The Effects Of Macrophytes And Different Age Of Nile Tilapia(oreochromis Niloticus) On The Phytoplankton

Experiments were carried out to study the effects of macrophytes and Nile tilapia(Orechromis niloticus) in different age on phytoplankton in aquatic mesocosmsystems. Nitrogen, phosphorus, suspended solid and Chl a in water and communitystructure of phytoplankton were measured. Four treatment groups with each grouprepeated four times as: control(C), vallisneria natans(VN), juvenile tilapia(JT:including vallisneria natans and juvenile fish) and adult tilapia treatment (AT:including vallisneria natans and adult fish). By comparing the data from control andV. natans treatment, the effects of submerged macrophytes on phytoplankton wereevaluated. Meanwhile, by comparing the data from V. natans, juvenile tilapia andadult tilapia treatment, the effects of tilapia in different age on shallow lake ecosystemdominated by submersed plants were also evaluated.The results showed that:(1) Chl a concentration of phytoplankton in V. natanstreatment groups were significantly lower than in the control; The speciescomposition of phytoplankton had no significance difference between V. natans andcontrol treatment groups (29species in V. natans groups and33species in controlgroups); The dominant group of phytoplankton was green algae in control treatmentgroups (the percentage abundance of cells was75％-96％), while the dominant groupof phytoplankton succeeded from green algae (78％) to diatom (88％) in V. natansexperiments.(2) The concentration of Chl a and species composition ofphytoplankton were no significance difference in adult tilapia, juvenile tilapia and V.natans treatment groups; The dominant group of algae in adult tilapia and juveniletilapia treatment groups was green algae, the percentage abundance of cells was57％、52％respectively; The cell abundance of cryptomonas and dinoflagellate werehigher in adult tilapia than in V. natans and juvenile tilapia treatment groups, thepercentage abundance of cryptomonas and dinoflagellate cells was13％、5％in adulttilapia, while in juvenile tilapia and V. natans was5％,4％and5％,0％respectively; In addition, the Shannon’s diversity index in adult tilapia was significantly higher than in V. natans and juvenile tilapia treatment groups.The study shows that submerged macrophytes can effectively reduce thephytoplankton growth in maintaining water clarity; Juvenile tilapia and adult tilapiacan affect phytoplankton community structure, in which adult tilapia can improvebiodiversity of phytoplankton, promotes stability of community structure. But thetilapia did not have a significantly impact on phytoplankton biomass.