The study was conducted on net phytoplankton in the urban Lake Jeziorak Mały (Mazurian Lakeland) in the littoral zone in the years 1998 – 2003. The blue-green algae community was dominated by three species: Planktolyngbya brevicellularis (Cronberg &Komarek), Limnothrix redekei (Van Goor) Meffert and Aphanizomenon gracile Lemm. Changes in the numbers of blue-green algae were analyzed in the annual cycle, with respect to water temperature and orthophosphate concentration. One abundance peak of Planktolyngbya brevicellularis was recorded in July, of Limnothrix redekei in May, and for Aphanizomenon gracile there were two peaks in numbers in May and in August. The relationship between water temperature and the occurrence of blue-greens was statistically significant, whereas in the case of orthophosphate concentration, the coefficient of correlation was statistically significant only for Planktolyngbya brevicellularis. On the basis of equations of multiple regression the proportion of orthophosphate concentration was affirmed to be higher than water temperature. This may suggest that the blue-green species contributed to reducing the phosphorus content of the water, and the largest part in this phenomenon could be played by Planktolyngbya brevicellularis, which developed in the widest temperature range and in this way was a competitor species for remaining species.
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