Light absorption is an inherent optical property, which determines the transmission and distribution of underwater light.Knowledge about the inherent and apparent optical properties of water is very important for the development of bio-optical algorithms for remote sensing, and of bio-optical models for estimation of primary production as well as ecological restoration of eutrophic water bodies. In shallow eutrophic lakes, phytoplankton blooms and continuous sediment resuspension by benthivorous fish and wind lower the underwater light, which results in loss of submerged vegetation. The light absorption properties of an aquatic medium are characterized by the medium’s absorption coefficient. The total absorption coefficient a(λ) includes the coefficients for total particulate matter (phytoplankton and nonalgal particulates) ap(λ), chromophoric dissolved organic matter (CDOM) ag(λ) and pure water aw(λ). Phytoplankton absorption influences the maximum photosynthetic rate as well as the rate of photosynthesis integrated over depth. CDOM limits the penetration of biologically damaging UV-B radiation (280320 nm) in the water column, thus shielding aquatic organisms by its strong absorption in the UV spectral range. Absorption of nonalgal particles and CDOM often disturb the remote sensing of the biomass and primary production of phytoplankton. The present study documents spectral absorption variations and relative contributions of the optically active constituents in water, and presents correlations between absorption coefficients and concentrations of these components.
Spectral absorption coefficients of the total particles ap(λ), nonalgal particles ad(λ), phytoplankton aph(λ), and CDOM ag(λ) as well as the relative contribution of each component’s absorption to the total absorption coefficient integrated over the range of PAR (400700 nm) in Meiliang Bay of Lake Taihu were determined in summer 2004. CDOM absorption coefficients ag(λ) were obtained by measuring the optical density of filtered water. Particulate matter absorption coefficients ap(λ) were measured by applying the quantitative filter technique (QFT). Downward photosynthetically available irradiance (PAR, 400700 nm) at the water surface and at different depths was measured using an irradiance meter from Li-Cor (Lincoln, Nebraska, USA) equipped with a Li-192SA underwater cosine corrected sensor connected to a Li-1400 datalogger. Diffuse attenuation coefficients for downward irradiance were obtained from non-linear regression of the underwater irradiance profile.
The particulate matter absorption at 440 nm ap(440) ranged from 3.58 to 9.86 m^－１ with an average of （7.56±1.74） m^－１. The absorption coefficients of nonalgal particles ad(440), phytoplankton aph(440) and CDOM ag(440) were in the range 2.237.07 m^－１, 0.684.76 m－１ and 1.061.70 m^－１,respectively. The red absorption peak of phytoplankton aph(675) ranged from 0.58 to 2.84 m^－１. Absorption coefficients of nonalgal particles and CDOM decreased exponentially from short to long wavelength. The exponential spectral slope coefficients of nonalgal particles Sd over the 400～700 nm interval and of CDOM Sg over 280～500 nm were （10.91±0.62）, （15.52±0.49） μm^－１, respectively. In most cases, the absorption spectra of nonalgal particles were similar to those of the total particles, which demonstrated that the absorption of the total particulate matter was primarily due to the absorption of nonalgal particles.
Significant linear relationships were found between ap(440) and total suspended solids (TSS), organic suspended solids (OSS) and inorganic suspended solids (ISS). Significant linear relationships were also found between ad(440) and the suspended solid fractions, as was true as well for aph(440) with respect to OSS and chlorophyll a (Chla). Significant positive relationship was found between ag(440) and Chla, but not with dissolved organic carbon (DOC), which indicated that decomposition of phytoplankton was an important source of CDOM. Total particulate matter absorption dominated the total absorption. The relative contribution of ap to a integrated over the range of PAR (400700 nm) exceeded 70% and the relative contribution of ad to a generally exceeded 40% except for station M8, M9 and M13. Finally, the ratio of a to PAR diffuse attenuation Kd(PAR) also generally exceeded 40%.