The seasonal abundance and vertical distribution of the <3‐μm phytoplakton in the north basin of Lake Biwa

The seasonal changes in the size‐fractionated chlorophyll a concentrations (<3 μm, 3 to 25 μm, and >25 μm) were investigated at a pelagic site of the north basin of Lake Biwa during June to December 1985. Autofluorescing plankton cells in the <3‐μm fractions were also examined using the fluorescein isothiocyanate staining epifluorescence microscopic technique. The <3‐μm phytoplankton (usually dominated by chroococcoid cyanobacteria except for a few cases dominated by small eukaryotes) showed a clearly different pattern of seasonal change compared with the larger fractions. That is, from August to early September, chlorophyll a of the larger fractions declined considerably, while the <3‐μm chlorophyll a did not decrease significantly. Moreover, cyanobacterial cell density in the <3‐μm fraction showed a maximum value (2–3.5×10 5 cells·ml −1 ) during this period. The relative contribution of the <3‐μm chlorophyll a to the total chlorophyll a increased from <5% to 45% during the course of this change. No clear vertical trend in the distribution and composition of the <3‐μm phytoplankton was found, except that relatively large cyanobacteria (>4 μm 3 ) appeared at a depth of 15m but not at 0,5 and 10 m from late July to August. These large cells were also found in November and December. The drastic seasonal change of phytoplankton size structure occurring in this basin was discussed in relation to grazing, nutrient depletion and sinking.