| Land/inland water ecotones are the transition areas between aquatic and terrestrial ecosystems located along rivers,streams,and lakes.Aquatic plants play an important role in the function of the land/inland water ecotones between aquatic and terrestrial ecosystems,such as reducing pollutants transferring from terrestrial to aquatic ecosystems.The local government in Kunming,Yunnan province decided to restore the hydrologic connectivity between the eutrophic Lake Dianchi,and its associated ponds and farmlands in the land/inland water ecotones,to decrease the water quality deterioration in Lake Dianchi and restore the structural and functional role of this aquatic ecosystem.This thesis investigated the characteristic of the land/inland water ecotones for Lake Dianchi,including the macrophytes distribution and their role of storing heavy metals,and nitrogen removal potentials in the land/inland water ecotones,to provide scientific support for restoring the hydrologic connectivity for Lake Dianchi.The main findings are as follows:1.There was a substantial abundance of aquatic species in the land/inland water ecotones for Lake Dianchi according to our investigation.This included 251 species that belong to 182 genera and 78 families.The main families of these aquatic species were Gramineae,Compositae,Leguminosae,Labiatae,Potamogetonaceae and Umbelliferae.These species belonged to thirteen of the fifteen areal-types for Chinese seed plants and dominated by the Widespread areal-type(39 genera),Pantropic areal-type(18 genera)and North Temperate areal-type(16 genera).Therefore,the aquatic species in the land/inland water ecotones for Lake Dianchi showed a subtropical floristic characteristic and has the shifting characteristics from sub-tropical to warm temperate flora.We divided these aquatic plants into five groups according to the TWINSPAN classification.These five groups were dominated by five ecological associations,including 1)Nymphaea tetragona + Nelumbo nucifera + Thalia dealbata;2)Azolla imbricate + Trapa bispinosa;3)Eichhornia crassipes;4)Alternanthera philoxeroides;and 5)Lemna minor ecological associations.We also analyzed the correlation between the distribution of aquatic plants and nine environmental variables in the water column.The results showed that the distribution of aquatic plants was relatively more important in determining the dissolved oxygen,chemical oxygen demand and total phosphorus concentrations in the water column,compared with the water column ammonium,nitrate,total nitrogen,and chlorophyll a concentrations.2.We investigated the best water depth and substrate requirements for the growth of common aquatic plants in the land/inland water ecotones for Lake Dianchi,which could provide useful instructions for the selection of suitable aquatic plants for vegetation restoration in the land/inland water ecotones.Our results showed that Lythrum salicaria and Typha orientalis had better water depth adaptability than other species,as they grow better at all tested water depths.The water depth of 50 cm was more suitable than other water depths,as six species showed their greatest growth rates at this water depth,including Echinochloa crusgalli,Zizania latifolia,Acorus calamus,Iris tectorum,and Polygonum hydropiper.Additionally,sandy substrates significantly decreased the growth of all tested species,compared with the muddy substrates.Specifically,Sagittaria trifolia,Alisma plantago-aquatica,and Iris tectorum showed significantly higher requirements for substrate conditions,compared with Polygonum lapathifolium,Scirpus y agar a,Acorus calamus,Scirpus validus,Phragmites austral,Zizania latifolia,and Monochoria vaginalis.3.In order to examine the role of aquatic plants in reducing and storing pollutants,such as heavy metals,we investigated the concentrations of ten heavy metals(i.e.,As,Cd,Co,Cr,Cu,Fe,Mn,Ni,Pb,and Zn)in the water column,sediments,and submerged macrophytes collected from 37 ponds around Lake Dianchi.Our results showed that both the water column and sediments of these ponds were polluted by Pb.The maximum values of As,Cr,Fe,Mn,and Zn in water of ponds were 0.034,0.010,1.140,1.330,and 0.096 mg L-1.Mean concentrations of all metals,except Pb,were lower than the values of grade I of the environmental quality standards for surface water in China.In the cases of Co and Pb,the average concentrations determined in sediments substantially exceed the corresponding soil background values in Yunnan Province.In contrast with our hypothesis,heavy metal concentrations in water and sediments of ponds did not differ among land-use types.A significant difference was only found in Cd concentrations among the submerged plants(p<0.05).The Cd concentration in Ceratophyllum demersum was significantly higher than that in Myriophyllum spicatum,but not significantly higher than that in Potamogeton maackianus and Stuckenia pectinata.Except for Mn and Zn,plant metal concentrations had no significant relationships with metal concentrations in the water column.For all heavy metals,no significant correlations were found between submerged macrophytes and sediments.The maximum concentrations of Cr,Fe,and Ni in C.demersum were 4242,16429 and 2662 mg kg-1,respectively.The result suggests that C.demersum could be a good candidate species for removing heavy metals from polluted aquatic environments.4.Sediment denitrification was an important pathway for nitrogen(N)removal in eutrophic lakes.We demonstrated that the littoral zone was the hotspot to remove N through sediment denitrification in eutrophic lakes.In this study,we measured the potential denitrification,unamended denitrification,and nitrous oxide(N2O)production rates in surface sediments of 20 Chinese plateau lakes with different watershed land uses.The result showed that the potential denitrification rate of human-dominated lakes(37.94 ± 8.91 ng N g-1 h-1)was significantly higher than that of reference lakes(18.50 ± 3.22 ng N g-1 h-1).Potential denitrification rates were positively related to the proportion of human land uses in watersheds.At the lake level,unamended denitrification and N2O production rates were significantly related to water chemistry or sediment properties(e.g.,conductivity and sediment total nitrogen).Water chemistry and sediment properties together accounted for 0-69%of the variance in denitrification and N2O production rates.Variance partitioning showed that unamended denitrification and N2O production rates in human-dominated lakes were controlled primarily by sediment properties,while in reference lakes were generally controlled by water chemistry.Our findings suggest that Chinese plateau lakes can remove large quantities of N through sediment denitrification and produce small amounts of N2O.The relative contributions of water chemistry and sediment properties to the lake denitrification and N2O production vary widely.Littoral zones had an unamended denitrification rate considerably higher than profundal zones,which suggested that the littoral zones in plateau lakes were hotspots for sediment denitrification and N removal.Therefore,the littoral zones should be considered when characterizing N dynamics and restoring eutrophic lakes.The quantity of N exported from agricultural and urban land uses in watersheds should be controlled to improve the water quality of these plateau lakes,although our findings suggest that lake sediments have the capacity to remove much of the anthropogenic N. |
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