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

这项研究是在低营养的沿海海湾进行的，该海湾位于生产力普遍较高的亚北极海洋。这是由于对马暖流、副热带洋流的强烈影响。本研究的目的是阐明河流营养通量及其组成对少营养沿海海湾石卡里湾低营养生态系统的影响。对沉积物岩心进行了化学分析，以了解初级生产力和河流营养盐通量的历史变化。对河流羽流中沉积物岩心的化学分析结果表明：1)1970年S层有机碳含量高，C/N比高，表明这一时期由于农田开发和洪涝灾害频繁，泥炭土负荷较高；2)鼠河附近沉积物芯中生物硅含量在30~40年前呈上升趋势。生物硅是硅藻产生的一个很好的指示器，因为只有硅藻才能产生生物硅作为其锥体。硅藻是初级生产力高的主要浮游植物。因此，生物硅含量高表明硅藻产量(初级生产力)较高。初级生产力呈上升趋势的原因可能是由于堤坝的修建而使石卡里河的悬浮物负荷减少，或者由于城市化和过度使用化肥而增加了石卡里河的营养盐通量。

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.