Effects Of Water Depth And Nitrogen Load On Growth And Competition Of Submerged Macrophytes

With the increasing eutrophication of fresh water in China,aquatic vegetation in aquatic ecosystems gradually decreases or even declines and disappears,which makes the grass-type clear water body with complete structure and complete function turn into algae-type turbidity water body.In recent years,the technology of ecological restoration of eutrophic water with submerged macrophytes restoration as the core has become an important ecological practice in the field of aquatic ecology.Submerged macrophytes play an important role in aquatic ecosystems and are essential components.However,there are many difficulties in the recovery process of submerged macrophytes.For example,high water depth,low light and high nitrogen(N)load will affect the recovery of submerged macrophytes.To this end,the paper choose the middle and lower reaches of the Yangtze river in shallow lakes and two kinds of submerged macrophytes Vallisneria natans and Hydrilla verticillata as the research object,by setting up a series of field simulation experiment,the different environmental factors on the influence of different submerged macrophytes growth,thus for ecological restoration and management of eutrophic shallow lakes to provide theoretical support.The main research contents and results are as follows:1.The effect of water depth on the growth of V.natans and H.verticillata.Water depth is one of the main factors affecting the growth of submerged macrophytes in shallow lakes.Rosette V.natans and crown rosette H.verticillata are common submerged plant species in shallow lakes of middle and lower reaches of Yangtze river in China.In natural water body,the influence of water depth change on the growth and competition pattern of these two plants remains to be studied.Therefore,in this study an outdoor mesocosm experiment was carried out using there levels of water depths(50 cm,150 cm,250 cm)to explore the response of the growth and competition pattern to the change of water depth under mixed cropping condition.The results showed that the response of the two submerged plants to the change of water depth was different under the mixed culture condition.With the increase of water depth,the biomass and relative growth rate of V.natans decreased significantly.In the condition of high water depth(250 cm),the biomass and relative growth rate of H.verticillata were also significantly lower than those in the low water depth(50 cm)and the middle water depth(150 cm)groups.In the latter two groups,there was no significant difference in the growth of H.verticillata.The morphological adaptation of the two submerged macrophytes to the variation of water depth is also different.With the increase of water depth,the plant height of H.verticillata increased significantly,indicating that H.verticillata adapted to the change of habitat water depth by increasing plant height.On the contrary,the plant height of V.natans at high water depth was significantly lower than that at medium water depth,indicating that the morphological strategy of V.natans to water depth was different from that of H.verticillata.In this experiment,with the increase of water depth,the biomass ratio between V.natans and H.verticillata increased gradually,indicating that the increase of water depth would increase the competitive advantage of rosette submerged plant-V.natans.2.The influence of water depth on the competition between the species of V.natans and H.verticillata.Competition often occurs between and within submerged macrophytes.When environmental conditions change,plants with competitive advantages can adapt to the changing environment and survive,while plants with competitive disadvantages will suffer more adverse effects or even decline.The effect of water depth on the competitive relationship between the two species remains to be studied.Two water depth gradients(50cm and 150cm),two planting modes(single species of V.natans and mixed planting of V.natans and H.verticillata)were designed to explore the influence of water depth change on the competition between V.natans and H.verticillata.The results showed that the biomass,relative growth rate and plant number of the single species were significantly higher than those of the mixed species,indicating that the H.verticillata had significant inhibitory effect on the growth of the V.natans.The mechanism may be that the H.verticillata formed a dense canopy on the upper layer of the V.natans,and the shading effect of the canopy restricted the growth and reproduction of the V.natans.Meanwhile,the competition of a large number of H.verticillata on nutrients also hindered the growth of the V.natans.In addition,the plant height of V.natans under higher water depth was significantly higher than that under high water depth,and the plant height of V.natans in the mixed species of V.natans and H.verticillata was higher than that of the single species of V.natans,which reflected the growth adaptation of V.natans to low light caused by water depth and H.verticillata canopy shading by increasing plant height.3.Effects of nitrogen load increase on submerged macophytes growth under different light conditions.In the natural water body,there is a strong competitive relationship between V.natans and H.verticillata in nutrient and light.However,in the case of coexistence,the response of the two to the increase of water N load remains to be studied.At the same time,the response of the two to the increase of water N load under different illumination has not been reported.Based on this,we designed an outdoor simulation experiment to explore the influence of exogenous N load increase(input N:P ratio increased from 5:1 to 100:1)on the growth of two submerged macrophytes under different light intensity(high light(sunlight)and low light(shading 75%))by mixing two submerged macrophytes.The results showed that under high light condition,the increase of N load had no significant effect on the biomass,relative growth rate,plant height and root-leaf ratio of V.natans.The increase of N load significantly reduced the biomass,relative growth rate,plant height and plant number of H.verticillata.In the low light condition,the increase of N load did not cause significant difference between the two species.Correspondingly,under high-light condition,the increase of N load significantly reduced the total biomass of V.natans and H.verticillata.In the low light condition,the N load increase had no significant effect on the total biomass of the two species.The results showed that there was no significant difference in the adaptation to the increase of N load under different light conditions.However,the adaptability of H.verticillata to the increase of N load was significantly different under low light and high light conditions.Under high light conditions,the increase of N load significantly reduced the total biomass of submerged plants and increased the concentration of Chl-a in water.This result indicates that the increase of N load may be conducive to the transformation of the ecosystem from grassy clear water state to algal turbid water state.From the perspective of lake management and ecological restoration,this study supports the view that both N and P need to be controlled.