Ecological adaptation and acclimatization of natural freshwater phytoplankters with a nutrient gradient.

Ecological adaptation and acclimatization of natural phytoplankters within a nutrient gradient were studied in an oligotrophic pond with special reference to the diel fluctuations of cell volumes and frequency of dividing cells (FDC) at each season using an in situ gradostat. The predominant phytoplankters (Cryptomonas ovata in spring and autumn, Crucigenia rectangularis in summer and Chlamydomonas elongus in winter) exhibited moderately phased division during each season. These diel cycles could be regressed onto a sine curve to give a highly significant fit. The diel patterns in all gradostat chambers and periods were almost the same except for slight variations in the FDC. This shows that the diel pattern of cell division was governed mainly by the light:dark cycle during the 24 h, without regard to trophic differences of the aquatic environment. The mean growth rates of most phytoplankters were higher in the mesotrophic zone than in the eutrophic zone during each season. In winter, the growth rate of Chlamydomonas elongus was evidently greater during the seventh day of the experiment (Period III) than during the first day (Period I) in the mesotrophic chamber. As Chlamydomonas species occur as mesotrophic species in natural waters, Chlamydomonas elongus must acclimatize easily from the oligotrophic habitat to the mesotrophic environmental condition.