Two planktonic communities in rivermouths-their origin, behavior and effects of a rivermouth barrage on planktonic communities

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  • 河口域に見られる二つのプランクトン群集　　起源，挙動，及び河口堰がプランクトン群集に及ぼした影響
  • カコウイキ ニ ミラレル 2ツ ノ プランクトン グンシュウ キゲン キョドウ オヨビ カコウゼキ ガ プランクトン グンシュウ ニ オヨボシタ エイキョウ

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Abstract

Data continuously recorded for one hour on chlorophyll a at lower reaches of the Nagara and Kiso Rivers made it clear that two planktonic communities with different origin have developed at their rivermouths. One was a community dominated by Cryptophyceae or Dinophyceae originating in a brackish or marine environment. Changes in their biomass synchronized with that in salinity. The other was a potamoplanktonic diatom community drifting down from upstream, and the maximal algal biomass was recorded at low tide.<BR>A rivermouth barrage in the Nagara enhanced potamoplanktonic algal production in the newly created freshwater region by prolongation of the river water retention time. Moreover a large algal biomass downstream of a construction site in the Kiso suggested that brackish or marine planktonic algae accumulation resulted from obstruction of dilution with freshwater and inhibition of exchange of the rivermouth water. The mechanism by which the algal biomass of each community was controlled after construction was different. For assessment and improvement of river eutrophication after barrage construction, a clear distinction between the two planktonic communities should be considered.

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