Specific absorption and backscattering coefficients of the main water constituents in Poyang Lake, China.

Obtaining and analyzing the specific inherent optical properties (SIOPs) of water bodies is necessary for bio-optical model development and remote sensing-based water quality retrievals and, further, for related ecological studies of aquatic ecosystems. This study aimed to measure and analyze the specific absorption and backscattering coefficients of the main water constituents in Poyang Lake, China. The specific absorption and/or backscattering coefficients of the main water constituents at 85 sampling sites (47 in 2010 and 38 in 2011) were measured and analyzed as follows: (1) the concentrations of chlorophyll a (C(CHL)), suspended particulate matter (C(SPM)) (including suspended particulate inorganic matter (C(SPIM)) and suspended particulate organic matter (C(SPOM))), and the absorption coefficients of total particulate (a(p)), phytoplankton (a(ph)), and non-pigment particulate (a(d)) were measured in the laboratory; (2) the total backscattering coefficients at six wavelengths of 420, 442, 470, 510, 590, and 700 nm, including the contribution of pure water, were measured in the field with a HydroScat-6 backscattering sensor, and the backscattering coefficients without the contribution of pure water (b(b)) were then derived by subtracting the backscattering coefficients of pure water from the total backscattering coefficients; (3) the specific absorption coefficients of total particulate (a*(p)), phytoplankton (a(ph)*), and non-pigment particulate (a*(d)) were calculated by dividing a(p), a(ph), and ad by C(SPM), C(CHL), and C(SPIM), respectively, while the specific backscattering coefficients of total suspended particulate matter (b*(b)) were calculated by dividing b(b) by CSPM; and (4) the a(ph)*, a*(d), a*(p) and b*(b) of the remaining samples (46 in 2010 and 36 in 2011) were visualized and analyzed, and their relations to CCHL, CSPIM or CSPM were studied, respectively. The main results are summarized as follows: (1) the a(ph)* values at 440 nm were 0.0367-0.7203 m(2) mg(-1) with a mean of 0.1623 ± 0.1426 m(2) mg(-1) in 2010 and 0.0319-0.7735 m(2) mg(-1) with a mean of 0.3145 ± 0.1961 m(2) mg(-1) in 2011; there existed significant, negative, and moderate correlations between a(ph)* and C(CHL) at 400-700 nm in 2010 and 2011 (p<0.05); (2) The a*(d) values at 440 nm were 0.0672-0.2043 m(2) g(-1) with a mean of 0.1022 ± 0.0326 m(2) g-1) in 2010 and 0.0559-0.1347 m(2) g(-1) with a mean of 0.0953 ± 0.0196 m(2) g(-1) in 2011; there existed negative correlations between a*(d) and C(SPIM), while the correlations showed overall decreasing and increasing trends before and after around 575 nm with increasing wavelengths, respectively; (3) The a*(p) values at 440 nm were 0.0690-0.1929 m(2) g(-1) with a mean of 0.1036 ± 0.0298 m(2) g(-1) in 2010 and 0.0571-0.1321 m(2) g(-1) with a mean of 0.1014 ± 0.0191 m(2) g(-1) in 2011, and the negative correlations between a*(p) and C(SPM) were found in both years; (4) The b*(b) at the six wavelengths generally decreased with increasing wavelengths, while the b*(b) values at 420 nm were lower than those at 442 nm for some samples; the correlation between b*(b) and C(SPM) increased with increasing wavelength. Such results can only represent the SIOPs during the sampling time periods, and more measurements and analyses considering different seasons need to be carried out in the future to comprehensively understand the SIOPs of Poyang Lake.