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

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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.