Nutrient Control Of Clonal Growth Of Invasive Eichhornia Crassipes, And Its Spatiotemporal Distribution Pattern In Shanghai

The invasion of exotic species and its impact on environments,ecosystems and society have been widely recognized.Eichhornia crassipes(Mart.) Solms,a free-floating aquatic macrophyte,is thought to be native to the Amazon River Basin.It possesses specialized growth habits,physiological characteristics,and reproductive strategies that allow for rapid growth and range expansion in freshwater environments.E.crassipes was widely introduced to other tropic and subtropic districts in the world,and is now recognized as one of the 10 most harmful weeds.E. crassipes is now distributed in Africa,Asia,North America and Oceania,where it causes many ecological,economic and social problems in 62 countries between 40~QN and 45~QS.E.crassipes usually forms large,free-floating monocultures(mats) that considerably alter the physical and chemical environments of invaded freshwater ecosystems,and compete with native aquatic species for light,nutrients and oxygen.At the same time,E.crassipes mats reduce dissolved oxygen levels and light and significantly alter invertebrate and vertebrate communities.Moreover, organic input to sediments increases dramatically as biomass from mats decomposes.Fish spawning areas may also be reduced and critical waterfowl habitat may be degraded.Therefore, the structure and function of the invaded ecosystems are greatly affected by E.crassipes invasions. E.crassipes interferes with water uses and tourism.It also resulted in flooding and damage to canal banks and structures,which causes substantial economic loss to the invaded areas. Furthermore,E.crassipes may also affect human health.Several methods are used to control E.crassipes infestations,including physical,chemical and biological controls,but herbicidal and biological cbntrols are more effective.Herbicides,however, damage or kill other aquatic organisms,and may have only limited usefulness in wildland settings. Biological control of using insects,pathogens and allelopathicals seems most promising,which can be strengthened by using physical method and water eutrophication control simultaneously.Extensive research on the ecology and management of this species has been done and is still ongoing.However,knowledge gaps still exist in its invasion mechanisms,ecosystem effects and population control.Therefore,further work is needed on how its biological traits and freshwater ecosystem properties interact to result in its rapid growth and range expansion.Work is also needed on how the ecosystem processes are altered by E.crassipes invasions.Moreover,work is needed on the integrated management strategies that combine biological control with other control methods and restoration techniques.We studied the relationship between clonal growth of E.crassipes and N and P nutrient in aquatic environments under glasshouse and field conditions.The results showed that under glasshouse conditions,when the concentrations of N and P increased from 0.5 to 62.5mg/L for N and from 0.05 to 6.25mg/L for P,the numbers of ramets and live leaves of E.crassipes increased significantly.In the rivers in Shanghai,however,N concentration ranged from 1.68±0.118 to 3.65±0.217 mg/L,and P concentration from 0.28±0.03 to 0.43±0.08 mg/L,indicating that the rivers were more or less eutrophic.For this reason,E.crassipes could grow well in all the rivers, thus there were not significant difference in ramet production among the E.crassipes populations cultured in different rivers.Consequently,biomass and clonal growth of E.crassipes were uncorrelated with O.M.,TN,and TP content in the rivers.It was also found that the total biomass of E.crassipes in a district did not reflect the discharge of sewage and contamination of that district although it was correlated with the water area.The results obtained here does not mean that the successful invasion of E.crassipes in the aquatic ecosystems has zero correlation with water eutrophication.The observed zero correlation reflects the fact that all the freshwater aquatic ecosystems in Shanghai are eutrophic,giving a low variability in N and P concentration among the water bodies.In this case,the control of E.crassipes by reducing eutrophication is currently impractical,and other integrated approach is obviously needed.In order to understand the growth response of E.crassipes to nutrient availability and stolon severing,E.crassipes was cultivated at different nutrient levels,and was subject to stolon severing.The results showed that after stolon severing,the total number of ramets,live leaves and biomass of all generations haven't changed significantly,but the ramet number,live leaf number, biomass of parent have increased greatly.It implied that daughter ramet could constraint the clonal growth of parent.On the contrary,the leaf number,root length and shoot height of the first generation haven't changed greatly contrasting with control,only when the nutrient was low, splitting stolon in early phase,the biomass,live leaves.of the first generation decreased significantly.It showed that the constraint effect of ramet to parent will decrease along with the growth time,and also,the integration between ramet and parent has expressed,when the nutrient is low;In low nutrient level,after cutting the stolons,the root length of parent elongated significantly,and E.crassipes represented phenotypic plasticity after removed the constraint effect.So that,in uniform aquatic environment,ramet can constrain parent's clonal growth,and also,E.crassipes trends to split it's ramet for spreading and invading other spaces.Investigations of the distribution and growth of E.crassipes in the field in Shanghai,showed that the abundance of E.crassipes varied among the districts of Shanghai.In particular,Jiading, Qingpu,Songjiang and Jinshan were more heavily infested with E.crassipes than the other seven districts.The river width was found to be an important determinant of the cover of E.crassipes, i.e.the wider the river,the lower the cover.Interestingly,the presence of other aquatic plants(e.g. Alternathera philoceroides,Phragmites communis,Zizania latifolia,Eleusine indica,Leptochloa chinensis) could facilitate E.crassipes establishment,and hence promotes its invasion. Traditionally,it is believed that E.crassipes may not overwinter in Shanghai as it is cold in winter. Field observations revealed that E.crassipes could survive the winter in the field in Shanghai. Therefore,to make control of E.crassipes more effective,it is better to take the control measures in late spring and early summer.At the same time,the effective management of other aquatic plants is also critical to the effective control of E.crassipes.An attempt was made to use GIS for managing the information about E.crassipes distribution and the environmental conditions that are related to the invasibility of aquatic ecosystems. Although very few studies have been conducted to explore the use of GIS in the management of invasive species,the results obtained here show that GIS may provide important information for the managers and decision-makers,thus it can be an important tool for effective management of invasive species like E.crassipes.More studies are obviously needed to further examine the utility of this tool inthe management of invasive species.