend? gradient was 0.70℃/m. In summer there will be a strong themocline in the depth range from 10 to 25 m, the max temperature descend?gradient was 1.19℃/m. Then in autumn and winter, the water colum was represent to be homogeneous.(2) In the investigation we indentified 5 families 8 species of cladocera, in which the Daphnia pulex and D. hyalina was dominated species all year around, and Diaphanosoma brachyurum and? Bosmina coregoni was the dominated species mainly in summer. Besides, we indentified 3 orders 5 families 12 genera 8 species of copepod, in which Microcyctops sp., Thermocyclops dybowskii, Mesocyclops leuckarti, Tropocyclops prasinus prasinus was dominated species all year around, and T. hyalinus was the dominated specie mainly in spring and summer, Neutrodiaptomus genogtbbosus, T. taihokuensis was dominated mainly in spring, Neodiaptomus yangtsekiangensis was the dominated specie only in summer. The mainly dominated species was present in each layers, and be agminated, but not prominent. The main resident depth was varied in different season and time. In spring and summer the pelagic crustacean group was mainly stay at the depth of epilimnion (10-25m), while in autumn and winter rhe main resident depth was extend to the whole water column, and was totally accompanying with the phenomenon of diel vertical migration (DVM), which the amplitude was winter > autumn > spring and summer. Also, there were differences in the pattern among groups, such as cladoceras were normal migration in spring, summer and winter, while reverse migration in autumn, copepods were normal migration in 4 seasons. Overall, the thermocline has little influence to most pelagic crustacean species, only had reduced the amplitude of DVM, but for D. pulex, particularly, maybe was astricted under the thermocline.(3) The data of the vertical water temperature in the mainly tributary of Three Gorges Reservior (Daning Bay, Xiangxi Bay) show that, there was a linear correlation between the depth and water temperature in Daning Bay, the water column temperature difference was intensified with the rise of water temperature. Otherwise, in summer, water column temperature was descent fast from 0 to 5 m, but the therocline wasn't shaped in Xiangxi Bay. Further more, in spring, autumn and winter the water column was homogeneous in temperature.(4) In the investigate period of Daning Bay, we indentified 5 families 5 gemus 6 species of cladocera, in which B. coregoni and D. hyalina was dominated in 4 seasons, and D. brachyurum was the dominated specie in summer and autumn. we also indentified 3 orders 6 families 12 species in copepod, in which, Sinocalanus dorrii was dominated all year around, Schmackeria forbesi and T. vermifer was the dominated species mainly in summer and autumn. In the investigate period of Xiangxi Bay, we indentified 5 families 5 genus 12 species in cladocera, except winter. B. coregoni was the dominated specie in the three seasons, D. brachyurum, D. leuchtenbergianum, D. sarsu, B. fatalis was the dominated species only in summer. And there was 3 order 3 families 7 genus 8 species in copepod, but didn't have dominated species.The patterns in vertical distribution of pelagic crustacean in Daning Bay was varied each month, and the cladocera group represent to be successive in water column in spring and summer, while the copepod group was in spring, summer and autumn, varying in species. The pattern of vertical distribution was aggregated, more with the descend of water temperature. Spectacularly, the same as in Xiangxi Bay, indeed, the degree of agregatiton was highist in 10:00. The main resident depth in water column was varied in species, but mainly stay in the depth of 10-25m in spring and summer. In autumn and winter there was no clear MRD in water column, and the D. brachyurum was in deeper layer when the tenperature wans high. The nomal migration of cladocera was obviously and almost migration to all layers, different in species. The amplitude of DVM by Diaphanosoma and?B. fatalis was more in high temperature, B. coregoni was show no significant changes in any season.Totally speaking, the vertical distribution of pelagic crustacean in Three Gorges Rerservior changed little by the influence of water column temperature difference, and the vertical distribution of the group correlated with the temperature just to be seasonal changes in individual diapause or food level. What's more, the seasonal dispatching of the Three Gorges Reservior was make for the vertical migration of pelagic crustacean.(5) The spatially structured of water temperature in different reserviors result in the different vertical migration of pelagic crustacean, it can be due to the hydrological dynamics facters. In this habitat, for copepod groups, they will gather into the surface layer where has plenty of food, and for cladocera, they will gather into the thermocline. The reserponse of pelagic crustacean in DVM are all reprensent to be earlier the migration time to the surface layer (to lengthen the migration time).