Study on the improvement of underwater light field in Wuli lake ecological demonstration project

On July 20,2004,the underwater irradiance was measured and the water samples were collected at 9 sampling sites in Wuli lake ecological demonstration project.The concentrations of total suspended mater(TSM),organic suspended matter(OSM),inorganic suspended mater(ISM),chlorophyll a(Chla),dissolved organic carbon(DOC)and absorption coefficients of total particles,phytoplankton,nonalgal part icles and chromophoric dissolved organic matter(CDOM)were mea sured and calculated TSM,Chla and DOC are in the range of 5.1-38.7 mg/L,25.5-86.4μ/L,7.43-8.74 mg/L,respectively.The ecological demonstration project can signif icantly decrease the concentrations of TSM and Chla TSM,Chla of type region decrease by 53.4%,33.0% and TSM,Chla of Ⅲ type region decrease 77.4%,65.0% relative to type region,photosytheically available radiation(PAR)diffuse at enuation coefficients range from 1.81 to 4.93 m^-1,and the corre sponding euphotic depths range from 0.93 to 2.54 m PAR diffuse attenuation coefficients of Ⅱ and Ⅲ type region decrease by 42.2%,52.8%,respectively,and the corresponding euphotic depths increase 73%,112%,respectively. Absorption coefficient of CDOM at 440 nm(a_g(440)),absorption coefficient of nonalgal particles at 400 nm(a_d(440))and absorption coefficient of phytoplankton at 440 nm(a_ph(440))are in the range of 0.79-1.31 m^-1,0.41-2.22 m^-1 and 0.73-2.12m^-1 The relative contribution of pure water(a_w)to total absorption coefficient(a)integrated over the range of PAR(400-700 nm)is less than 15%.The relative contributions of CDOM(a_g),nonalgal particles(a_d)to total absorption coefficient (a)are in the range of 18.16-40.28%,14.40-37.88%,respectively.The relative contributions of phytoplankton(a_ph) to total absorption coefficient(a)is the biggest,ranging from 31.05 to 45.28%.Based on the regression analysis between PAR attenuation coefficients,euphotic depths,secci disc and main light attenuators and the relative contribution of every component,phytoplankton and organic suspended solid play a main role in regulating the attenuation of light in ecological demonstration project in Wuli lake However,the roles of phytoplankton and nonagal particles in regulating the attenuation of light are almost equal to type region of the periphery of ecological demonstration project.