| Nitrogen(N)and phosphorus(P)are essential elements for the synthesis of nucleic acids and other substances used for cell division and growth.However,human activities have changed the contents of N and P in the freshwater ecosystems,leading to serious eutrophication,which is one of the most common and greatest threats to aquatic ecosystems.It is predicted that the content of N and P will continue to increase in the coming decades due to population growth,further development and greater demand for food production and energy production.Phytoplankton is an important primary producer of water environment and plays a key role in the biogeochemical cycle and the structure of food webs.Zooplankton can consume phytoplankton through predation and can provide food sources for higher trophic organisms.It is in a key position in the energy transfer process of microbial loop and classical food chain.Therefore,it is very important to clarify the effects of nutrients on plankton communities to understand the ecological effects of nutrients in the whole aquatic ecosystem.In order to improve the ecological reality,the evaluation of the environmental effects of N and P should be at community and ecosystem level.To sum up,we chose total nitrogen(TN)and total phosphorus(TP)as the main research objects.We used the freshwater microcosm technology to evaluate the ecological effects of TN and TP under environmental concentrations.We hope that this study can provide a scientific basis for the environmental management of TN and TP.In the first part of this paper,the effects of TN and TP under environmental concentrations on the structure and function of plankton community were analyzed through microcosm experiment containing complex trophic cascades.The test concentrations were based on the Chinese National Environmental Standard for Surface Water Quality GB3838-2002,and the thresholds of TN and TP in freshwater ecosystems were determined.The main results are as follows:(1)For TN threshold analysis,compared with the control,most community structure parameters changed significantly in the TN ≥ 1.0 mg/L groups(p ≤ 0.05).The potential harm of TN was the obvious increase of phytoplankton abundance(p ≤ 0.05).The abundances of Chlorophyta responded to nutrients most positively.The increase of the abundance of small-sized green algae resulted in the reduce of phytoplankton community diversity and promoted the development of zooplankton to miniaturized species,that was,the increase of the abundances of small rotifers and copepod nauplius.However,it was difficult for zooplankton to control the continuous growth of phytoplankton.The analysis results of Principal Response Curve(PRC)showed that TN had significant impacts on the species composition of plankton communities(p ≤ 0.05).In terms of community functional endpoints,the photosynthetic activity of phytoplankton was not sensitive to TN concentration(p > 0.05).Threshold Indicator Taxa ANalysis(TITAN)showed that the negative response threshold of TN was 0.9 mg/L and the positive response threshold of TN was 1.0 mg/L.(2)For TP threshold analysis,compared with the control,the plankton community structure and function in TP ≥0.10 mg/L groups had changed significantly(p ≤ 0.05).In terms of community structure,the potential harm of TP was its promotion effects on cyanobacteria.Due to the predation of zooplankton and interspecific competition of phytoplankton,the abundances of green algae and diatoms decreased.TP indirectly affected the zooplankton community through phytoplankton.The abundance of Cladocera increased significantly from 42 d(p ≤ 0.05).TP inhibited the plankton community diversity consistently.The results of PRC analysis showed that TP significantly affected the community composition of plankton(p ≤ 0.05).In terms of community functional endpoints,the addition of TP led to the decline of phytoplankton photosynthesis.When TP concentration reached 0.40 mg/L,chlorophyll fluorescence parameters were significantly inhibited(p ≤ 0.05).TITAN analysis showed that the negative response threshold of TP was 0.07 mg/L and the positive response threshold of TP was 0.10 mg/L.In the second part of this paper,the effects of nutrients(TN+TP)on the structure and function of plankton communities under different cadmium(Cd)pollution conditions were compared and analyzed.We also clarified the effects of nutrients on the transmission of Cd in the plankton food chain.The main results showed that nutrients significantly changed the community composition of plankton under both low and high Cd conditions(p ≤ 0.05).Under low Cd condition,nutrients promoted the abundances of cyanobacteria and green algae and declined the community diversity.These effects were transmitted to the zooplankton community,resulting in the development of community species composition towards miniaturization.The changes of community structure were transmitted to the community function,which were manifested by the increases of dissolved organic carbon(DOC)and pH in the microcosms.However,under the condition of high Cd,nutrients had no consistent effect on almost all dominant species.The species composition of zooplankton still showed a trend of miniaturization under the indirect influence of nutrients.Due to the toxicity of Cd,nutrients had a slight inhibitory effect on plankton abundance and function.Chlorophyll fluorescence parameters were generally insensitive to nutrients,and were more suitable for long-term evaluation of heavy metal.Our study also found that under high Cd pollution,phytoplankton and zooplankton showed bioconcentration effect or potential bioconcentration effect.The addition of nutrients intensified the biomagnification effect of Cd along the food chain.Furthermore,we tried to build the freshwater microcosm contained phytoplankton-zooplankton-fish,and evaluated the ecological effects of nutrients under different Cd pollution.The results showed that under low Cd condition,nutrients promoted the development of plankton.At the species level,the zooplankton species had been miniaturized.The DOC and pH increased with the development of phytoplankton biomass.The above indicators can be used to characterize the long-term responses of community function to nutrients.Under the high Cd condition,different from the results obtained from the second part,the nutrients showed a promoting effect on the biomass of phytoplankton from day 21 to 28.The omnivorous fish had obvious regulatory effect on the zooplankton community,which was manifested by the indirect improvement of the competitive advantage of rotifers in the community through the predation on cladocerans and copepods.The proportion of rotifers in community was more than 90%.In addition,in both low and high Cd condition,nutrients exacerbated the transfer of Cd along the phytoplankton-zooplankton food chain.However,due to the low bioconcentration factor(BCF)of fish,there was no biomagnification effect along the zooplankton-fish food chain. |
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