Occurrence of viable photoautotrophic picoplankton in the aphotic zone of Lake Biwa, Japan

The distribution and abundance of photoautotrophic picoplankton (PPP, Synechococcus group) in the aphotic bottom sediments of Lake Biwa were investigated by direct counting and viable counting (most probable number, MPN) methods. In the surface layer of bottom sediments (0-1 cm), where large PPP blooms occurred in the past 5 years, >10(5) cells cm(-3) of PPP were found to be viable throughout the year. Furthermore, the density of PPP deposited on the sediment surface (0-0.1 cm) was one order of magnitude higher (MPN = 1.3 x 10(6) cells cm(-3), direct count = 9.9 x 10(6) cells cm(-3)) than that of bulked surface sediments (0-1 cm). Even in the deeper layer (13-14 cm) of bottom mud, viable PPP were still found (10(1) cells cm(-3)). In winter, viable PPP in the aphotic bottom sediments were 10(4)-10(5) times greater per unit volume than those in the euphotic lake water. Since the aphotic bottom sediments have high levels of PPP, as well as high growth potential (high ratio of viable count/total direct count), they are likely to seed PPP blooms in the North Basin of Lake Biwa.