Seasonal Abundance, Grazing Impacts on Bacteria, and Vertical Distribution of Heterotrophic Microflagellates in the South Basin of Lake Biwa

Abstract

Seasonal abundance of heterotrophic microflagellates and bacteria was investigated at an offshore station (water depth, ca. 4 m) in the south basin of Lake Biwa (eutrophic water basin) during May to October 1984. Cell abundance of flagellates and bacteria ranged from 1.7×10³ to 9.7×10³ cells·ml^-1 and 2.5×10⁶ to 1.5×10⁷ cells·ml^-1, respectively. The instantaneous grazing rate (g) of flagellates on bacteria was estimated from the total cell volume of flagellates and the volume specific clearance rate of flagellates reported in the literature. The g values ranged from 0.003 to 0.020 h^-1. The instantaneous growth rate (μ) of bacteria estimated from the frequency of dividing cells method ranged from 0.009 to 0.034 h^-1. From the values of g and μ, it was estimated that 13-130% of bacterial daily production was consumed by flagellates. Vertical distribution of flagellates at the dredged area (water depth, ca. 12 m) of the same water basin revealed that flagellates reached maximum abundance in a subsurface layer (6 m-depth) during the stratification. The significance of flagellates as bacterial consumers and some characteristics of their occurrence in the eutrophic water body are discussed.