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

本研究是在贫营养的海岸湾，这是位于亚北冰洋，生产力一般较高。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盐通量及其组成对石狩湾低营养生态系统的影响，贫营养海岸海湾。对沉积物柱样进行化学分析，以了解初级生产力和河流营养盐通量的历史变化。研究结果表明：（1）20世纪70年代沉积物有机碳含量和C：N比值较高，表明该时期农田开发和洪水频繁，泥炭土负荷较高;（2）30 ~ 40年前，河鼠附近沉积物生物硅含量有增加的趋势。生物硅是硅藻生产的一个很好的指标，因为只有硅藻产生生物硅作为其硅藻壳。硅藻是初级生产力高的主要植物群落。因此，高含量的生物硅表明硅藻产量（初级生产力）高。初级生产力呈上升趋势的原因可能是由于筑堤减少了石狩川的悬浮固体负荷，或由于城市化和过度使用化肥增加了石狩川的营养盐通量。

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.