Study On Physiological Mechanism Of Submerged Macrophyte Distributed In Waters Based On Carbon And Nitrogen Metabolism

Submerged macrophytes are an important part of lake ecosystem as they can purify water quality and maintain ecosystem stability,and are often been used in restoration of water eutrophication.Submerged macrophytes could respond to heterogeneous environments by adjusting the carbon and nitrogen metabolism,subsequently influencing their growth and distribution.In this study,the response mechanism of carbon and nitrogen metabolism in submerged plants to different water environments was explored from the perspectives of water depth gradient,local water environment and water temperature levels by field investigation and controlled experiments.The results of the study are as follows:(1)Submerged macrophytes were sampled every 0.5m along the water depth gradient in 7 transects in Erhai Lake.The water depth gradient had a significant effect on the distribution and community structure of submerged plants.The biomass of Hydrilla verticillata and Myriophyllum spicatum decreased with the increase of water depth,while that of Vallisneria natans and Potamogeton maackianus increased with increasing water depth.The biomass proportion of H.verticillate,Potamogeton lucens,P.maackianus and V.natans were highest at a water depth of 0.5m,1.0m,2m and 3m,respectively.The contents of total nitrogen(TN)and total phosphorus(TP)in H.verticillata increased with the increase of water depth,while those in Potamogeton malaianus were decreased.The carbon and nitrogen metabolite contents did not change significantly within 1.5m for submerged plants.The starch and soluble carbohydrate(SC)contents decreased in H.verticillata,P.maackianus,V.natans and P.lucens,while the free amino acid(FAA)content increased from1.5m to the water depth where plants established.(2)There were significant differences in ability to adaptation to heterogeneous environments among different submerged plants.H.verticillata and P.maackianus distributed more widely in heterogeneous environments,occupying more than 5% of the submerged plant community.The biomass of V.natans in each lake bay accounted for more than 14% of total community biomass,which may be benefit from its strong reproductive ability.No significant correlations were found between the nitrogen and phosphorus concentrations in the water and the nitrogen and phosphorus contents in the submerged plant tissues.The nitrogen and phosphorus contents in the submerged plants were significantly higher in the XJ and WS than those in the HC,HD and HS bays,even though the nitrogen and phosphorus contents in the XJ and WS bays were not higher.The starch and SC contents of submerged plants were lower in the XJ,SP and HS bays on the east than west lakeside zone,and the content of FAA was higher than that in the west lakeside zone.(3)Carbon and nitrogen metabolism were evaluated for submerged macrophytes under three water temperature levels(T1: ambient temperature,T2: ambient temperature+5 °C,T3: ambient temperature +10 °C).Results showed that FAA content in Elodea nuttallii increased significantly under the T1 and T2 treatments from October to December2019 when the temperature decreased from autumn to winter.After the spring in March 2020,the content of starch and SC continued to increase,and the content of FAA continued to decrease.Under the T3 treatment,the FAA content continued to decrease from October 2019 to August 2020.Potamogeton crispus consumed a lot of starch in advance when it was about to enter winter in October 2019,and accumulated SC and FAA.After the winter in November 2019 the FAA content declined,and the starch and SC increased,but from April2020 starch,FAA and soluble reducing sugar have continued to decline.At the same spatiotemporal scale,E.nuttallii can accumulate more starch under low temperature conditions and more carbohydrates under high temperature conditions.On the other hand,under the same temporal and spatial scale conditions,the starch,FAA and SC did not change significantly for P.crispus under different temperature,indicating its high adaptability to temperature variation.In conclusion,the effects of water depth on different submerged plants are different.The biomass of H.verticillate and M.spicatum decreased with the increase of water depth,and the biomass of V.natans The biomass of P.maackianus increased with the increase of water depth,and V.natans could grow well in all water depths of 0-3m.Within 1.5m,the inhibition of carbon and nitrogen metabolism of submerged plants is not obvious,and when the water depth exceeds 1.5m,the inhibition effect of submerged plants by water depth is more significant.Different submerged plants have certain differences in their ability to adapt to water environment.H.verticillate and P.maackianus have strong adaptability to different environments.They are distributed in every lake and bay.V.natans)has the strongest reproductive ability and occupies a high proportion in all lake and bay communities.Water flow rate and human activities will have certain effects on the carbon and nitrogen metabolism of submerged plants.Submerged plants will consume a lot of starch and soluble reducing sugar to accumulate free amino acids to cope with the harsh winter environment when winter comes.When entering spring,the content of starch and soluble reducing sugar will increase again.There are also differences in the response of different submerged plants to temperature.E.nuttallii can accumulate more starch under low temperature conditions and more carbohydrates under high temperature conditions.P.crispus has a wide range of adaptation to temperature,and the difference in carbon and nitrogen metabolism is not obvious at different temperatures.Therefore,when we use submerged plants to restore water bodies,we must fully consider factors such as water depth,water environment and temperature at the site to select suitable submerged plants.