Studies on biological activity in marine euphotic layr using ^<13>C and ^<15>N of sedimentary organic matters.

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Stable carbon and nitrogen isotopic compositions have been used as a proxy of biological activity or CO2 availability in sea water, although still many questions and argument are retained. The purpose of this project is to reveal the interrelation between such proxies and observational biological change based on tracing chemical and isotopic components of biogeochemical samples in a semi-closed sea.Biogeochemical researches as well as hydrographic observations have been performed in the Funka Bay, Hokkaido, for 1994 fall to 1996 spring. In the period of spring bloom, the delta^<13>C and delta^<15>N of suspended particles showed significant enrichment in heavy isotopes as expected from previous model. However, we found that nitrogen isotopic change could not be interpreted by the spontaneous enrichment of ^<15>N as described by Reighley distilllation model. The delta^<13>C of total inorganic carbon in sea water risen with progress of primary production, but it was not sufficient factor to explain the isotopic change of particulate organic carbon. We estimated that isotopic fractionation associated with photosynthesis might be changed with stratification of water density.We found that ^<13>C of POC becomes light by -28%_0 and simultaneously delta^<15>N rises from 5 to + 10%_0 above bottom layr in summer. This change appears to be related with decomposition of particulate matters in near bottom and progress of reduction condition. These recults strongly suggest that delta^<15>N and delta^<13>C values found in bottom sediment should suffer from such isotopic change, accordingly emphasize that it is essential to understand the as a proxy for reconstruction of biological process in the water column.