Effects Of Abiotic Environmental Factors And Interspecific Interactions On The Ecological Characteristics Of Submerged Plants In Shallow Lakes

Submerged vegetation is widely recognized to play a vital role in maintaining ecological structure and function in aquatic ecosystems,particularly in shallow lakes.The water eutrophication and extreme water levels,which were caused by human activities intensifying and extreme weather happening frequently,have given rise to the degradation of submerged vegetation.In previous studies,most studies paid attention to the effect of a single biotic environmental or two abiotic environmental factors on functional characteristics of submerged macrophytes by laboratory incubation experiments.Nonetheless,there is little attention about how the interaction of biotic-and abiotic factors affect the traits of submerged macrophyte.Besides,only a few researchers observed the vertical patterns of submerged macrophyte's traits.It is necessary to illustrate the vertical patterns of submerged macrophyte's characteristics between segments and community composition on those vertical patterns for revealing the adaption mechanism of submerged macrophytes to water depth on one hand and species interaction.To reveal the distribution pattern of submerged macrophytes and their reason,illustrating the response of submerged macrophytes to environmental conditions,such as eutrophic water and low light condition,also has critical effects.We collected Hydrilla verticillata individuals from a single community and a mixed community which was composed of the target species and Ceratophyllum demersum in different water depths of Xukou Bay,including the depths of 1.0 m,2.2 m,then measured leaf characteristics after cutting individuals into segments,to illustrate the vertical pattern of submerged macrophyte adapting different water depth and community composition in a shallow lake.We chose Vallisneria natans(Lour.)Hara and Potamogeton crispus L.as experimental subjects,and conducted an incubation experiment about the effect of different environmental conditions on submerged macrophytes'ecological characteristics.The condition includes different nutrient concentrations(TN₁:1.0 mg L^-1,TP₁:0.1 mg L^-1;TN₂:3.0 mg L^-1,TP₂:0.3 mg L^-1),different light intensities(shading 0%,shading 50%)and interspecific interactions(no interspecific competition,interspecific aboveground competition,and interspecific aboveground+underground competition),in order to reveal the response rules of growth characteristics of submerged plants to changes in community composition and water environment and explore the mechanism of interspecific interaction affecting community structure during vegetation community succession.(1)The upper segments of H.verticillata had fewer leaf-associated matters than that in nether segments(P<0.05),and community composition had a significant effect in leaf-associated matters between a single community and a mixed community(P<0.05).An in-situ survey showed that:when H.verticillata distributed in shallow areas(1.0 m),compared with bottom segments,upper segments attached more inorganic mattes,more organic matters,more total matters,and less epiphyte biomass;compared with mixed communities,H.verticillata which collected from single communities had more leaf-associated matters.In addition,when H.verticillata was distributed in deep areas(2.2 m),the leaf-associated matters on H.verticillata of mixed communities were 2.06±1.10 times more than that in single communities.(2)Community composition had a significant effect on the vertical pattern of leaf physiological traits(P<0.05),no matter which water level that H.verticillata distributed.Moreover,the difference of leaf physiological traits of H.verticillata among segments was bigger in shallow areas than in deep areas.In addition,leaf functional trait characteristics were closely related to both underwater light intensity and leaf-associated matters,and community composition could change the correlation.An in-situ survey showed that:in shallow areas,only the physiological trait of H.verticillata in the mixed community had differences among segments,that is the leaf pigment concentrations and the enzymatic specific activity of peroxidase(POD-ESA)of upper segments were 0.78(±0.04)times and2.82 times than that in bottom segments,respectively.At the same time,the chlorophyll content and POD-ESA in the mixed community were 0.61±0.10 times and 2.39±1.07 times of those in the single community,respectively.In addition,community composition had the greatest effect on POD-ESA,that was POD-ESA among segments showed an increasing trend with an increasing water depth in the single community while lower segments had the lowest POD-ESA and upper segments had higher POD-ESA.In a mixed community,the correlation between leaf pigment and underwater light intensity was more close than that of the single community,and the leaf-associated matters were closely related to POD-ESA when the two submerged macrophytes grew together at 1.0 m water depth.However,when they growing in a water depth of 2.2 m,the leaf POD-ESA was closely related to the leaf-associated matters and light intensity,while the correlation between leaf pigment and light intensity was not as close as that in the single community.(3)The effects of light conditions on interspecies interaction were more significant than those resulted from nutrient concentration.(1)Under the relatively sufficient light condition(no shading),nutrient concentration(TN₁:1.0 mg L^-1,TP₁:0.1 mg L^-1;TN₂:3.0 mg L^-1,TP₂:0.3 mg L^-1)had no significant impacts on the total biomass of Vallisneria natans(Lour.)Hara and youth Potamogeton crispus L..There was no significant competition or mutual facilitation between the two submerged macrophytes.(2)Under the low light condition(50%shading of light intensity),nutrient concentration had a significant effect on the growth of V.natans and P.crispus.Specifically,the lower nutrient concentration would suppress V.natans but facilitated P.crispus,when higher nutrient concentration suppressed both V.natans and P.crispus.Therefore,light intensity is the major factor affecting the seasonal succession of submerged vegetation.Eutrophication would intensify the interspecific competition and hinder the seasonal succession of the two submerged plants when submerged macrophytes grew under low light intensity.