Predicting the light attenuation coefficient through Secchi disk depth and beam attenuation coefficient in a large, shallow, freshwater lake

The diffuse attenuation coefficient of photosynthetically active radiation (PAR) (400–700 nm) ( K d (PAR)) is one of the most important optical properties of water. Our purpose was to create K d (PAR) prediction models from the Secchi disk depth (SDD) and beam attenuation coefficient of particulate and dissolved organic matter ( C t−w (PAR), excluding pure water) in the PAR range. We compare their performance and prediction precision by using the determination coefficient ( r 2 ), relative root mean square error (RRMSE), and mean relative error (MRE). Our dataset comprised 1,067 measurements, including K d (PAR), SDD, and C t−w (PAR) taken in shallow, eutrophic, Lake Taihu, China, from 2005 to 2010. The prediction models of K d (PAR) were based on the linear model with an intercept of zero, using the inverse SDD, and the nonlinear model using SDD. The linear model generated a slope of 1.369, which was not significantly different from 1.7, the index used worldwide, but significantly lower than the value of 2.26. The nonlinear model gave a slightly more reliable prediction of K d (PAR) with a r 2 of 0.804. Compared to the SDD, C t−w (PAR) was more significantly correlated to K d (PAR) based on the linear model, with a significantly higher r 2 and lower RMSE and RE. Considering the measurement simplicity of C t−w (PAR) and data acquisition feasibility from high-frequency autonomous buoys and satellites, our results demonstrated that this prediction model reliably estimates K d (PAR), and could be used to significantly expand optical observations in an environment where the conditions for underwater PAR measurement are limited.