Abstract
The water quality environment from autumn until early spring with small discharge were discussed in a natural river estuary, Nakasuji River, Japan. Eutrophication processes and longitudinal distributions of inorganic nutrient salts during drought season were observed in detail, and the characteristics of the considerable phytoplankton blooming was clarified. The spatially averaged dissolved inorganic nitrogen concentration in winter becomes four times as large as that in summer in the lower reach of the river owing to the lack of convection effect of river flow. The typical longitudinal distribution of inorganicnitrogen is established; the concentration peak appears at the upstream limit of the sea water intrusion throughout a year and the concentration gradually decreases to the river mouth. The spatial profiles are strongly affected by the mixing and dilution of tidally generated alternating river flow.
The phytoplankton (dinoflagellate) blooming is observed only in winter season. It is formed between the river mouth and the high nutrient salt concentration area. The long term variation of colored water distribution correspondsto the river discharge, and the short term, diurnal variation is strongly affected by the flow conditions governed by the tidal motion. Colored water becomes most conspicuous from the flood stage until the high tide stage while it disappears in the ebb. The convergence and divergence of water body due to tidal motion and the vertical migration of the phytoplankton cells are the primal factors of the short term variation. The phytoplankton blooming in the river is maintained because the growth rate of plankton exceeds the river flow convection and because the inorganic nutrients supporting the plankton growth are continuously supplied from surrounding areas.
