Growth Characteristics and Occurrence of Staurastrum dorsidentiferum var. ornatum (Charophyceae), a Large Green Alga, in Northern Basin of Lake Biwa, Japan

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The growth characteristics and occurence of Staurastrum dorsidentiferum (Charophyceae), a large green alga, in the northern basin of Lake Biwa were studied by laboratory culture and the analysis of data obtained through 30 year of phytoplankton and water quality monitoring. The culture result showed that the lowest and highest bounds of temperatures that support growth were 6.1°C and in the range of 30 to 35°C, and that the estimated half-saturation constants for nitrogen and phosphorus were 0.091 mgN·L-1 and 0.0075 mgP·L-1, respectively. The monitoring data analysis result revealed that the population density of the alga at Imazuokichuo station 17B showed a decrease over the monitoring period, and that seasonal patterns of apparent specific growth rate also changed from a "one-peak type" with a single growth rate peak in June (1979-1988) to a "two-peak type" (1999-2008). The stronger thermocline development in the autumn of 1999 to 2008 might have reduced the sedimentation loss of S. dorsidentiferum, resulting in a relatively high apparent specific growth rate in the autumn of the same period. Nitrate limitation in the northern basin of Lake Biwa was thought to be responsible for the decrease in the apparent specific growth rate of S. dorsidentiferum in late summer and early autumn.