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

Abstract

The diffuse attenuation coefficient of photosynthetically active radiation (PAR) (400–700 nm) (Kd(PAR)) is one of the most important optical properties of water. Our purpose was to create Kd(PAR) prediction models from the Secchi disk depth (SDD) and beam attenuation coefficient of particulate and dissolved organic matter (Ct−w(PAR), excluding pure water) in the PAR range. We compare their performance and prediction precision by using the determination coefficient (r2), relative root mean square error (RRMSE), and mean relative error (MRE). Our dataset comprised 1,067 measurements, including Kd(PAR), SDD, and Ct−w(PAR) taken in shallow, eutrophic, Lake Taihu, China, from 2005 to 2010. The prediction models of Kd(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 Kd(PAR) with a r2 of 0.804. Compared to the SDD, Ct−w(PAR) was more significantly correlated to Kd(PAR) based on the linear model, with a significantly higher r2 and lower RMSE and RE. Considering the measurement simplicity of Ct−w(PAR) and data acquisition feasibility from high-frequency autonomous buoys and satellites, our results demonstrated that this prediction model reliably estimates Kd(PAR), and could be used to significantly expand optical observations in an environment where the conditions for underwater PAR measurement are limited.