Impacts of faeces from microcystis-dietary silver carp (Hypophthalmichthys molitrix) and tilapia (Oreochromis niloticus) on the aquatic environments and the transferring of faeces-sourced nitrogen

Stable isotope tracer technique was employed to study faeces-sourced nitrogen transferring from silver carp( Hypophthalmichthys molitrix) and tilapia( Oreochromis niloticus) after feeding on Microcystis in aquatic ecosystem,when nutrients concentrations and plankton biomass were determined by the tracers. Nutrients analysis indicated that nutrients contents in tilapia group increased rapidly at the end of the experiment,while there was no obvious difference between silver carp group and control group.Some nutrient contents in silver carp group were even less than those in control group. What's more,generally concentrations of suspended solid,chlorophyll-a content and phytoplankton cell density in fish group were higher than those in control group. The three parameters mentioned above of tilapia group became significantly higher than those of the rest groups in the mid-stage of the experiment. Phytoplankton biomass showed a slight decrease after the introduction of fish,and then phytoplankton biomass in-creased rapidly. The maximum biomass of tilapia and silver carp were 7. 4 and 4. 5 times higher than those of control group when cyanobacteria and green algae were the main food. With filter behavior prolonged,the biomass of zooplankton in both fish groups reduced sharply. At the end of the experiment,zooplankton became scarce in the tilapia group. Stable isotope analysis indicated that,after microcystis-feeding,15 N stable isotope ratio increased significantly in fish muscle,dissolved NH+4,phytoplankton,zooplankton,sedimentary debris and snail( Bellamya aeruginosa) on the first day. During the experiment,15 N isotope ratio of tilapia muscle was consistently higher than that of silver carp,and15 N isotope ratios of above parameters in fish group were invariably higher than those in control group.15 N stable isotope ratio of dissolved NH+4,phytoplankton and zooplankton in both fish groups reached their maximum values at the beginning of the experiment,while15 N isotope ratios of sedimentary debris and snail reached their peak point on day 5 and day 10,respectively. Significant difference was found in absorption efficiency of microcystis-feed between two types of fish. The maximum15 N isotope ratios of silver carp appeared in day 5 with the value of 36. 22‰,while tilapia reached its peak value of 151. 53‰ in day 15 and the peak value is 4. 2 times higher than that of silver carp group. The result of the experiment indicated that faeces-sourced nitrogen went directly into water column after feeding on microcystis,taking part in nutrition cycle,finally stimulated the bloom of algae.