Variability of bio-optical factors influencing the seasonal attenuation ofultraviolet radiation in temperate coastal waters of Japan

The study identifies the relative contribution of various bio-optical facto rs to the total attenuation of ultraviolet radiation (UVR) wavelengths and photosynthetically active radiation (PAR) in temperate coastal waters of Ja pan by surveying the physical properties of the water column, UVR and PAR p enetration, and the absorption characteristics of dissolved and particulate material. Spectral absorbance properties of pigment (a(ph)), detritus (a(d )) and chromophoric dissolved organic material (a(CDOM)) displayed both sea sonal and wavelength specific variability, On an annual basis, absorbance b y a(CDOM) I was the highest absorbing fraction (47-59%) for the UVR wavelen gths measured (305, 320, 340 and 380 nm) hut decreased (32%) at 450 nm, Con tribution of pigments to total absorbance was highest (40-60%) during a spr ing bloom for both UVR and PAR. A large variability (C.V. > 42%) for annual average attenuation coefficients (K-d[lambda]) at respective wavelengths o bserved suggests that the spectral composition of the water column changes throughout the year in this region, A significant relationship was observed between K-d(lambda) and a(CDOM) at 305, 320, 340 and 380 nm only (P < 0.01 ) but not for 450 nm (PAR) indicating the role of CDOM in regulating variat ions in K-d(lambda), particularly in the UVR range, The slope S, obtained f rom a natural-log plot of the absorption coefficient of CDOM against wavele ngth, ranged between 0.014 and 0.036 nm(-1) annually (average = 0.020+/-0.0 07, C.V. = 35%) and suggests seasonal changes in the origin of CDOM between terrestrial (low S) and biogenous thigh S) CDOM.