| Research Abstract |
Variation of carbon and nitrogen isotope ratios in suspended particulate materials (POM) were examined in a wide variety of the ocean environment. The study area extends to the northern North Pacific, equatorial central Pacific, north and south Atlantic Ocean, as well as coastal seas in Japan. It has been found that ; 1) In the northern North Pacific in summer, nitrate in the sea surface was depleted around the subarctic front, where phytoplankton abundance formed its maximum. To the north of the front, sea surface nitrate existed inrelatively high concentration, but the phytoplankton was less abundant than in frontal region. Nitrogen isotope ratio in the POM was low in the frontal region, and the carbon isotope ratio was higher than the expected value of non-blooming conditions. 2) In the equatorial Pacific, POM showed higher nitrogen isotope ratio in the central region compared to the eastern region. Nitrate nitrogen also showed the same trend in the isotope ratio. The reason for the
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above trend was speculated as follows : Nitrate supplied by the equatorial upwelling which is intense in the eastern region are translated to the west by the south equatorial current. During the translation to the west, the sea surface nitrate are taken up by phytoplankton with isotope fractionation, leaving nitrate and POM rich in ^<15>N.3) In the Atlantic, fron 50ﾟ N to 50ﾟ S,carbon isotope ratio of sea surface POM varies with isotope ratio of total inorganic carbon, as well as with primary productivity. It was shown that the variability of carbon isotope ratio, corrected for variability in isotope ratio of total inorganic carbon, correlated inversely with primary productivity per unit carbon biomass. 4) A new method to measure nitrogen isotope ratio in very low amount of nitrate was developed. 5) In the coastal front region, time scale for the vertical mixing of seawater was estimated to be about 2 days. It was found that carbon isotope ratio of the POM varied with a longer time scale than nitrogen. Less
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