Adaptation Mechanism Of Submerged Macrophytes In Eutrophic Lakes To Low Underwater Light

Eutrophication is a common problem in lakes along the middle-lower reaches of Yangtze River,and the low underwater light caused by this phenomenon can threaten the survival of submerged macrophytes.In these lakes,various substances can affect the absorption and scattering of red and blue spectra,and the intensity and quality of the incident light are changed.The changed light environment will directly affect the growth and distribution of submerged macrophytes.Submerged macrophytes are important primary producers in aquatic ecosystem,and their growth and distribution affect the ecological service function.In this study,by investigating the underwater light environment and submerged macrophyte growth and distribution characteristics in 19 shallow lakes along the middle-lower reaches of Yangtze River,we analyzed the influence factors of the underwater light environment,the interaction and feedback effect of the underwater light environment and the growth and distribution of submerged macrophytes.Combining the results of field experiments with indoor experiments,we analyzed the physiological responses of common submerged macrophytes to different light environment to throw light upon the ecological adaptation mechanism of submerged macrophytes in eutrophic lakes with low-light environments.The results suggested:（1）In eutrophic lakes,nitrogen,phosphorus,suspended solids and chlorophyll a were the key environmental factors affecting the underwater light environment.In 19shallow lakes along the middle-lower reaches of Yangtze River,the underwater spectrum showed the strongest decline in blue light,followed by green and red light.In lower-transparency lakes,the attenuation rate of blue light was faster than that of red light,and the ratio of red to blue light increased exponentially with water depth(Red/Blue=0.5974?^{0.3596（2）,R2=0.9913}.The results of structural equation modeling（SEM）showed that the increase of nitrogen and phosphorus concentration did not directly affect the underwater light environment（Estimates=-0.033,p>0.05）,but led to the increase of suspended matter s（TSS,WChl a）（Estimates=0.801,p<0.01）and then reduced the transparency of the water column(SD,Z_eu/WD)（Estimates=-1.518,p<0.001）,which eventually increased the light attenuation and affected the underwater light spectral distribution（Estimates=-0.918,p<0.001）.（2）Underwater light environment could affect the growth and distribution of submerged macrophytes.Underwater light environment（K_d,Red/Blue）had no significant effect on the stoichiometric traits（total carbon,total nitrogen,total phosphorus,ratio of nitrogen to phosphorus）and photosynthetic traits（plant chlorophyll a,ratio of Chl a to Chl b,Fv/Fm,alpha,ETRmax）of submerged macrophytes（p>0.05）,but the deterioration of underwater light environment led to the decrease of morphological traits（leaf area,leaf dry weight）（Estimates=-0.261,p<0.001）,which led to the decrease of species richness of submerged macrophytes（Estimates=0.18,p<0.01）.The underwater light environment could directly affect the abundance of submerged macrophytes（Estimates=-0.533,p<0.001）.（3）Submerged macrophytes in eutrophic lakes had a positive feedback on the underwater light environment.This study found that the concentrations of nitrogen,phosphorus and chlorophyll in the areas with submerged macrophytes distributed were significantly lower than those without submerged macrophytes（p<0.05）,indicating that submerged macrophytes have a significant improvement effect on the underwater light environment of lakes.When the numbers of submerged macrophyte species reached 3 or more,the water transparency and light attenuation coefficient increased significantly（p<0.05）.Different types of submerged macrophytes have different effects on the improvement of underwater light environment.From the perspective of growth form,the underwater light environment in the areas with rosette-form submerged macrophytes was better than that of canopy-form submerged macrophytes（p<0.05）.In terms of leaf type,the underwater light environment in the areas with flat-leaf submerged macrophytes was better than needle-leaf type submerged macrophytes,and the distribution depth of flat-leaf type submerged macrophytes was shallower than that of needle-leaf type submerged macrophytes.（4）H.verticillata and V.Natans showed significant differences in morphology,biomass,plant height,and tiller number under different light quality conditions.There were no significant differences in stoichiometric and photosynthetic traits between H.verticillata and V.natans under different red-blue ratio LED light（p<0.05）.With the increasing of the proportion of red light,the leaf area,H.verticillata and V.Natans showed the same survival strategy under different ratios of red and blue light.As the proportion of red light increased,the height of the two macrophytes decreased（p<0.05）,and the number of tillers and biomass increased（p<0.05）.