Underwater light field determined by water constituents in highly turbid water: the case of Taihu lake

The relationships between optical properties and water constituents in highly turbid productive water were studied on the basis of the multiple bio-optical measurements and samplings of water constituents made during five cruises from 2006 to 2008 in Taihu lake. Taihu lake is a high dynamic ratio [(square root of area)/depth] inland shallow lake. The spatial and temporal variation of water constituents and optical properties is significant. The inorganic suspended matter (ISM) has become the primary constituent in Taihu lake: its average percentage can reach 65.21%. The concentration of ISM is highly correlated to the optical properties in Taihu lake due to the sediment resuspension. Consequently, the ISM can be taken into account as an important optically-active constituent in Taihu lake. Resuspended sediments also lead to a poor correlation between scattering optical property and chlorophyll-a concentration (C_Chl-a). However, empirical relationship between the C_Chl-a and phytoplankton absorption coefficient at 675 nm is still valid when the package effect is removed. The parameters of linear equation in the present study have slight temporal variation, especially for the relationship between inherent optical properties (IOPs) and concentration of total suspended matter (TSM). The relationship between apparent optical property (AOP) (diffuse attenuation coefficient of particle, K_dbio) and ISM has been examined as well. The K_dbio is strongly affected by ISM, and correlates to it with linear function. The difference between specific diffuse attenuation coefficients of organic [K^*_dOSM(l)] and inorganic [K^*_dISM(l)] particles is significant. K^*_dOSM(l) includes the absorption property of chlorophyll-a (chl-a) at 675 nm, which is much higher than that of K^*_dISM(l). This indicates that the attenuation ability of OSM is stronger than that of ISM although the K_dbio induced by large concentration of ISM is bigger than the K_dbio induced by small concentration of OSM. Overall, the knowledge of the bio-optical properties obtained in this study extends our understanding of water optics and can be used to predict the optical properties via water constituent concentration, regardless of measurement type (in situ or simulated by hydrodynamic model).