Development of Potamoplanktonic Diatoms in Downreaches of Japanese Rivers

Abstract

Occurrence of potamoplanktonic diatoms was recorded in three northeastern Japanese rivers during low discharge in summer: the Omono, the Mogami and the Shinano. The potamoplanktonic diatom communities are characterized by the following: 1) universality of appearance of specific species, such as Cyclotella atomus and C. meneghiniana. 2) longitudinal increase in number of living cells along the stream. 3) replacement by green or blue-green algae in a lentic environment, such as a river mouth or still waters made by barrage. The distinction between potamoplanktonic species and exfoliated periphytic species in river waters was clearly based on differences in the living-cell/dead-frustule ratio. Organic carbon/nitrogen ratio of particulate matter in the river water decreased with an increase in chlorophyll a concentration passing downstream. Just the opposite was observed in the Tenryu, into which allochthonous limnoplankton flows from eutrophic Lake Suwa. In the Nagara, potamoplanktonic diatoms occurred each summer during low discharge for three years. One of the causes for the development of potamoplankton is considered to be the increase in flow time related to a change in volume/discharge ratio, which may be enhanced by channeling of the riverbed and widening of the river. River eutrophication by artificial changes in the river environment will be an important consideration in water conservation and use. © 1994, The Japanese Society of Limnology. All rights reserved.