Effect Of Artificial Light Intensity On The Growth Of Vallisneria Natans And The Quantitative Relationship

Urban lakes are important natural assets,with the functions of controlling flood and drainage,regulating climate,and beautifying urban environment.Due to the multiple stressors of human disturbances,most urban lakes are in severe eutrophication,faced with frequent algal blooming,and serious ecosystems degrading.Submersed macrophytes are fundamental to maintain the clear-water state in lakes,and the reconstruction of submersed macrophytes is thus currently one of the main measures for ecosystem restoration in eutrophic lakes.However,due to the poor growth conditions of damaged lakes,the restoration of submersed macrophytes has long been one of the biggest challenges for lake ecological restoration.Impaired underwater light climate in eutrophic lakes is a major stimulus causing the loss of submersed macrophytes.Therefore,enhancing the underwater light conditions with light supplementation by light-emitting diodes（LEDs）might be a potential effective method to restore submersed macrophytes in urban lakes.At present,researches on LED light supplementation promoting plant growth mainly focuses on terrestrial plants,and only a few studies explain its effects on submersed macrophytes under low and medium nutrient conditions,and quantitative relationship between light intensity and the growth of submersed macrophytes is unclear.Therefore,whether LED supplementation could promote the growth of submersed macrophytes in more severe eutrophic waters remains unclear.In this study,the Vallisneria natans,a widely distributed submersed macrophyte in the middle reach of the Yangtze River,was selected as the tested plant.There were two simulation experiments respectively performed in two mesocosm systems.The first experiment was conducted in cement tanks（1 m×1 m×1 m）with LED supplementation intensities setting between 0～100μmol m^-2 s^-1,To explore the relationship between LED supplementation intensity and the growth of submersed macrophytes in eutrophic conditions.The second experiment was conducted in polyethylene buckets（1 m in diameter and 0.9 m in height）with LED supplementation intensities setting between 0～250μmol m^-2 s^-1,to further analyze the relationship between LED supplementation intensity and the growth of submersed macrophyte and establish a quantitative model.Combined with the quantitative model between light attenuation coefficient and turbidity of water,the quantitative model between light intensity of LED light,the effective light and turbidity of water was constructed.Finally,the ecological restoration scheme of submersed macrophytes restoration assisted by LED light supplementation was designed in eutrophic urban lakes（Lake Nanhu）.The main results are as follows:1)LED supplementation can promote the growth of submersed macrophytes in eutrophic conditions.In the mesocosm systems,most growth variables（shoot number,leaf number,maximum leaf length,average leaf length,total leaf dry mass,and total root dry mass）of submersed macrophytes in the high light-supplemented group were 1.6～11.1times higher than those in the control group.2)LED supplementation did not promote the growth of phytoplankton,and the increase of periphyton had no significant effect on the growth of submersed macrophytes.The results showed that the content of chlorophyll a of phytoplankton maintained at a low level(6.2～13.4μg L^-1),and was not significantly correlated with other environmental factors or plant growth variables.Although the periphyton on the surface of leaves increased to a certain extent(32.4±8.51 mg m^-2 in the highest light group at the terminal stage of experiment),there was no negative correlation between the periphyton and growth variables,and no inhibition effect on the growth of Vallisneria natans.3)A quantitative model between the relative growth rate（RGR）（×10²）and the received effective light intensity（F）was established:RGR=0.90 ln（F）-2.15（R²=0.53,p<0.01）.The minimum effective light intensity for promoting the growth of submersed macrophytes was 10.9μmol m^-2 s^-1.When the effective light was in the range of 10.9～100.6μmol m^-2 s^-1,the RGR increased rapidly from 0 to 0.02 g d^-1,while it increased slowly when it was greater than 150μmol m^-2 s^-1.Therefore,the optimal effective light intensity to promote the growth of submersed macrophytes was 150μmol m^-2 s^-1.4)The relationship between the actual light intensity（F₀）at the top of the leaves,the effective light intensity（F）and the water turbidity（Turb）was established:F₀=12 F{[1/（?）（0.034 Turb+0.14）]–1}/{[1/（?）（0.408 Turb+1.68）]–1}.The quantitative model between light attenuation coefficient（K）and water turbidity（Turb）in the study was K=0.68 Turb+2.80（R²=0.58,p<0.01）.Combined with the quantitative model between relative growth rate（RGR）and the effective light intensity（F）of submersed macrophytes,the relationship between LED light intensity（F₀）,the effective light intensity（F）and water turbidity（Turb）was determined.4)The ecological restoration scheme of submersed macrophytes restoration assisted by LED light supplementation was designed in eutrophic urban lakes（Lake Nanhu）.After the construction of enclosure,sediment improvement,and fish community structure adjustment,LED light supplementation was applied to reconstruct the submersed macrophytes community.When the turbidity of Lake Nanhu is 47.6 NTU,the light intensity at the top of submersed macrophytes should not be lower than 55.1μmol m^-2 s^-1to promote the growth of plants.The results showed that LED light supplementation could promote the growth of submersed macrophytes under eutrophic conditions.LED supplementation did not promote the growth of phytoplankton,and the increase of periphyton had no significant effect on the growth of submersed macrophytes.The lowest light supplement intensity was 10.9μmol m^-2 s^-1,and the optimum intensity was about 150μmol m^-2 s^-1.In a certain range,the light intensity was proportional to the plant growth variables,but when the light intensity approached or exceeded the optimum intensity,the plant growth did not change significantly with the increase of light intensity.Main innovation points:a quantitative model between the relative growth rate of submersed macrophytes,the light intensity and water turbidity under eutrophic conditions was established.The relevant research results can provide technic principles for the application of LED light supplement to submersed macrophytes restoration in urban eutrophic lakes.