Ecological Effects Of Submersed Macrophytes Diversity Under Eutrophic Conditions

Nowadays,eutrophication has been a critical environmental issue in China,which has caused remarkable changes in the structure and functions of aquatic ecosystems,and led to the degradation of submersed macrophyte community.This will necessarily hinder the construction of aquatic ecological civilization in our country.Physics,chemistry and biological remediation technologies are widely applied in the treatment of eutrophication,but the restoration of aquatic plant has gradually become an indispensable means to control water pollution.The presence of submerged plants and the restoration of their diversity have crucial implications for aquatic ecosystem function and structure,but we lack a mechanistic understanding of how it affects the aquatic environment and ecosystem function in the long term.In this experiment,we selected 22 common submerged plants as the species pool,and assigned randomly them into five levels of species diversity（i.e.,1,2,4,8,and 16）and three levels of functional diversity（i.e.,1,2,3 functional groups）.For this point,seven functional structures were randomly formed from three functional groups to explore the ecological effects of specific species and specific functional groups under eutrophic conditions.Seven functional structures are specified as follows:FG1._p（species are randomly selected from functional group1）,FG2._p（species are randomly selected from functional group 2）,FG3._p（species are randomly selected from functional group 3）,FG12._p（species are randomly selected from functional groups 1 and 2）,FG13._p（species are randomly selected from functional groups 1 and 3）,FG23._p（species are randomly selected from functional groups 2 and 3）and FG123._p（species were randomly extracted from functional groups 1,2,and 3）.Then we measured dissolved oxygen（DO）,conductivity（Cond）,p H,chlorophyll a（Chl-a）,total nitrogen（TN）,total phosphorus（TP）,ammonium nitrogen（NH₄-N）,nitrate nitrogen（nitrate nitrogen,NO₃-N）,orthophosphate phosphorus（PO₄-P）,chemical oxygen demand（COD）,submerged plant biomass（dry weight,DW）,soil carbon（soil carbon,soil C）,soil nitrogen（soil nitrogen,soil N）,soil phosphorus（soil phosphorus,soil P）concentration dynamic changes.By comparing the effects of different treatments（CK:no fertilizer+ambient light,T1:slow-release fertilizer+ambient light,T2:no fertilizer+low light,T3:slow-release fertilizer+low light）the diversity of submerged plant community on aquatic ecosystem functions,with the following main results:（1）With the increase of species diversity,the contents of TN,NH₄-N,NO₃-N and PO₄-P in T1 decreased significantly by 21.3%,44.0%,27.0%and 66.7%respectively,and the DO content increased significantly by 8.2%.However,the contents of TN,TP,NO₃-N and PO₄-P in T3 decreased significantly by 42.3%,55.5%,48%and 75%respectively.Among the treatments,the ecological effect of enhanced taxonomic diversity of submerged plants was most pronounced in T3 treatment.（2）With the increase of functional diversity,the contents of TN,NO₃-N,NH₄-N and PO₄-P in T1 decreased significantly by 15.5%,30.3%,18.4%and 50.0%respectively,the DO content increased significantly by 6.1%,and the Cond decreased significantly by 4.1%.The contents of TP,NO₃-N and PO₄-P in T3 decreased significantly by 55.6%,30.0%and 50.0%respectively.Among the treatments,the ecological effect of enhanced diversity of submerged plants was the most pronounced in T3.（3）Among the 22 submerged plants,Vallisneria natans.,Elodea nuttallii.,Ceratophyllum demersum L.and Utricularia aurea Lour.etc.can lead to significant decrease for TN,TP,NO₃-N,NH₄-N,PO₄-P and COD,and was the most pronounced in T3.（4）The FG3 functional group was characterized by plants with long flowering during and large specific leaf area,among which Potamogeton pectinatus L.,V.natans.,C.demersum L.,Hydrilla verticillata.,Potamogeton maackianus.,Myriophyllum verticillatum L.,Potamogeton perfoliatus L,Potamogeton pusillus L.,Ottelia acuminata var.Acuminata.,Potamogeton crispus L.etc.could significantly reduce Cond,TN,TP,NO₃-N and PO₄-P in T3,and was the best among the four treatments,indicating that species in this functional group（FG3）can be preferentially used in eutrophication.（5）The communities with species selected from FG3（FG3.p）had significant effects on DO,Cond,p H,Chl-a,TN,TP,NH₄-N,NO₃-N,and the removal effect of nitrogen in T1 was the best.This provides further evidences for that species from FG3 can be used for restoration.In addition,the positive biodiversity-ecosystem functioning relationship is largely attributable to the cooperation of selection and complementary effects.For example,compared with a single functional group,FG123._p in T3 can significantly reduce the NO₃-N,because in a submerged plant community,the more different functional groups,the more fully the utilization of resources in space or time,thereby enhancing the function of the ecosystem（complementary effect）;while the functional trait of the most dominant species/functional group is closely related to the ecosystem functions,that is the functional characteristics of the dominant species in the community determine the function of the ecosystem（selection effect）,such as compared with multiple functional groups,FG3._p in T1 can significantly reduce the NH₄-N,which further indicates that the selection effect and complementary effect of functional structures have a better effect on the treatment of eutrophic water bodies.In summary,enhanced submerged plant species/functional diversity significantly improved the water quality of eutrophication,and its diversity effect was more pronounced,while specific functional groups（FG3）or specific species such as V.natans.,C.demersum L.,E.nuttallii.and other species were introduced,will help to rebuild and restore plant communities under eutrophication conditions.