To assess the seasonal and spatial variations and long-term trends in water optical properties in Lake Fuxian,investigations based on field work in four seasons and a long-term analysis of data from 1980 to 2014 were conducted.The results show that there was no significant variation in the euphotic depth (Zeu) across the four seasons,and no significant correlations between Zeu and potential influencing factors in seasons other than summer,suggesting that the water itself may be a major factor regulating the Zeu in general.Nevertheless,significant differences in Zeu between the north region (NR) and the south region (SR) were observed in all seasonal tests except spring.This finding relates to a higher abundance of chromophoric dissolved organic matter (CDOM) in the NR due to runoff,especially in the rainy seasons (summer and autumn).CDOM and its terrigenous component had an important impact on Zeu in summer,with the highest precipitation,and impacts from suspended solids and non-algal particles were also found in the NR in summer.The Secchi disk depth in the lake decreased clearly over the years,with significantly negative correlations with the increasing permanganate index and air temperature,implying that organic contaminants (CDOM and/or phytoplankton) are important regulators of water transparency.We estimate that the combined effects of climate warming and changes in land use and land cover are also indirect regulating factors.These findings should be considered in the protection of Lake Fuxian,owing to the importance of light penetration in aquatic ecosystems.