Limnological features of Lake Puma Yum Co, a typical alpine lake located at the altitude of 5030 m in the Tibetan Plateau, China, are described based on the fi ndings of the second expedition in September 2004. The lake surface is about 280 km 2 and maximal depth is 65 m. Total infl ow just after the rainy season was estimated to be about 860 000 m 3 day, and the lake water was drained from a newly excavated channel at a rate of 960 000 m 3 day −1. We may have to expect undesirable lowering of the water level by this new drainage, especially in the dry season. Thermocline developed from 20 to 30 m depth, and the euphotic zone reached the 50 m depth. Dissolved oxygen in surface water was supersaturated as in productive lakes, although there were no large point and nonpoint sources of nutrient in the catchment. Vertical distribution of phytoplankton biomass and primary production suggested the presence of photoinhibition. What should be noted about the fl ora and fauna is that a Chara zone and a shell zone were distributed at about 30 or 40 m of depth.
The fluctuations of drift diatom assemblage and water quality in the lower reaches of the Nagara river part of a river system without large reservoirs, were surveyed
Occurrenceof 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.
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