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

Abstract

The seasonal changes in the size-fractionated chlorophyll a concentrations (<3 I~m, 3 to 25 ~tm, and >25 ~tm) were investigated at a pelagic site of the north basin of Lake Biwa during June to December 1985. Autofluorescing plankton cells in the <3-1am fractions were also examined using the fluorescein isothiocyanate staining epifluorescence microscopic technique. The < 3-~tm 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 <3-1am chlorophyll a did not decrease significantly. Moreover, cyanobacterial cell density in the <34tm fraction showed a maximum value (2--3.5 • 10~cells'm1-1) during this period. The relative contribution of the <3-gm 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 <3-1~m phytoplankton was found, except that relatively large cyanobacteria (>4 ~tm 3) appeared at a depth of 15 m 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.

High biomass and production of picoplankton in a Chinese integrated fish culture pond

“…The percentage contribution of picoplankton to the total biomass in the present study was lower than previous values, but the absolute chlorophyll a concentrations (mg m -3 ) were 3-14 times higher than the maximum value in previous reports . A high value such as that observed in this study has apparently not been reported elsewhere (Nagata, 1986 ;Weisse, 1988) . Weisse (1988) suggested that a positive relationship between the trophic state of waters and picoplankton abundance is likely .…”

Seasonal variation in viral-induced mortality of bacterioplankton in the water column of a large mesotrophic lake (Lake Biwa, Japan)

“…5), suggesting that the majority of viruses in both the upper and deeper layers of Lake Biwa are coupled with bacteria. However, temporal resolutions of our data (sampling with a monthly interval) were probably too low to capture temporal dynamics of cyanophages and their hosts (picoplanktonic cyanobacteria display seasonal peaks during warm seasons in Lake Biwa; Nagata 1986, Maeda et al 1992. Thus, we do not exclude the possibility that cyanophages are important components of viral communities in Lake Biwa, as suggested in other lakes (Maranger & Bird 1995, Clasen et al 2008.…”

The water bloom of Cyanobacterial picoplankton in Lake Biwa, Japan

“…In Lake Biwa, however, densities of up to lo6 cells ml-' are considerably higher than in mesotrophic lake in general. Nagata (1986Nagata ( , 1988 reported that in Lake Biwa, the highest cell density of unicellular chroococcoid cyanobacteria (< 3 ym) was 3 x lo5 cells ml-' in late summer 1984 and 1985. Hence, the unusually high abundance up to lo6 cells ml-' should be checked from now on in order to find out whether it is a common phenomenon in this lake.…”