Study On Interaction Of Submerged Macrophytes And Nutrients In The Water And Sediment

Submerged macrophytes are an important components in lake ecosystems,and have an important ecological function.The disappearance of submerged macrophytes would lead to deterioration of lake ecosystems.The restoration and maintaining of submerged vegetation play important roles in lake procection and restoration.The major factor causing disappearance of submerged macrophytes is eutrophication of water,especially the increase of nitrogen(N) and phosphorus(P) in water.It is important to study the interaction of submersed macrophytes and N,P of water environment in lakes.In this thesis,based on the experimental approach,effects of growth of submersed macrophytes Hydrilla verticilla and Vallisneria natans on dynamic change of the TN,TP and suspended solids in water body was studied firstly.And then the variation of chemical forms of phosphorous in sediments brought by the submersed macrophytes was analyzed.Thirdly,effect of submerged macrophytes on the allocation of external phosphorus in simulative aquatic ecosystems was studied. Lastly,the responses of submersed macrophytes to external nutrient source and of sediment type were investigated.The main results are summarized as following:(1) The results showed that the existing of submerged aquatic plant can improve the water quality obviously when receiving three different nutrient loadings(4 mg·N L^-1 and 200μg·P L^-1,1 mg·N L^-1 and 50μg·P L^-1,or 0.2 mg·N L^-1,10μg·P L^-1 every tow weeks).Effects on water quality differed depending on nutrient loadings and the submerged macrophytes species.Under the low nutrient loading,compared with CK, the concentration of TN,TP,Chl-a and SS in Vallisneria natans tanks were reduced by 56.9%,39.1%,17.2%and 20.5%at the end of the experiments.The concentration of TN,TP,Chl-a and SS in Hydrilla verticilla tanks were reduced by 82.4%,93.4%, 94.2%and 93.5%.Under the medium nutrient loading,compared with CK,the concentration of TN,TP,Chl-a and SS in Vallisneria natans tanks were reduced by 37.1%,28.6%,38.0%and 8.6%at the end of the experiments.The concentration of TN,TP,Chl-a and SS in Hydrilla verticilla tanks were reduced by 73.2%,93.5%, 96.6%and 90.2%.Under the high nutrient loading,compared with CK,the concentration of TN and Chl-a in Vallisneria natans tanks were reduced by 9.7%and 14.0%.The concentration of TN and Chl-a in the waterof Hydrilla verticilla tanks were reduced by 24.8%and 75.9%.(2) Submersed macrophytes could effectively decrease the content of total nitrogen(TN),total phosphorus(TP) and different chemical forms of phosphorus in the sediment,especially Vallisneria natans.The two submersed macrophytes can change the phosphorus in the sediment through shoot oxygenation and nutrition allocation or some other mechanisms.Though the mechanism is complicated,the submersed macrophytes really reduce the content of phosphorus to the different extend during their growing period.(3) Submerged macrophytes have an important role on the allocation of external phosphorus.In the aquatic ecosystems where submerged macrophytes grew well, submerged macrophytes took up lots of phosphorus.In such ecosystems,submerged macrophytes are a nutrient sink.In the other ecosystems where phytoplankton dominated,sediment were the main sink of external phosphorus,and lots of external phosphorus remained in the water as PP.Effects of submerged macrophytes on the allocation of phosphorus was influenced by the degree of phosphorus loading.When the nutrient loading was low,the phosphorus sunk on the submerged macrophytes mainly.Submerged macrophytes were limited with the increase of nutrient loading. And the exogenous phosphorus sunk on the sediment mostly.(4) The results suggested that nutrient loading could have a nagetive effect on the growth of submersed macrophytes,and such effect differs among different species. Vallisneria natans grew well when the nutrient loading was low.The growth of Vallisneria natans got limited under medium nutrient loading,the biomass of Vallisneria natans in medium nutrient loading is 72.3%of which in low nutrient loading.In the high nutrient loading,the growth of Vallisneria natans got most limited.In definite limit concentration,Hydrilla verticilla grew better with the increase of nutrient loading.However,when the nutrient loading was too high,which a nagetive effect on the growth of Hydrilla verticilla was also detected.When the experiment ended,the biomass of Hydrilla verticilla was 11.3 g,18.1 g and 8.9 g under low,medium and high nutrient loading.(5) To determine the effect of different sediments on the morphology,biomass accumulation and allocation of Vallisneria natans,plants were planted in four different sediments:lake sediment,bank sediment,sandy loam and river sediment. After growth of 7 weeks,plants were dried,separated into above and below ground parts and weighed.The results showed that among the test sediments,the biomass, ramet number and total stolon length of V.natans grown in the lake sediment was significantly higher than the plants grown in the sandy loam,bank sediment or river sediment.Compared with the plants grown in the other sediments,V.natans grown in sandy loam allocated more biomass to roots and had the highest r:s,0.48.But root diameter of V.natans grown in sandy loam was lowest.Chlorophyll content of V. natans grown in sandy loam and river sediment were significantly lower than the plants grown in the lake sediment and bank sediment.The results suggest that sediment type plays an important role in growth of this submerged macrophyte:to some extent,V.natans is able to grow normally under conditions of the fertile sediments,but the heavily eutrophic sediment as well as sterile sediment poses a negative effect on the growth of V.natans.