Optical Properties of the Red Tide in Isahaya Bay, South-western Japan: Influence of Chlorophyll a Concentration

摘要

Remote sensing reflectance [R_(rs)(λ)] and absorption coefficients of red tides were measured in Isahaya Bay, southwestern Japan, to investigate differences in the optical properties of red tide and non-red tide waters. We defined colored areas of the sea surface, visualized from shipboard, as "red tides". Peaks of the R_(rs)(λ) spectra of non-red tide waters were at 565 nm, while those of red tides shifted to longer wavelengths (589 nm). The spectral shape of R_(rs)(λ) was close to that of the reciprocal of the total absorption coefficient [l/a(λ)], implying that the R_(rs)(λ) peak is determined by absorption. Absorption coefficients of phytoplankton [a_(ph)(λ)], non-pigment particles and colored dissolved organic matter increased with increasing chlorophyll a concentration (Chl a), and those coefficients were correlated with Chl a for both red tide and non-red tide waters. Using these relationships between absorption coefficients and Chl a, variation in the spectrum of 1/a(λ) as a function of Chl a was calculated. The peak of 1/a(λ) shifted to longer wavelengths with increasing Chl a. Furthermore, the relative contribution of a_(ph)(λ) to the total absorption in red tide water was significantly higher than in non-red tide water in the wavelength range 550-600 nm, including the peak. Our results show that the variation of a_(ph)(λ) with Chl a dominates the behavior of the R_(rs)(λ) peak, and utilization of R_(rs)(λ) peaks at 589 and 565 nm may be useful to discriminate between red tide and non-red tide waters by remote sensing.