| As a typical seasonal gulp type lake,Poyang Lake has become more and more serious in recent years,the problem of seasonal dry water has been aggravated,and the decline of sub-merged plants has been getting obviously.To explore the causes of the decline of submerged plants in Poyang Lake at the current stage,a survey was conducted on the water quality in the main channel of Poyang Lake and some saucer-shaped sub-lake areas,meanwhile,the outdoor simulation experiment was designed with the V.spinulosa as the study target;the influence of water level and nitrogen concentration changes on the V.spinulosa was investigated in terms of biomass,morphological characteristics,carbon and nitrogen metabolism,etc.,combined with statistical methods to clarify the mechanism of the effect of growth environment factor altera-tion on the V.spinulosa.Based on the literature reading and data analysis,it is clear that the decline of water level and the prolongation of dry period of Poyang Lake are the dominant factors in the evolution of aquatic vegetation,the water condition determines the spatial and temporal distribution and changes of aquatic vegetation,and the spiny bitterbrush is generally distributed in the water depth range of 20～180cm;in addition,the water quality survey found that the nitrogen concen-tration of Poyang Lake ranges from 1.8～3.82.0mg·L-1 with the mean value of 2.0～3.02.0mg·L-1,and the nitrogen concentration in a few areas is too high even eutrophication occurs.There-fore,in the experiments of outdoor simulations,two types of nitrogen concentrations,high(5.0mg·L-1)and low(2.0mg·L-1),and three water level（control,60 cm,60±30 cm）variations were designed;the experiments lasted 96 d,during which destructive sampling was conducted three times,and the ANOVA two-factor ANOVA method was applied to assess the main effects of water level,nitrogen concentration,and the The results of the experiment showed that the water level declined,the nitrogen concentration decreased,and the biomass of the tissues was significantly reduced.The results showed that lower water level,higher nitrogen concentration and higher con-centration of suspended algae in the water column all affected the growth of V.spinulosa,among which,higher nitrogen concentration was the main cause of the decline of V.spinulosa.The main results were as follows.1.In terms of biomass:（1）both decreasing water level and increasing N concentration will affect the biomass of V.spinulosa;at low N concentration,V.spinulosa has certain adaptability to water level changes,and in response to water level changes,it will significantly increase the biomass of stems,leaves and winter buds to maintain its own growth and development;however,as N concentration increases,the growth of V.spinulosa is inhibited and the biomass of each organ decreases significantly;（2）high N concentration will not only destroy the adaptability of V.spinulosa to water level changes,but also produce toxic effects on the growth of V.spinulosa,and the effects on stems,leaves and winter buds are greater than those on roots.2.As for morphological characteristics:（1）Under the near-natural water situation（low N control group）,V.spinulosa will respond to the natural water level change of Poyang Lake through the growth of plant height,and when it is at low water level,it will also adapt to the low water level change by increasing leaf width,number of winter buds,number of divisions,plant height,number of stolons and number of leaves,but the increase of N concentration will inhibit the adaptation of V.spinulosa to the low water level change;（3）When resources are limited,V.spinulosa achieves its own maximum suitability by reducing the number of subplants and lowering the subplant density to secure the growth and development of offspring.3.Carbon and nitrogen metabolism:（1）Both the decrease of water level and the increase of N concentration will affect the C-N metabolic indexes of all organs of the spiny bitterbrush,and the toxic effect of high ammonia nitrogen is the main reason for the decrease of spiny bitterbrush.（2）The analysis of the C-N metabolic indexes of each organ of V.spinulosa clarified that the C-N metabolic response of leaves to environmental stress was more sensitive than that of root and stem cells.（3）At high water level,in order to release the toxic effect of high am-monia nitrogen on itself,V.spinulosa will convert the ammonia nitrogen in its body into FAA rapidly,but this detoxification mechanism will be significantly weakened after the water level decreases.It was also found that chlorophyll b content of V.spinulosa had a significant negative correlation with root biomass,leaf biomass and winter shoot biomass,and leaf FAA and chlo-rophyll b also showed a significant negative correlation with stem biomass.In summary,V.spinulosa showed various adaptation strategies under different water level and N concentration changes.It is important to avoid large changes in water level and to control nitrogen nutrient inflow into the lake strictly to prevent the decline of V.spinulosa. |
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