Abstract: The behavior of light in water, particularly its attenuation with depth, has important ecological and water quality implications. The underwater light climate ultimately determines the depth distribution, abundance and primary production of phytoplankton and macrophyte. Light attenuation is regulated by the composition and concentration of various attenuating constituents, which include water itself, gelbstoff, phytoplankton and tripton. Here the vertical attenuation coefficients of downward visible irradiance were measured in 10 different stations in Hangzhou West Lake using 192SA and 193SA light sensors and a SR9910PC scanning spectroradiometer at the water surface and at different depths. Water chemistry variables such as total suspended solids (TSS), organic suspended solids (OSS), inorganic suspended solids (ISS), chlorophylla (Chla), dissolved organic carbon (DOC), absorbance of chromophoric dissolved organic matter (a) were measured to determine the relative contribution of dissolved and particulate components in explaining the variation in diffuse attenuation coefficients.The attenuation properties of Hangzhou West Lake vary greatly though it is a small shallow lake. The main attenuators such as TSS, Chla and DOC vary from 3.68 to 42.76 mg/L, 4.64 to 85.95 μg/L, 5.19 to 9.22 mg/L, respectively. CDOM absorption coefficients at 440 nm are in the range of 0.30～1.46 m-1. PAR attenuation coefficients range from 1.13 to 6.04 m-1 with an average value of 4.00±1.69 m-1, the corresponding euphotic depth ranging from 0.76 to 4.08 m with an average value of 1.54±1.11 m. Only small south lake and Maojiabu lake regions the euphotic depths are larger than water depths. Therefore, the restoration of macrophyte is difficult under present underwater light condition and water level. Based on regression analysis between PAR attenuation coefficients, euphotic depths, secci disc and main light attenuators, phytoplankton and organic suspended solid play a main role in regulating the attenuation of light.

Key words: Hangzhou West Lake, diffuse attenuation coefficient, euphotic depth, chlorophylla, total suspended solids