Journal of Plankton Research Vol.18 no.4 pp.539-550.1996
Occurrence of viable photoautotrophic picoplankton in the
aphotic zone of Lake Biwa, Japan
Mitsuru Eguchi1, Takemi Oketa u , Nobukazu Miyamoto1, Hiroto Maeda2-4 and
Akira Kawai1
'Department of Fisheries, Kinki University, Nara 631 and2 Lake Biwa Research
Institute, Otsu, Shiga 520, Japan
•'Present address: Matsushita Electric Industrial Co., Ltd, Nara 639-11, Japan
4
'Present address: Faculty of Fisheries, Kagoshima University, Kagoshima 890,
Japan
Abstract. 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, >105 cells cnr' 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 106 cells cm 3. direct count = 9.9 x 106 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' cells cm 1 ). In winter, viable PPP in the aphotic bottom sediments were 104—105
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.
Introduction
Lake Biwa, the largest lake in Japan, consists of two basins: the North Basin (surface area 618 km2, average depth 44 m) and the South Basin (57 km 2 ,3 m). Recent
trophic states of the North and South Basins are mesotrophic and eutrophic,
respectively. In early summer of 1989, a dense bloom of photoautotrophic picoplankton (PPP) of up to 106 cells cm"3 occurred in the North Basin (Maeda et ai,
1992). During this bloom, the Secchi disc transparency decreased to less than half
(1.2 m) of that observed during the same period in 1988. Although the existence of
the same type of picoplankton in Lake Biwa had been reported (Nagata, 1986,
1988a), this was the first time such a large bloom of PPP had been reported and
similar blooms of PPP have occurred in early July 1990 and late June 1991. In 1992,
1993 and 1994, the same type of PPP blooms were recognized, but the maximum
population densities were in the range of 5-8 x 105 cells cnr 3 (data not shown). The
PPP population was composed of three different types of cyanobacteria, two were
rich in phycoerythrin and one was rich in phycocyanin. All the types belonged to
the Synechococcus group (Maeda et ai, 1992).
Although sedimentation is one of the important processes in the dynamics of
phytoplankton populations, it could be argued that it is of less importance to PPP
compared to larger phytoplankton due to their minute size (0.4-1.5 jim in
diameter; Weisse, 1988). For instance, a cell 1.0 |xm in diameter is predicted to sink
at a rate of only 2.6 mm day 1 (Raven, 1986). In contrast, it is well known that PPP
© Oxford University Press
539
M.Eguchi el al.
have the ability to survive in the dark (Anita and Cheng, 1970; Smayda, 1974;
Anita, 1976; Plat, 1983) and dormant phytoplankton in the sediment, existing as
cysts and akinetes, are often inoculum sources for water blooms in various regions
(Preston et al., 1980; Huber, 1984; Takamura et al., 1984; Imai and Itoh, 1987;
Takamura and Yasuno, 1988). Little attention has been paid to the occurrence of
PPP in bottom sediments of Lake Biwa, hence it is important to determine whether
viable PPP are in the bottom sediments in order to clarify the population dynamics
of PPP in Lake Biwa.
In this study, we investigated the abundance and distribution of viable PPP in
the aphotic zone of Lake Biwa and discussed the loss process of PPP. Also, viable
PPP deposited on the sediment surface could be the 'seed population' for PPP
blooms, therefore consideration is given to the abundance of viable PPP on the
sediment surface which are easily resuspended.
Method
Sampling
PPP in the bottom sediments were examined 11 times from 1990 to 1992. The
location of sampling sites (Figure 1) and the types of samples are summarized in
Table I.
Samples of bottom sediment were collected by a K-K type core sampler of 5 cm
diameter (original model; Kimata et al., 1960; Figure 2). The sediment column was
sliced immediately at 1 cm intervals from the surface with a sterilized spatula, and
the sliced samples were carefully placed into sterilized plastic bottles. In March
1992, we investigated the easily resuspended layer of the sediment core (0-0.1 cm)
by gently stirring the overlying water (200 ml) of the core sample and resuspended
the PPP cells deposited on the sediment surface (Figure 2). The resuspended sediments were slowly and gently collected by a sterilized syphon into a sterilized glass
bottle (Figure 2). Lake water samples were collected by a Van Dorn water
sampler, and poured into sterilized glass bottles. Each sample was immediately
divided into two subsamples aseptically, one for viable counting (most probable
number, MPN) and the other fixed in 0.5% (w/v) glutaraldehyde for total direct
counting. All sediment and water samples were stored in dark and cool conditions
(~4°C), and brought back to the laboratory. The experimental procedure for
viable counting and that for total direct counting were completed within 5 and 24 h
(Waterbury et al., 1986) after sampling, respectively.
We denned the aphotic zone as the zone below the euphotic depths (2.7 times as
deep as Secchi depth; Tsuda, 1980) and since the Secchi depth in the North Basin
varied from 2.0 m (euphotic depth = 5.4 m; Station 1, 27 June 1991) to 10.0 m
(euphotic depth = 27.0 m; Station 1, 13 December 1990) (data not shown), all
sediment samples except the sample at Station 3 (Secchi depth = 3.0 m, water depth
= 4.0 m) were regarded as being from the aphotic zone. Bottom sediments at Stations 11 and 12 (Secchi depths = 1.9 m, water depths = 3.0 m) in the South Basin
were in the euphotic zone. The Secchi depth at Station 1 on 12 March 1992 was
6.0 m (euphotic depth = 16.2 m), hence the water sample collected from 30 m can
also be treated as being from the aphotic zone. All water samples for direct counts
540
Viable photoautotrophic picoplankton in aphotic zone
135
W
145*E
45 N
. 1 0 km
.
Fig. 1. Sampling locations (Stations 1-12) in Lake Biwa, Japan.
Table I. Sampling program
Sampling date
Locations"
(station no.)
Sampling procedure
14 November 1990
13 December 1990
28 February 1991
26 March 1991
2 May 1991
15 May 1991
15 June 1991
27 June 1991
29 August 1991
1
1
1
1
1
1
1
1
11 September 1991
12 March 1992
1,4-12
Bulked surface sediments (0-1 cm)
Bulked surface sediments (0-1 cm)
Bulked surface sediments (0-1 cm)
Bulked surface sediments (0-1 cm)
Bulked surface sediments (0-1 cm)
Bulked surface sediments (0-1 cm)
Bulked surface sediments (0-1 cm)
Bulked surface sediments (0-1 cm)
Sectioned sediment cores (0-1,2-3,
5-6,9-10 and 13-14 cm)
Bulked surface sediments (0-1 cm)
Water column (0.5,6.0 and 30.0 m),
resuspended surface sediments (0-0.1
cm), bulked surface sediments (0-1 cm)
1,2 and 3
1
"Refer to Figure 1.
541
M.Eguchi el al.
Syphon
Core Sample
Fig. 2. Schematic diagram of the sample collection of resuspended bottom sediments.
of PPP in the euphotic water were collected from the Secchi depth at each sampling
time and site.
Enumeration of PPP
The number of viable PPP in sediment or water samples was estimated by an
extinction dilution method (MPN) using a CT culture medium (modified C
medium; Ichimura, 1979) with five replicate test tubes each. Among 12 kinds of
culture media for cyanobacteria and other phytoplankton, the CT medium showed
the best growth for PPP (Synechococcus group) isolated from Lake Biwa (data not
shown). The composition of CT medium is as follows (per liter): Ca(NO3)2-4H2O,
150 mg; KNO3, 100 mg; MgSO4-7H2O, 40 mg; P-Na2 glycerophosphate-5H2O, 50
mg; K2HPO4, 50 mg; vitamin B,, 10 u.g; vitamin B12, 0.1 u.g; biotin, 0.1 jxg; TAPS
(GOOD buffer), 400 mg; Provasoli's PIV metal solution (Provasoli and Pintner,
1960) 3 ml; pH 7.5. As the water temperature in Lake Biwa at the time of PPP
blooms was around 20°C, all culture tubes were incubated at 20°C under relatively
lowfluorescentlight (Panasonic, white type, 50 |iE nr 2 s-'; Maeda et al., 1992) with
a 14:10 h light/dark (L/D) cycle for more than 1 month.
The growth of PPP in culture tubes (4 ml each) was checked after filtration onto
a 0.2 n.m pore size Nuclepore membrane filter under low vacuum pressure (<60
542
Viable photoautotrophic picoplankton in aphotic zone
mmHg) by epifluorescence microscopy using a Nikon Microscope equipped with a
mercury short arc lamp (HBO 100W/2). A Nikon Standard green-excitation
system (main wavelength = 546 nm, exciter filter IF535-550, dichroic mirror
DM580, barrier filter 580W) was used for detecting PPP cells on the filter. Under
the green excitation, PPP (<2 jim) rich in phycoerythrin and phycocyanin were
seen as strong orange and dark red autofluorescent cells, respectively.
The criterion for judging positive growth in diluted culture tubes (4 ml each) for
MPN estimation was an average count of >25 cells per microscope field (10-15
microscope fields were selected randomly) at xlOOO total magnification (Kirchman
etal., 1982). This is equivalent to >7.5 x 104 cells ml-', so to avoid overestimation,
test tubes with >105 cells ml 1 were judged as growth positive for PPP.
Total direct counts of PPP in water and sediment subsamples fixed in 0.5%
glutaraldehyde were also estimated by a direct counting method using epifluorescence microscopy (Nikon Standard green-excitation system). The procedure of
the direct counting method is basically the same as that described above for culture
tubes for MPN estimation. Duplicate filters were prepared within 24 h for each
sample and counted. Sediment samples were diluted appropriately 10-100 times
with 0.2 Jim-filtered sterilized natural lake water and then filtered onto a 0.2 \x.m
Nuclepore filter. PPP biomass on each filter was estimated by enumerating a minimum of 500 units (<5% counting error assuming Poisson statistics; Lund et at.,
1958).
PPP cells which could grow in the CT medium were regarded as 'viable' and the
ratio of viable PPP (MPN) to total PPP (total direct count, DC), MPN/DC, was
used as an indicator of growth potential in this study.
All cell numbers, both MPN and total direct counts were expressed as cells cm-3.
Results and discussion
Seasonal change of PPP abundance in the bottom sediments
In winter 1990, at Station 1 (water depth = 35 m), the abundance of total PPP in
water as estimated from the direct counting method was in the range of lOMO4
cells cnr3 (water temperature <10°C). At the beginning of June, the abundance of
total PPP in the euphotic zone started to increase (104 cells cnr3, water temperature 12-15°C). From late June to early July, just after stratification, the total PPP
reached a maximum (2.3 x 106 cells cm"3). At that time, water temperature in the
euphotic zone reached 20°C. Thereafter, the abundance of PPP declined and stabilized at —105 cells cm 3 until the end of September. During October, the estimated
total PPP decreased to KP-IO4 cells cnr 3 (Maeda et al., 1992). This was the typical
seasonal change in PPP abundance during 1989-1991 in the North Basin of Lake
Biwa.
We investigated the abundance of PPP in the surface layer (0-1 cm) of bottom
sediments at Station 1 about once a month from 14 November 1990 to 27 June 1991
by both MPN and direct counting methods (Figure 3). It was observed by MPN
estimates that at least 105 cells cnr 3 of PPP in the bottom sediments were always in
a viable state. Station 1 was located at the center of the area where large PPP
blooms occurred in 1989,1990 and 1991. In almost all cases, direct counts of PPP in
543
M.Eguchi et al.
MPN in the sediment
DC in the sediment
D DC in the euphotic water
Nov.14
Dec.13
Feb.28
Mar.26
May2
May15
Jun.15
Jun.27
Date
Fig. 3. Changes in viable (most probable number, MPN) and total (direct count, DC) PPP abundance in
the surface layer of bottom sediments (0-1 cm) at Station 1 in Lake Biwa from 14 November 1990 to 27
June 1991. DC in the euphotic water was the number of PPP at the Secchi depth on each sampling data.
*, no data.
bottom sediments were significantly higher (P = 0.05) than MPN values. However,
sampling on 15 May and 15 June 1991 revealed that viable counts (MPN) and
direct counts were in close agreement (~3 x 106 cells cnr3) (Figure 3). This
phenomenon was not caused by changes in water temperature. The temperature of
the hypolimnion and bottom sediments did not change throughout the year from
around 6°C. Although it was not clear why the MPN estimates rose by about one
order of magnitude to give values close to those of direct counts during the period
between 15 May and 15 June, it was clear that PPP with a really high growth potential (MPN/DC « 100%) existed in the surface layer of bottom sediments.
Except for early summer (the bloom of PPP on 27 June), direct counts in bottom
sediments were significantly higher (P=0.05) per unit volume by one or two orders
of magnitude than those of PPP in the euphotic zone of water (Figure 3).
From the difference in autofluorescent colors under the Nikon green-excitation
system, two groups of PPP were identified and enumerated: phycoerythrin
-rich (strong orange autofluorescence) and phycocyanin-rich (dark red autofluor
escence) types. In all water and sediment samples, >90% of total PPP were
represented by the phycoerythrin-rich types. The predominance of the
544
Viable photoautotrophic picoplankton in aphotic zone
phycoerythrin-rich type in the water bloom of the North Basin was also reported
by Nagata (1986) and Maeda et al. (1992). The phycoerythrin-rich type often
comes to be predominant in oligo-mesotrophic lakes, such as Lake Ontario (Caron
et al., 1985), Lake Huron, Lake Michigan (Fahnenstiel and Carrick, 1992) and
Lake Constance (Weisse and Schweizer, 1991).
PPP in the deeper layer of bottom sediments
Vertical distributions of viable PPP in bottom sediments at Stations 1, 2 and 3 in
August 1991 are shown in Figure 4. The water depths at Stations 1,2 and 3 were 35,
32 and 4 m, respectively. Stations 1 and 2 were located in the bloom area of PPP.
Station 3 was located in a small, shallow bay, out of the PPP bloom area. Estimates
of the viable PPP by MPN at Stations 1 and 2 showed little variation in the same
range (105 cells cm-3) from the surface to a depth of 3 cm. Samples collected from
deeper sediments (5-14 cm) contained fewer viable PPP, with the number of viable
PPP decreasing with depth. The number of viable PPP at Station 3 was two orders
of magnitude lower than those at Stations 1 and 2. Even at a depth of 13-14 cm in
the bottom mud at Stations 1 and 2, there were 101 cells cm 3 of viable PPP. In
comparison, in the Peel-Harvey Estuary (Western Australia), viable Nodularia
Viable picoplankton (cells/cm 3 )
10
Fig. 4. Vertical distributions of viable PPP at Stations 1,2 and 3 bottom sediments of Lake Biwa on 29
August, 1991. *, not detected.
545
M.Eguchi el al.
(Cyanobacteriaceae) akinetes were found in sediments to a depth of 35 cm
(Huber, 1984). Considering the sedimentation rate (0.3 mm year 1 ) in PeelHarvey Estuary, these viable cells are estimated to have survived for >1000 years
(Huber, 1984). Similarly, the sedimentation rate of the North Basin of Lake Biwa
has been estimated to be —1.5 mm year 1 (Nakamura et al., 1983). Thus, the viable
PPP in the 13-14 cm layer of sediment can be estimated to have survived for >90
years. While we cannot deny the possibility of such a long survival of PPP in these
sediments, it is more probable that the occurrence of viable PPP in the deeper
layers was due to the bioturbation caused by endbenthos such as polychaete and
oligochaete worms (Stockner and Lund, 1970; Huber, 1984).
Horizontal distribution of PPP
On 11 September 1991, ~2 months after a large PPP bloom, we investigated the
horizontal distribution of viable PPP in the surface layer of bottom sediments (0-1
cm) at Stations 1-12, excluding Stations 2 and 3 (Table II). Among the examined
stations in the North Basin, the shallowest point was Station 1 (depth = 35 m) and
the depths of the other stations were >40 m. The depths of stations in the South
Basin (Stations 11 and 12) were ~3 m. PPP in bottom sediments of Lake Biwa were
not distributed uniformly. At Stations 1, 4, 5, 7 and 8, the numbers of viable PPP
were >105 cells cm-3, while at Stations 6,9 and 10 in the North Basin and stations in
the South Basin, the abundances of viable PPP in the bottom sediments (0-1 cm)
were significantly lower (P = 0.05) by one or two orders of magnitude than those at
other stations.
The North Basin has three large horizontal circular currents during periods of
stratification. The currents are called, from north to south, thefirst(anticlockwise),
the second (clockwise) and the third (anticlockwise) circular currents. The first and
second currents are stable at least from June to the beginning of December; however, the third disappears at times (Endoh et al., 1981). Transportation of suspended substances in the North Basin, especially the horizontal movement, is
Table II. Viable PPP (MPN) in bulked surface sediments (0-1 cm) of Lake Biwa on 11 September 1991
at Stations 1,4,5,6, 7,8,9 and 10 in the North Basin, and Stations 11 and 12 in the South Basin
Sampling
North Basin
South Basin
546
Station
Viable PPP (xlu3 cells cm')
1
4
160
350
5
6
7
8
170
24
220
540
9
10
12
6
11
12
39
32
Viable photoautotrophic picoplankton in aphotic zone
strongly influenced by these circular currents (Endoh et al., 1981). Stations 1,4 and
5 were within the sphere of the first current, and Stations 7 and 8 were within the
sphere of the second current. The number of viable PPP in bottom sediments at
Station 6 was one order of magnitude lower than those at Stations 1,4, 5,7 and 8.
Station 6 was located in the gap between the sphere of thefirstcurrent and that of
the second current. Additionally, a large river, Ado River,flowsinto the lake near
Station 6. Thus, the differences in the horizontal distribution of PPP among the
stations in the North Basin are probably affected by these circular currents and the
large rivers which flow into Lake Biwa.
PPP deposited on the sediment surface
On 12 March 1992, we investigated the amount of viable and total PPP on the
sediment surface at Station 1 (Table III). The viable PPP (MPN) in the resuspended part of bottom sediments (0-0.1 cm) was 1.3 x 106 cells cnr3 (total PPP = 9.9
x 106 cells cm~3), and the MPN value was significantly higher (P - 0.05) by one order
of magnitude than the gross count (MPN) of the 0-1 cm layer of the bottom sediments. In the euphotic lake water, the amount of viable PPP was in the range of
lO'-lO2 cells cnr3 and total PPP was around 103 cells cm-3 (Table III). In March,
water temperature was homogeneous around 6-10°C from the water surface to the
bottom of the North Basin due to vertical mixing. The MPN/DC values in the 0-0.1
cm layer of the bottom sediments and in the aphotic zone of water near the bottom
(depth = 30 m) were 10-20 times higher than those in the euphotic zone (Table III).
This indicates the high growth potential of PPP in the aphotic zone (30 m depth and
bottom sediments) of Lake Biwa.
It was reported that the turbid benthic boundary layer (TBBL) of thickness 10 m
was usually formed near the bottom of the North Basin in May and the TBBL was
supported by an intermittent resuspension of bottom sediments with a patchy
structure moving with a period of several hours (Kumagai et al., 1995). Once PPP
cells deposited on the sediment surface (0-0.1 cm) are resuspended before strong
stratification, they are probably able to go up to the euphotic zone by upwarddirected turbulence and become a seed population for the PPP blooms.
Dark survival and vertical transportation of PPP
Numerous PPP recovered in bottom sediments showed a high ability to survive in
the dark and confirmed the dark survival of isolated PPP in laboratory experiments. Under dark and cold (5°C) conditions, as exist in the aphotic zone of the
North Basin, isolated PPP could survive with no loss of viability for >6 months in
sterilized natural lake water (M.Eguchi, unpublished data). Similar results have
been observed in resting cells of the diatom Melosira italica (Lund, 1954) and marine phototrophic picoplankton (Anita, 1976; Plat, 1983). However, under cold
temperature but light/dark (14:10 h L/D cycle) conditions, isolated PPP could not
survive for such a long period (Jitts et al., 1964; M.Eguchi, unpublished data). This
finding supports the result that the MPN/DC values in the euphotic zone during
winter time (cold temperature but light/dark) were less than one-tenth of those of
PPP in the aphotic zone (cold and dark; Table III).
547
M.Eguchi et al.
Table III. Viable (MPN) and total (direct count) PPP in the water column [0.5,6.0 (Secchi depth) and
30.0 m] and resuspended (0-0.1 cm) and bulked surface (0-1 cm) sediments on 12 March 1992 at Station
1 (water depth = 35 m)
Sample
Euphotic zone (water)
0.5 m
6.0 m
Aphotic zone (water)
30.0 m
Aphotic bottom sediments
Resuspended 0-0.1 cm
Bulked 0-1 cm
MPN1 (cells cm-3)
D C (cells cm 3 )
2.1 x 10'
1.8 x 102
1.6 x 103
9.2 x 103
1.3
1.9
1.4 x 102
5.3 x 102
26.4
1.3 x 10"
2.2 x 105
9.9 x 106
7.1 x 10"
13.1
3.9
MPN/DC
(%)
•Most probable number.
b
Direct count.
In order to explain the large abundance of viable PPP in bottom sediments of the
North Basin, PPP must have sunk from the euphotic zone, since they are believed
to be obligate photoautotrophs (Waterbury et al., 1986). It is highly unlikely that
PPP deposited on the sediment surface (water depths = 35-100 m) can grow photosynthetically because (i) the euphotic depth Z,% (defined as the depth to which 1 %
of incident light penetrates) measured with a multichannel instrument in the North
Basin of Lake Biwa varied from 10 to 17 m (Tsuda and Nakanishi, 1995) and (ii) the
intensity of the downwelling irradiance decreased exponentially (even the green
light decreased four orders of magnitude at 40 m depth; Tsuda, 1980).
PPP (0.4-1.5 (Jim in diameter) probably use some way to accelerate the sinking
velocity. The best way for fast sedimentation is the transformation of viable PPP
cells via larger, faster sinking fecal or detrital particles (Lochte and Turley, 1988;
Stockner, 1991). Viable PPP cells can sink rapidly to the bottom by getting on the
express train: 'fecal pellets' (Smayda, 1971; Malone et al., 1973; Stockner, 1988,
1991). Undigested and viable cyanobacterial picoplankton have been found in the
guts and fecal pellets of both marine and freshwater copepods (Silver and
Aldredge, 1981; Johnson et al., 1982; Caron et al., 1985; Iturriaga and Mitchell,
1986). Even in Lake Biwa, it was reported that Daphnia feeds on picoplankton
(Nagata and Okamoto, 1988) and grazing pressure may be important not only for
the loss process (Nagata and Okamoto, 1988; Stockner, 1988; Weisse, 1988), but
also for the survival process of PPP in dark and cold bottom sediments of Lake
Biwa. Turbulent diffusion is also believed to transport viable PPP cells to the
aphotic zone. As mentioned above, in the converse process, turbulent diffusion
may be important for the transportation of viable PPP from the aphotic zone to the
euphotic zone.
Conclusions
It was revealed that at least 105 cells cm-3 of PPP in the aphotic bottom sediments
(0-1 cm) of the North Basin were viable throughout the year. Furthermore, the
density of viable PPP cells deposited on the sediment surface (0-0.1 cm) was one
548
Viable photoautotrophic picoplankton in aphotic zone
order of magnitude higher than that in the bulked surface sediments (0-1 cm). The
MPN/DC values in the aphotic zone (30 m depth and the 0-0.1 cm layer of the
bottom sediments) were 10-20 times higher than those in the euphotic zone in
March. This indicates the high growth potential of PPP in the aphotic zone of Lake
Biwa. A single cell of PPP (<2 ujn) cannot sink fast and PPP is believed to be an
obligate photoautotroph. Thus, the large abundance of PPP in the aphotic bottom
sediments may be explained by transportation of viable PPP cells via larger, fastersinking fecal or detrital particles (Stockner, 1991). Once viable PPP in the bottom
sediments are resuspended (Kumagai etal., 1995) before the beginning of summer,
they can be a 'seed population' for the PPP bloom. In order to study the population
dynamics of PPP in Lake Biwa, much more attention should be paid to the resuspension process of viable PPP from the aphotic bottom sediments.
Acknowledgements
We are grateful to Dr D.R.Nelson of the University of Rhode Island, USA, and Dr
S.Kjelleberg of the University of New South Wales, Australia, for their critical
reading and valuable comments on an earlier version of the manuscript. We also
thank Dr P.T.Smith of the University of Western Sydney, Australia, for his critical
reading of the manuscript. This research was partly supported by grants from the
Ministry of Education, Japan (grant no. 03856051 and 07760194), the Japan
Private School Promotion Foundation and Kinki University, Japan (grant no.
GG82,1991; Environ. Sci. Res. Inst. 002,1992) to M.E.
References
Anita,N.J. (1976) Effects of temperature on the darkness survival of marine microplankton algae.
Microb. Ecol., 3,41-54.
Anita.N.J. and ChengJ.Y. (1970) The survival of axenic culture of marine planktonic algae from prolonged exposure to darkness at 20°C. Phycologia, 9,179-183.
Caron.D.A., Pick,F.R. and Lean.D.R.S. (1985) Chroococcoid cyanobacteria in Lake Ontario: vertical
and seasonal distributions during 1982. J. Phycol., 21,171-175.
Endoh.S., OkamotoJ. and Nakai,M. (1981) Circular currents in the North Basin of Lake Biwa (I):
Seasonal variation of circular currents deduced from water temperature distributions. Jpn J. Limnol., 42,144-153.
Fahnenstiel.G.L. and Carrick,H.J. (1992) Phototrophic picoplankton in Lakes Huron and Michigan:
abundance, distribution, composition, and contribution to biomass and production. Can. J. Fish.
Aquat. Sci., 49,379-388.
Huber,A.L. (1984) Nodularia (Cyanobacteriaceae) akinetes in the sediments of the Peel-Harvey Estuary, Western Australia: potential inoculum source for Nodularia blooms. Appl. Environ. Microbiol.,
47, 234-238.
Ichimura,T. (1979) Media for freshwater cyanobacteria. In Nishizawa,K. and Chihara.M. (eds),
Methods in Phycology. Kyouritsu shuppan, Tokyo, pp. 295-296 (in Japanese).
Imai,I. and Itoh.K. (1987) Annual life cycle of Chattonella spp., causativeflagellaof noxious red tides in
the Inland Sea of Japan. Mar. Biol., 94, 287-292.
Iturriaga.R. and Mitchell.B.G. (1986) Chroococcoid cyanobacteria: A significant component in the
food web dynamics of the open ocean. Mar. Ecol. Prog. Ser., 28, 291-297.
Jitts,H.R., McAllister J.H.R., Stephens.K. and StricklandJ.D.H. (1964) The cell division rates of some
marine phytoplankters as a function of light and temperature. /. Fish. Res. Board Can., 21,139-157.
Johnson.P.W., Xu,H.S. and Sieburth J.M. (1982) The utilization of chroococcoid cyanobacteria by
marine protozooplankters but not by calanoid copepods. Ann. Inst. Oceanogr. Paris, 58,297-308.
Kimata.M., Kawai, A. and Ishida,Y. (1960) The method for sampling of marine bottom muds. Nippon
Suisan Cakkaishi, 24,1227-1230.
549
M.Eguchi el al.
Kirchman.D., SigdaJ., Kapuscinski,R. and Mitchell,R. (1982) Statistical analysis of the direct count
method for enumerating bacteria. Appl. Environ. Microbiol., 44, 376-382.
Kumagai,M., Tsuda.R. and Fukagae.K. (1995) Dynamics of the turbid benthic boundary layer. In
Okuda,S.. Imberger J. and Kumagai.M. (eds). Physical Process in a Large Lake: Lake Biwa, Japan.
Coastal and Estuarine Studies. 48. American Geophysical Union. Washington, DC. pp. 87-99.
Lochte.K. and Turley,C.M. (1988) Bacteria and cyanobacteria associated with phytodetritus in the
deep sea. Nature, 333,67-69.
LundJ.W.G. (1954) The seasonal cycle of the plankton diatom Metosira italica (Ehr.) Kiitz. subsp.
subarctica O. Mull. J. Ecol., 42, 151-179.
LundJ.W.G., Kilpling,C. and LeCren.E.D. (1958) The inverted microscope method of estimating algal
numbers, and the statistical basis of estimation by counting. Hydrobiologia, 11,143-170.
Maeda.H., Kawai,A. and Tilzer.M.M. (1992) The water bloom of cyanobacterial picoplankton in Lake
Biwa. Hydrobiologia, 248, 93-103.
Malone,T.C, Garside,C. Anderson,R. and Roels,O.A. (1973) The possible occurrence of photosynthetic microorganisms in deep-sea sediments of the North Atlantic. /. Phycoi, 9,482-488.
Nagata,T. (1986) The seasonal abundance and vertical distribution of the <3 u.m phytoplankton in the
north basin of Lake Biwa. Ecol. Res., 1, 207-221.
Nagata,T. (1988a) The microflagellate-picoplankton food linkage in the water column of Lake Biwa.
Limnol. Oceanogr., 33, 504-517.
Nagata,T. and Okamoto.K. (1988b) Filtering rates on natural bacteria by Daphnia longispina and
Eudiaplomus japonicus in Lake Biwa. J. Plankton Res., 10, 835-850.
Nakamura.T., Nakai.T., Kikura.M., Kojima.S. and Maeda,H. (1986) Geological studies on radionuclides distributed in the bottom sediments of Lake Biwa. J. Sedimentol. Soc. Jpn, 25,1-14.
Plat,T. (1983) Photosynthetically competent phytoplankton from the aphotic zone of the deep ocean.
Mar. Ecol. Prog. Ser., 10, 105-110.
Preston,T., Stewart,W.D.P. and Reynolds,C.S. (1980) Bloom-forming cyanobacterium Mycrocystis
aeruginosa overwinters on sediment surface. Nature, 288, 365-367.
Provasoli.L. and Pintner.I.J. (1960) Artificial media for fresh-water algae. In Tryon,C.A. and
Hartman.R. (ed.), The Ecology of Algae. University of Pittsburgh Press, Pittsburgh, pp. 84-96.
Raven J. A. (1986) Physiological consequences of extremely small size for autotrophic organisms in the
sea. In Platt.T. and Li.W.K.W. (eds), Photosynthetk Picoplankton. Can. Bull. Fish. Aquat. Sci., 214,
pp. 1-70.
Silver.M.W. and Aldredge,A.L. (1981) Bathypelagic marine snow: Deep-sea algal and detrital community./ Mar. Res., 39, 501-530.
Smayda,T.J. (1971) Normal and accelerated sinking of phytoplankton in the sea. Mar. Geol., 11,
105-122.
Smayda,T.J. (1974) Dark survival of autotrophic, planktonic marine diatoms. Mar. Biol., 25, 195-202.
Stockner,J.G. (1988) Phototrophic picoplankton: an overview from marine and freshwater ecosystems.
Limnol. Oceanogr.. 33. 765-775.
StocknerJ.G. (1991) Autotrophic picoplankton in freshwater ecosystems: the view from the summit.
Int. Rev. Ces. Hydrobiol., 76, 483-492.
StocknerJ.G. and LundJ.W.G. (1970) Live algae in postglacial lake deposits. Limnol. Oceanogr., 15,
41-58.
Takamura.N. and Yasuno.M. (1988) Sedimentation of phytoplankton population dominated by
Mycrocystis aeruginosa Kiitzin a shallow lake. J. Plankton Res., 10, 283-299.
Takamura.N., Yasuno.M. and Sugahara.K. (1984) Overwintering of Microcystis aeruginosa Kiitz. in a
shallow lake./ Plankton Res., 6,1019-1029.
Tsuda,R. (1980) Measurement of underwater spectral irradiance in Lake Biwa. Jpn J. Limnol., 41,
57-67.
Tsuda.R. and Nakanishi.M. (1995) Spectral irradiance and optical properties. In Okuda.S., ImbergerJ.
and Kumagai.M. (eds), Physical Process in a Large Lake: Lake Biwa, Japan. Coastal and Estuarine
Studies, 48. American Geophysical Union, Washington, DC, pp. 65-76.
WaterburyJ.B., Watson.S.W., Valois.F.W. and Franks,D.G. (1986) Biological and ecological characterization of the marine unicellular cyanobacterium Synechococcus. In Platt.T. and Li.W.K.W. (eds),
Photosynthetic Picoplankton. Can. Bull Fish. Aquat. Sci., 214, pp. 77-120.
Weisse.T. (1988) Dynamics of autotrophic picoplankton in Lake Constance. /. Plankton Res., 10,
1179-1188.
Weisse,T. and Schweizer,A. (1991) Seasonal and interannual variation of autotrophic picoplankton in a
large prealpine lake (Lake Constance). Verh. Int. Ver. Limnol., 24,821-825.
Received on July 6,1995; accepted on November 20, 1995
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