Regional patterns of particulate spectral absorption in the Pearl River estuary

Abstract

Spectral absorption coefficients of the total particulate, a(p)(lambda), nonalgal particulate, a(d)(lambda), and phytoplankton pigment, a(ph)(lambda), in the Pearl River estuary and its vicinity waters were determined using the quantitative filter technique. The particulate absorption ap(443) ranged from 0.04 to 1.82 m(-1), with the corresponding a(ph)(443) ranging from 0.016 to 0.484 m(-1). Two typical spectral patterns are found for the total particulate absorption. For the first typical spectral pattern, the total particulate absorption spectra are similar to that of nonalgal particulate, with values of absorption coefficient decreasing with wavelength. In contrast, for the second spectral pattern the spectral absorptions by total particulate are very similar to that of phytoplankton pigment. The spectral dependency of absorption by nonalgal particulate follows an exponential increase toward short wavelengths, with an average slope of 0.012 +/- 0.002 nm(-1). The nonalgal absorption and the fraction of the nonalgal particulate absorption to the total particulate absorption exhibit a distinct trend of decreasing with salinity of the surface water. Phytoplankton pigment absorption exhibits a clear trend of increasing nonlinearly with chlorophyll a concentration. The relationships between the phytoplankton pigment absorption and chlorophyll a concentration can be described by power law, with the determination coefficient r(2) of 0.82. But only weak relationships between a(p)(lambda) and chlorophyll a concentration are observed, with the determination coefficient r(2) of 0.42. The relatively large scatter around ap(443) versus chl-a relationship would be attributed to the effects of loading of the nonalgal particulate absorption. Our analysis indicated that such relationships similar to that for Case I waters can be applicable to optically complex Case II waters if the effects caused by nonalgal are corrected. The chlorophyll-specific absorption coefficients of phytoplankton pigment are not constant, it increases with decreasing chlorophyll a level. To improve the accuracy of bio-optical algorithms for remote sensing in coastal waters, further investigations on the variations of specific absorption of chlorophyll pigment must be made.