Mechanisms of Remote-Sensing Reflectance Variability and Its Relation to Bio-Optical Processes in a Highly Turbid Eutrophic Lake: Lake Taihu (China)

During November 2006 and November 2007, two field bio-optical campaigns were conducted in Lake Taihu to assess the mechanisms regulating the water-color variability in a typically inland eutrophic lake. Various bio-optical parameters were acquired simultaneously with measurements of remote-sensing reflectance (R _rs), which generates a collection of 106 samples. The bio-optical parameters span a wide range of variability indicating the presence of complex water-column conditions, which also correspondingly brings about a great variability of the R _rs( ??) spectra. The R _rs(??) variability characteristics are extracted by an empirical orthogonal-function (EOF) analysis. The results show that 84% of the total variance of R _rs(??) is mainly attributed to particulate absorption (particularly chlorophyll a (Chl a) absorption) and pure-water absorption, which has been summarized as the first EOF mode, M ₁, and M ₂ can mainly reflect the other variance of R _rs(??), which can be attributed entirely to phytoplankton: absorption by pigments and scattering by particles. Further, the in situ R _rs(??) spectra are categorized into two types by a hierarchical cluster approach, that is, a universal type and a phytoplankton-dominant type. The phytoplankton-dominant stations are characterized by high Chl a concentration, organic suspended-matter concentrations, and phytoplankton absorption (a _ph) with a good regression relationship between R _rs (700) and Chl a concentration (R ² = 0.922), while the universal stations present the wider variation ranges of particulate scattering (b _p) and backscattering (b _bp) due to the great inorganic suspended-matter concentration variation.