TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 64 (No. 4), pp. 511-523, 2008

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

Hiroaki Sasaki1*, Akihiko Tanaka2, Mitsunori Iwataki3, Yasuharu Touke4, Eko Siswanto5, Chun Knee Tan6 and Joji Ishizaka7

1Seikai National Fisheries Research Institute, Fisheries Research Agency, Taira, Nagasaki, Nagasaki 851-2213, Japan
2School of Marine Science and Technology, Tokai University, Orido, Shimizu, Shizuoka, Shizuoka 424-8610, Japan
3Institute for East China Sea Research, Nagasaki University, Taira, Nagasaki, Nagasaki 851-2213, Japan
4Graduate School of Science and Technology, Nagasaki University, Bunkyo, Nagasaki, Nagasaki 852-8521, Japan
5Hydrospheric Atmospheric Research Center, Nagoya University, Furo, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
6Global Environment Information Centre, United Nations University, Jingumae, Shibuya-ku, Tokyo 150-8925, Japan
7Faculty of Fisheries, Nagasaki University, Bunkyo, Nagasaki, Nagasaki 852-8521, Japan

(Received 1 November 2007; in revised form 21 December 2007; accepted 22 February 2008)

Abstract: Remote sensing reflectance [Rrs(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 Rrs(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 Rrs(λ) was close to that of the reciprocal of the total absorption coefficient [1/a(λ)], implying that the Rrs(λ) peak is determined by absorption. Absorption coefficients of phytoplankton [aph(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 aph(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 aph(λ) with Chl a dominates the behavior of the Rrs(λ) peak, and utilization of Rrs(λ) peaks at 589 and 565 nm may be useful to discriminate between red tide and non-red tide waters by remote sensing.

*Corresponding author E-mail: hisasaki@affrc.go.jp

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