Optically active substances and their contributions to the underwater light climate in Lake Taihu, a large shallow lake in China

There are few data sets on optically active substances and optical conditions to partition the relative contribution of every substance on light attenuation in a lake. To address this need, underwater photosynthetically active radiation (PAR), optical parameters and concentrations of three optically active substances, tripton (non-phytoplankton particulate matter), chlorophyll-a (Chl-a), chromophoric dissolved organic matter (CDOM) were measured at 67 sites in Lake Taihu, a large shallow lake in China in October 2004. The spatial variation of optically active substances and their relative contributions to the underwater light climate were determined. The PAR diffuse attenuation coefficient K-d(PAR) at different sites varied between 0.87 and 12.43 m(-1), with a mean of 4.42 m(-1). Significant spatial differences were found for optically active substances and PAR attenuation, with low concentrations and weak light attenuation in macrophyte-dominated bays such as East Lake Taihu, Xukou Bay and Gongshan Bay, and high concentrations and strong light attenuation in the open water and algae-dominated areas such as the center of the lake, southwestern region, and Meiliang Bay. The mean relative contributions of tripton, Chl-a, and CDOM to light attenuation were 82.6 %, 9.7 %, and 6.8 % of K-d(PAR), respectively. Tripton was the dominant constituent of K-d(PAR), and mean of 97.5 % of the variation in K-d(PAR) could be explained by tripton. In autumn, the relative contribution of CDOM to light attenuation was lower than that of phytoplankton. We conclude that suspended particulate matter, rather than dissolved organic matter, is the dominant factor for PAR attenuation in Lake Taihu.