VERTICAL-DISTRIBUTION OF FLUORESCENT ORGANIC-MATTER ALONG WITH AOU AND NUTRIENTS IN THE EQUATORIAL CENTRAL PACIFIC

The vertical profiles of fluorescent organic matter were studied at five stations in the equatorial Central Pacific. At every station, seawater fluorescence was low in the surface layer, indicating photochemical decomposition of fluorescent organic matter. Seawater fluorescence increased steeply with depth to a maximum at about 300 m depth and then decreased slightly, showing a minimum at 500-600 m depth, and then increased again slightly, exhibiting a broad maximum around 1000 m depth; thereafter it was almost constant or decreased slightly. This vertical distribution is similar to that of AOU, nitrate and phosphate. The observed linear correlations between fluorescence intensity and AOU, nitrate and phosphate in the middle layer (300-1000 m depth) are consistent with results in the North Pacific and the coastal seas of Japan. Among the parameters, the linear relation between AOU and fluorescence intensity is highest. Since AOU is a parameter of oxygen consumption, the high correlation between AOU and fluorescence confirms our previous observation that fluorescent organic matter is regenerated in the water column by oxidation and remineralization of organic substances on the sinking particles, as in the case of nutrients. On the other hand, a contribution of the fluorescent organic matter to the dissolved organic matter with regard to oxygen consumption in the water column is discussed in conjunction with ''Redfield'' ratios in the ocean.