Effects of temporal variations in environmental conditions on phytoplankton in lakes.

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Phytoplankton in lakes is placed under constantly fluctuating environments. These fluctuations exert on the growth and competition of planktonic algae. In the present research, we examined effects of these fluctuations on photosynthetic activities and nitrogen uptake, and discussed growth of phytoplankton in lakes.First, we measured the activities of uptake of carbon and nitrogen during the mixing period in lake Nakanuma. Uptake of carbon did not occur in the dark, but that of ammonium did. This result was explained as an adaptation of phytoplankton to varying light conditons.Second, we examined the diel change of the uptake activities during stratification and mixing period. Garbon uptake showed maximum activity at noon and did not occur in the night. Ammonium uptake did not cease in the night. Furthermore, ammonium uptake showed a maximum in the evening in the dark incubation. Using the mathematical model, we could explain the results as follows. Phytoplankton may produce and store carbohydrates, and use these for uptake of nutrients in the night or dark. Thus, phytoplankton may grow even in the night or weak light conditions.Week-scale measurements of the uptake of carbon and nitrogen also exhibited the similar results, and thus sustained the above explanation.