The almost monospecific assemblages of planktonic diatoms that occurred in ancient lakes during the middle Brunhes Chron are an important research issue for lake ecosystems because their existence implies that the famous paradox of the plankton was no paradox at that time. To investigate their cause, I examined the assemblage structure and diversity of planktonic diatoms in the Paleo-Kathmandu Lake at Nepal Himalaya over the past 600. kyr. The assemblage structure changes frequently after 300. ka but remains nearly stable before then. Additionally, planktonic species show a relatively high average diversity after 300. ka but show an extremely low average before then. Those abnormal ecological conditions before 300. ka reflect the formation of almost monospecific planktonic diatom assemblages of Cyclotella kathmanduensis and Puncticulata versiformis. The assemblage structure and species diversity of the planktonic diatoms were controlled by lake-level fluctuations (especially lake-level declines) as ecological disturbances. After 300. ka, fluctuations in lake levels occur in about 20. ka intervals, which probably helped to sustain diverse planktonic diatom assemblages. In contrast, much longer intervals occurred between lake-level fluctuations before 300. ka, and these were of a frequency that was too low to sustain diverse planktonic diatom assemblages. This may have allowed C. kathmanduensis and P. versiformis to form monospecific assemblages.
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