Proceedings PaperCharacteristic Variations Of Bio-Optical Properties In The Central North Pacific
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Vertical profiles of diffuse attenuation coefficients K(λ) have been determined from measurements of spectral scalar irradiance profiles in the central North Pacific. K(λ) in the range of 410-488 nm show a pronounced maximum at the depth of deep chlorophyll maximum. This vertical pattern prevails in clear oceanic waters in which particle concentration shows a minimum vertical change in the euphotic zone. Thus, sunlight attenuation in 410-488 nm appears to be dominated by absorption by chlorophyll pigments. The deep fluorescence maximum was observed extensively, but it was due to higher pigment concentration rather than higher cell population. Chlorophyll pigments per cell increase with depth down to the fluorescence maximum, suggesting light limited conditions start from the fluorescence maximum. Above the fluorescence maximum, nutrient limited conditions prevailed according to earlier observations. In this upper layer the biomass and chlorophyll are not correlated; however, changes in the upper layer influence the depth of the fluorescence maximum. Observed depths of the deep chlorophyll maximum in the central North Pacific are deeper than those of winter mixed layer. Thus, temporal variations in bio-optical properties cannot be explained by variations of the mixed layer. An alternative hypothesis, nutrient flux associated with gyre circulation, is suggested.