中–後期更新世における古カトマンズ湖珪藻の古群集生態:―浮遊性・底生群集の生産性・種多様性変遷―

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Planktonic and benthic diatom assemblages in the Paleo-Kathmandu Lake, Central Himalaya, were examined to reveal predominant factors of productivity and species diversity between 600 and 15 ka. The productivity of planktonic diatoms fluctuated synchronously with Milankovitch cycles, and increased during interglacial and interstadial periods. This is probably because of variations in nutrient loadings from the catchment area controlled by the Indian monsoon climate (especially wet and dry) changes. In contrast to planktonic diatoms, the productivity of benthic diatoms tended to increase during glacial and stadial periods. The increases in benthic diatom productivity seem to be attributable to lake-level declines and associated habitat diversification (increases in resources) on the lake bottom. On the other hand, species diversity (number of taxa and evenness) of planktonic diatoms did not respond directly to global climate changes, and was characterized by a very low average from about 600 to 300 ka, accompanied by the formation of monospecific assemblages by Cyclotella kathmanduensis and Puncticulata versiformis. Probably, the very low average species diversity is attributable to less frequent lake-level declines (ecological disturbances), which may have allowed C. kathmanduensis and P. versiformis to dominate through competition processes on glacial and interglacial time scales. Species diversity of benthic diatoms remained high from 600 to 15 ka. However, the number of taxa and evenness (two component factors of species diversity) of benthic diatoms were negatively correlated and varied with lake-level fluctuations. For example, the number of taxa increased and evenness decreased with lake-level declines. It is suggested that benthic diatoms increased the number of taxa and decreased evenness when using and competing for increased resources in shallow habitats.

中–後期更新世における古カトマンズ湖珪藻の古群集生態:―浮遊性・底生群集の生産性・種多様性変遷―

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