Primary Research On Adaptation To Underwater Light Attenuation By Submerged Macrophytes In Lakes

Submerged macrophytes constitute a key element of aquatic ecosystems and are important for nutrient cycling,primary production and sustaining biodiversity.The availability of light and underwater light regime are widely considered the most important determinants controlling submerged macrophytes growth.Light availability depends on nutrient concentration,the abundance of phytoplankton,suspended solids and colour of the lake water.In this study,we investigated water environment,the underwater light regime and maximum colonization depth of submerged macrophytes on 11 shallow lakes in the middle and lower reaches of the Yangtze River.We conducted also the controlled experiments of response to light quality by 10 macrophytes.The main aim was to investigate the relationships among eutrophication,the underwater light regime,and the maximum colonization depth.The main results are showed as follows:1.Characterizing the underwater light field The photosynthetically active radiation(PAR)sharply decreased with the increase of water depth,and the attenuation coefficients of PAR(Kd(PAR))varied greatly among different lakes.Affected by eutrophication and floods,Kd(PAR)in some lake areas had increased in recent years.Due to TSS and CDOM absorption effects,the attenuation of blue light is stronger than the attenuation of red light.Compared with the spectrum of the atmosphere,the spectrum of underwater light climate accounts for more red light and less blue light.This phenomenon is strengthened with Kd(PAR)increase.2.The nature of the underwater light attenuation Essentially all the light absorption that takes place in natural waters is attributable to four components:the water nutrient,chromophoric dissolved organic matter(CDOM),the phytoplankton and total suspended solid(TSS).Total nitrogen(TN)and total phosphorus(TP)indirectly affect the underwater light attenuation in by promoting the growth and reproduction of phytoplankton and epiphyton,resulting in inhibited photosynthesis of rosette-type plants,which had no significant effect on the vitatte-type plants.3.Light acclimation mechanism of submerged macrophytes Only three species(Cabomba caroliniana,Myriophyllum spicatum and Vallisneria natans)among the 13 species showed leaf morphological plasticity to light climate.Cabomba caroliniana decreased its specific leaf area by decrease leaf dry weight to acclimate to lower PAR.The initial slope of photosynthesis of Myriophyllum spicatum and Vallisneria natans increased with the specific leaf area.Vitatte-type macrophytes(eg,Ceratophyllum)can utilize lower light illumination than rosette-type(eg,Vallisneria).Moreover,submerged angiosperms may be suit with "the theory of Chromatic adaptation",M.spicatum and Hydrilla verticillata occupy the niche in the deep water zone by adapt to the blue light decay strongly in the water colume of inland lakes.4.Macrophytes responed to water eutrophication The maximum colonization depth decreased and species diversity decreased obviously with the increase of TN,TP and Chl a.However,Cmax varies widely with a given concentration of TN,TP,Chl a or TSS.Therefore,our results do not support the maximum colonization depth(Cmax)as an indicator of eutrophication.Our studies suggest that the maximum colonization depth of submerged macrophytes in different lakes resulted in the competition and adaptation of plants to light intensity and light quality.In the lakes with low Kd(PAR),Cmax(Ceratophyllum.demersum),Cmax(P.wrightii)>Cmax(H.verticillata),Cmax(M.spicatum)>Cmax(N.marina),Cmax(V.natans),Cmax(P.maackianu);in the lake with high Kd(PAR),Cmax(H.verticillata),Cmax(M.spicatum)>Cmax(Ceratophyllum demersum),Cmax(P.wrightii).