Effects of fluvial nutrients and its historical change on lower trophic level productivity in subarctic coastal ecosystem

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This study was conducted in oligotrophic coastal bay, which is located in subarctic ocean where productivity is generally high. This is due to strong influence of Tsushima warm current, subtropical ocean current. The objectives of this study was to elucidate the effect of fluvial nutrient fluxes and its composition on the lower trophic ecosystems in Ishikari Bay, oligotrophic coastal bay. Chemical analyses on sediment cores were conducted to understand the historical changes in primary productivity and fluvial nutrient fluxes. The results of chemical analyses of sediment cores collected in the river plume suggested as follows.1) High organic carbon contents and high C:N ratios were found in 1970's layer, suggesting high loadings of peat soil due to development of farming ground and frequent flood in this period.2) Biogenic silica contents in the sediment core near the river mouse showed the increasing tendency toward present from 30 to 40 years ago. Biogenic silica is a good indicator of diatom production because only diatoms produce biogenic silica as their frustules. Diatoms are dominant phytoplankters where primary productivity is high. Therefore, high content of biogenic silica indicates diatom production (primary productivity) was high. The reason for the increasing trend of primary productivity seemed the decreasing load of suspended solids from the Ishikari River due to the construction of embankment or the increasing flux of nutrients from the Ishikari River due to the urbanization and over-use of fertilizers.