Remote sensing of diffuse attenuation coefficient of photosynthetically active radiation in Lake Taihu using MERIS data

Abstract

Especially for a shallow and extremely turbid lake, light availability in the water column can determine the euphotic zone, and the horizontal and vertical distribution of algae species. Light attenuation is traditionally quantified as the diffuse attenuation coefficient of the photosynthetically available radiation (Kd(PAR)). Global coverage of Kd(PAR) at high spatial and temporal resolution can be provided by satellite measurements, and these data can be used to improve our understanding of the physical, chemical and biological processes in Lake Taihu, a large shallow lake in China, of considerable importance as a source of drinking water for several large cities.The primary dominated contributor to Kd(PAR) was determined firstly using linear regression. There was a significant positive correlation between Kd(PAR) and concentrations of total suspended matter (TSM). The determination coefficient between Kd(PAR) and TSM (R2=0.91) was significantly higher than that between Kd(PAR) and chlorophyll-a concentrations (Chl-a) (R2=0.11), and between Kd(PAR) and absorption of chromophoric dissolved organic matter (CDOM) (R2=0.01). Our results therefore could demonstrate that TSM usually plays a dominant role in the attenuation of light in Lake Taihu.A retrieval model of Kd(PAR) values for Lake Taihu was subsequently developed using top-of-atmosphere (TOA) radiance of Medium Resolution Imaging Spectrometer (MERIS) image data at band 10, which was strongly correlated with in situ Kd(PAR) (R2=0.74, p<0.005, N=48). In contrast, the atmospheric corrected reflectance of MERIS image data was not strongly correlated with in situ Kd(PAR). Thus, atmospheric correction was not a prerequisite for estimations of Kd(PAR) in this highly turbid and hyper-eutrophic lake, and a simple empirical model of the Kd(PAR) estimation for Lake Taihu was effective.With the simple model, the seasonal and spatial distributions of Kd(PAR) in Lake Taihu were studied using the MERIS measurements from 2003 to 2010. Our results show distinct seasonal, spatial, and wind driven, Kd(PAR) values in Lake Taihu. The highest and the lowest Kd(PAR) values were found in summer and in winter, respectively. The increase of Kd(PAR) in summer can be attributed to the phytoplankton blooms, and wind-driven sediment resuspension. Spatially, the Kd(PAR) values were high in the southern part and the lake center, and low Kd(PAR) in East Lake Taihu. Based on the MERIS derived Kd(PAR) values, a significant correlation was found between Kd(PAR) and wind speeds, suggesting a critical role of wind speeds in the Kd(PAR) variations in Lake Taihu.