Dynamics of primary productivity in a subarctic coastal area affected by a large river discharge.

亚北极沿海地区从日本最大的河流之一竹内河获得淡水和营养物质，对该地区的化学物质和生化参数进行了测量。4月份以后，由于春季解冻，河流流量增加了10倍，一直持续到6月份。在沿海地区，在4月份发生春季水华后，营养物质枯竭。河流羽状区显示出来自Tokachi河的营养物质和铁的浓度升高。只有在河流羽流中才能观察到高浓度的叶绿素a，因为羽流外部的营养物质已经枯竭。竹内河干流及其支流表现出不同的营养盐浓度。这可能归因于不同类型的土地利用，即农业、奶牛场、森林和城市污水处理厂。我们还研究了与浮游植物产生的有机碳的去向有关的异养微生物过程(微生物环)。异养菌丰度河流羽流高于近岸海域。羽流中溶解有机碳含量也较高。河流有机质可促进羽流中异养菌的生长。采用稀释法测定细菌的致死率。病毒感染占死亡的80%以上。再生的有机碳可能有助于进一步的细菌生产。

Chemical substances and biochemical parameters were measured in the subarctic coastal region which receives fresh water and nutrients from the Tokachi River, one of the largest river in Japan. River discharge increased ten times after April due to a spring thaw and continued until June. In the coastal region, nutrients were depleted after a spring bloom which occurred in April. River plume area showed an elevated concentration of nutrients and iron originated from the Tokachi River. High concentration of chlorophyll a was observed only in the river plume because outside the plume nutrients were depleted. The mainstream of the Tokachi River and its tributaries showed different concentration of nutrients. This may be attributed to the different type of land use, i.e., farming, dairy farming, forest and city sewage plants. We also investigated the heterotrophic microbial process (microbial loop) which is related to the fate of organic carbon produced by phytoplankton. The abundance of heterotrophic bacteria was higher in the river plume than off shore region. Dissolved organic carbon was also higher in the plume. Riverine organic matter may accelerate the growth of heterotrophic bacteria in the plume. The mortality of bacteria was investigated by a dilution culture experiment. Virus infection was accounted for more than 80% of the mortality. The regenerated organic carbon may contribute to the further bacterial production.