115

Dynamics of 137Cs in Water and Phyto- and Zooplankton ina Reservoir Affected by the Fukushima Daiichi
Nuclear Power Plant Accident
Hideki TSUJI1*, Megumi NAKAGAWA2, Kazuki IIJIMA3, Hironori FUNAKI4,
Kazuya YOSHIMURA4, Kazuyuki SAKUMA3 and Seiji HAYASHI1
Fukushima Branch, National Institute for Environmental Studies,
10-2 Fukasaku, Miharu Town, Tamura-gun, Fukushima, 963-7700, Japan
2
Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies,
16-2 Onogawa, Tsukuba-shi, Ibaraki, 305-8506, Japan
3
Japan Atomic Energy Agency,
10-2 Fukasaku, Miharu Town, Tamura-gun, Fukushima, 963-7700, Japan
4
Japan Atomic Energy Agency,
45-169 Sukakeba, Kaibama, Haramachi-ku, Minamisoma-city, Fukushima, 975-0036, Japan*E-mail: tsuji.hideki@nies.go.jp*1

Abstract
Lake water, phytoplankton and zooplankton were sampled in a total of 12 quarterly surveys from August
2014 to May 2017 at a reservoir in the Fukushima nuclear disaster area, and variations in dissolved forms of137
Cs and planktonic 137Cs were observed. Seasonal variations in dissolved 137Cs concentration becoming highin summer and low in winter were observed in the upstream, midstream and downstream areas of thereservoir, but no seasonal or site-specific differences in planktonic 137Cs concentrations or dominant species
were found. The concentration factors of 137Cs for phytoplankton and zooplankton were 340 and 1,700, whichwere comparable to previously reported values. The amount of planktonic 137Cs in the water was less than
1.4% of the total 137Cs in the reservoir water; therefore the effect of plankton on the dynamics of 137Cs in thereservoir was minimal.
Key words :	 137Cs, concentration factor, dissolved form, phytoplankton, Yokokawa Reservoir, zooplankton

1. Introduction
Following the accident at the Fukushima Daiichi
Nuclear Power Plant (FDNPP) in 2011, a large amount ofradiocesium was deposited in the surrounding area. In themain river catchment of the area within 50 km north of
the FDNPP, radiocesium was deposited at a higher
density in the upper reaches (Ministry of Education,
Culture, Sports, Science and Technology, 2011), and there
are concerns that the future transport of highconcentrations of radiocesium into rivers may result inradioactive contamination of downstream areas. Indeed,in this area, some freshwater fish are still subject toshipping restrictions even nine years after the nuclear
accident, because their radioactivity levels from134
137
radiocesium (mainly Cs and Cs) are still above the−1national limit of 100 Bq kg . (Fukushima Prefecture,
2020a).
On most of the major rivers in this region, dams
were built in the upstream areas for storing irrigationwater. Dams also have the function of storing most of the
Global Environmental Research
24/2020: 115-127
printed in Japan

inflowing sediment and thus intercepting the discharge ofparticulate radiocesium to downstream. However,radiocesium in the bottom sediment of a reservoir can
gradually leach out, especially when promoted underanaerobic conditions (Evans et al., 1983; Kaminski et al.,1994), suggesting that it may act as a source of nonnegligible amounts of dissolved radiocesium in the longterm (Funaki et al., 2020). Furthermore, because
reservoirs usually have a wide distribution of areas withdeep euphotic zones, plankton as one of the main foodsources for fish breeds easily and the transfer ofradiocesium to plankton is promoted in this environment.
Considering these circumstances, dams may play a majorrole in the radioactive pollution of aquatic ecosystems asa source of bioavailable radiocesium. Therefore, it isimportant to focus on the dynamics of radiocesium inreservoirs to predict future ecological pollution in thiscontaminated area.
The fate of dissolved radiocesium, which is a
representative form of bioavailable radiocesium, infreshwater has been reported to show the following©2020 AIRES116

H TSUJI et al.

characteristics: its concentration is generally proportional
to the averaged 137Cs inventory in the catchment (Tsuji etal., 2014a; Ochiai et al., 2015; Yoshimura et al., 2015),
and shows seasonal variation within the environment
with a peak from summer to autumn (Alberts et al., 1979;
IAEA, 2006; Tsuji et al., 2016; Wang et al., 2017;
Wakiyama et al., 2017; Nakanishi & Sakuma., 2019;
Matsuzaki et al., 2020). In addition, dissolved 137Csconcentrations in natural water decrease faster than thephysical half-life (Smith et al., 2005; Onda et al., 2020),
but the decrease in concentration is reported to be slower
than that of particulate 137Cs in the area near the FDNPP(Nakanishi & Sakuma, 2019).
The concentration of radiocesium in plankton ismainly regulated by that in water, and the concentration
factor of radiocesium (ratio of the radiocesium
concentration in organisms to that in water) decreases
with increasing salinity (Ishii et al., 2020). The
concentration factor of plankton living in seawater isreported to be on the order of 100–101 (Tateda, 1998;
Kaeriyama et al., 2008), whereas the concentration factor
for plankton in freshwater environments is 102–104(Knapinska-Skiba et al., 2003; International Atomic
Energy Agency, 2004). Furthermore, because plankton
bodies have a high proportion of chemically bioavailable
components (Mori et al., 2017), their detritus could be afactor in the reproduction of bioavailable forms ofradiocesium.
As described above, the spatio-temporal variation indissolved radiocesium in the aquatic environment ofradioactive contaminated areas and its factors have been
elucidated, but the variation in planktonic radiocesium infreshwater lakes is not well understood. For example, it is
not known how the concentration and total amount ofradiocesium in plankton change seasonally, while theconcentration of dissolved radiocesium fluctuates
seasonally over a year. Because laboratory
137
experimentation has proven that the Cs concentration
factor of plankton increases with water temperature(Wolfe & Coburn, 1970), the hypothesis can be raisedthat the 137Cs concentration in plankton in reservoirs ishigh in summer and low in winter. In addition, the spatial
characteristics of planktonic radiocesium in reservoirs are
unknown because the environmental conditions of lightand nutrients vary spatially in reservoirs and there arelarge spatial diversities in planktonic species composition
and abundance (Seda & Devetter, 2000; Havel &Pattinson, 2004).
In this study, reservoir water, phytoplankton andzooplankton samples were collected from a reservoir
located in the nuclear disaster area. Through observationof radiocesium in these samples, the dynamics of
dissolved, particulate and planktonic forms ofradiocesium were investigated over a total of 12 surveys
during three years, mainly focusing on the characteristics
of temporal and locational variability of planktonic formsof radiocesium. Among the various isotopes of137
radiocesium, Cs was targeted in this article, because it
accounts for the largest residual amount of radioactivity
in the environment several years after the nuclear
accident in 2011.

2. Materials and Methods
2.1 Study Site
The reservoir behind Yokokawa Dam, located 23 km
northwest of the FDNPP in the mountainous region of the
Ota River, was selected as the site for this study (Fig. 1).
This reservoir has geological characteristics
representative of this area, as 99% of its catchment iscovered by forest (Ministry of Land, Infrastructure,
Transport and Tourism, 2020) and most of theradiocesium deposited at the bottom of the reservoir fromradiocesium fallout in March 2011 remains because thebottom sediment has not been dredged extensively since2011.
One of the topographic features of this dam is theshort average residence time of water, 65 days (estimatedfrom the dam discharge and storage water volume,
Takechi et al., in press), compared to natural lakes where
the dynamics of radiocesium have been reported so far,because this dam was constructed in a steep ravine. The
2137
dam’s catchment area is 44.2 km , and the average Cs

45°
NYokokawa
Reservoir

outflowStn.3(downstream)

40°
N

Stn.2
(midstream)

35°
N

130°
E

E
135°

0

E
140°

5

E
145°

N
30°

Stn.1
(upstream)
inflow

10 km

0

0.5

1 km

Pacific
Ocean

Ota Riverwatershed

137Cs deposition (kBq m-2)

＞3,0001,000 – 3,000
600 – 1,000
300 – 600
100 – 300
< 100

Fukushima Daiichi
Nuclear Power
StationFig. 1 
Study site. In the Yokogawa Dam catchment, the upper
reaches have higher 137Cs deposition.

Dynamics of 137Cs in water and phyto- and zooplankton in a reservoir affected by the Fukushima Daiichi nuclear power plant accident 117
inventory in the catchment area of Yokokawa Reservoir
as of July 2, 2011 is 1.8 MBq m−2 (Ministry of Education,
Culture, Sports, Science and Technology, 2011). The
dissolved 137Cs concentrations in the reservoir inflow riverin 2014–2015 ranged from 140 to 530 Bq m−3 (Tsuji etal., 2016). The annual precipitation for 2014–2017
observed at the Yokokawa weather station, which islocated in nearly the central part of the catchment area,
was 1,697–1,880 mm (Fukushima Prefecture, 2020b).
The suspended solids in the reservoir water are
thought to be composed of organic and inorganic mineralparticles derived from forest soils in the catchment area,
and also biological particles (seston) such as plankton andtheir detritus, which are generated in the reservoir. Since
these particles are expected to exhibit characteristic
behaviors for each season and location within thereservoir, the field survey was designed to be repeatedthree times in four seasons at three locations within thereservoir.

137
2.2 Collection of Dissolved/Particulate Cs andPlankton in the Reservoir
A total of 12 quarterly surveys were conducted at thereservoir from August 2014 to May 2017 (Table 1). The
survey dates were set to avoid the immediate aftermath ofheavy rainfall events, aiming for conditions where thereservoir water was not excessively turbid. Observation
stations were established in the upstream part of thereservoir (Stn. 1; 37.58492°N, 140.88993°E), at the centerof the reservoir (Stn. 2; 37.59186°N, 140.89536°E), andnear the embankment (Stn. 3; 37.59963°N, 140.90281°E;
Fig. 1). At each station the dissolved, particulate form of
137
Cs, and 137Cs in phyto- and zooplankton were observed.
Based on continuous data on water levels near theembankment measured by the reservoir management
office, the continuous water depth at Stns. 1– 3 estimatedfrom the measured depths at Stns. 1– 3 was as shown in
Fig. 2. When the reservoir bottom appeared at the surface
on some survey dates in Stn. 1, we moved the survey
station to a nearby location where the water depth wasmore than 1 m.
In the reservoir, a rubber boat was launched, andTable 1 Survey dates and water depth, euphotic layer, temperature, pH and electrical conductivity of the surface water at each station(Stns.1–3).
Year

Date

2014

7-Aug

13-Nov

2015

3-Mar

19-May4-Aug

17-Nov

201610-Mar17-May2-Aug

10-Nov

201712-Jan25-MayEC : electrical conductivity

Station No.
〔Stn.〕
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3

Water
depth(m)
5
16
54
9
17
41
4
16
39
6
16
43
2
13
38
5
16
46
8
17
50
2
12
42
1
8
43
9
22
44
9
21
48
6
16
46

Euphotic
layer(m)
5
5
6
7
11
14
4
15
15
6
11
14
2
10
11
5
15
16
7
15
10
2
10
16
1
6
8
5
12
14
5
10
10
6
9
12

Temperature
(°C)
29
29
29
13
13
13
4
4
4
18
19
19
28
29
30
13
14
14
7
6
6
16
17
17
22
27
27
13
13
13
6
6
7
21
21
20

PH
9.1
7.9
8.8
7.4
7.5
7.3
7.2
7.3
7.3
8.9
9.1
9.1
9.0
8.7
8.4
7.3
7.6
7.3
8.0
7.5
7.5
7.4
7.7
7.7
7.8
8.5
8.9
7.6
7.4
8.4
7.4
6.9
7.1
8.6
9.2
9.0

EC
(mS m−1)
4.8
4.6
4.5
5.7
5.6
5.6
5.5
6.2
6.5
5.5
5.5
5.4
5.8
5.6
5.6
3.3
3.3
3.3
3.2
3.2
3.2
3.8
3.8
4.4
4.3
4.5
3.2
3.1
3.2
3.3
3.2
3.3
3.3
3.7
3.8
3.8

118

H TSUJI et al.
60

Water depth (m)

50

Stn.3

40
30
20

Stn.2

10

Stn.1

0
-10
2014

2015

2016

2017

Fig. 2 Survey dates (arrow symbols) and estimated water depth at Stns.1–3 of Yokokawa Reservoir.
The depths estimated in this graph assume no lake water flow.

40 L of surface water was collected using a polyethylene
bucket. The reservoir water was pumped through a
cartridge filter device (Tsuji et al., 2014b; Yasutaka et al.,2015) on board immediately after collection, and solidliquid separation (collection of suspended 137Cs) using a
1 μm pore size filter and assessment of dissolved 137Csconcentration were conducted simultaneously. The
cartridge filters used to collect and concentrate theparticulate and dissolved 137Cs were brought back to thelaboratory, and the radioactivity from 137Cs was measured
with a high-purity germanium detector. The amount of137
Cs in the suspended filter [Bq] was converted to 137Csconcentration per unit weight [Bq kg−1] by dividing thedetected amount of 137Cs by the weight of the suspended
solids, which was calculated from the difference in dry
weight before and after the 48-hour drying process. The
vertical distribution of chlorophyll-a concentrations in thereservoir water was also measured using a multi water
quality profiler (AAQ-RINKO, JFE Advantech Co., Ltd.,
Hyogo, Japan) to observe the vertical distribution of thephytoplankton population.
At the same station, nylon plankton nets with meshsizes of 41 μm and 200 μm with an inner diameter of
30 cm at the top (NXXX25 and NXX7, Rigo Co., Ltd.,Tokyo, Japan) were submerged in the reservoir andrepeatedly hauled vertically to recover mainly
phytoplankton in the former and zooplankton in the latterdevice. Phytoplankton was recovered within the euphotic
zone (Table 1), which was determined at 2.5 times thevisible depth of a transparent plate (Visibility Secchi’s
disk, Rigo Co., Ltd., Tokyo, Japan). When the euphotic
layer reached the bottom of the reservoir, thephytoplankton recovery area was defined as from thesurface to a depth of 1 m above the bottom of thereservoir to prevent contamination by suspended solids
due to disturbance of bottom sediments. Zooplankton wasrecovered from the reservoir surface to a depth of 1 m
above the reservoir bottom. However, scanty volumes ofphytoplankton and zooplankton were collected at Stn. 1(upstream) in May 2016 and August 2016 due to theshallow water depths, so the samples served only for
137
Cs
identification of species composition, withconcentrations not being measured. During this collection
process, the plankton samples were adulterated by someabiotic suspended solids, and these impurities were later
removed manually as much as possible. When using thismethod, it is expected for zooplankton to be included in
the phytoplankton sample collected by the 41-μm net.
However, because the presence of phytoplankton is muchlarger than that of zooplankton and additionally it isdifficult to subtract the effect of zooplankton adulteration
due to the difference in collecting range, 137Cs detected in
the samples filtred at 41 μm was considered entirely due
to phytoplankton.
Plankton samples collected in polypropylene bottles
were frozen for at least 24 h to kill the plankton and afterthawing them, the supernatant was removed. 1 mL of
the supernatant-removed sample was prepared forobservation of plankton species, to which 5 mL ofneutral buffered formalin solution was added to fix thecells, and then the number of each plankton species wascounted using an inverted microscope (TS100, Nikon,
Tokyo, Japan) at ×400 magnification for phytoplankton
and ×40 –100 for zooplankton. The database of the
National Museum of Nature and Science (2011) was used
as a reference for the identification of phytoplankton,
following the method of Takamura & Nakagawa (2012).
For the identification and counting of zooplankton, we
followed the method of Takamura et al. (2017).
For the remaining plankton-enriched samples,particulate impurities that had sunk to the bottom of thebottle were removed manually with tweezers, and the137
amount of Cs radioactivity in the samples was measured
in a 100 mL polypropylene U8 container in a frozen
condition. However, fine mineral particles with high 137Csconcentrations (~5.0×104 Bq kg−1, Takechi et al., in press)
remained in the U8 container, which may have led tooverestimation of 137Cs in the plankton samples.
Therefore, as described below, the Al concentration in thesamples was measured to evaluate the purity of theplankton in the samples. Thereafter, the samples were
freeze-dried for more than three days (FDU-2200, Tokyo
Rikakikai Co. Ltd., Tokyo, Japan) to determine the 137Csconcentration per dry weight of plankton, and this value

Dynamics of 137Cs in water and phyto- and zooplankton in a reservoir affected by the Fukushima Daiichi nuclear power plant accident 119
was finally converted to 137Cs concentration per wet
weight of plankton assuming a plankton water content of
90% (Hoekstra et al., 2002; Hammerschmidt & Fitzgerald,2006).
2.3 A nalysis of 137Cs Radioactivity and the Al
Component137
Cs radioactivity [Bq kg−1] in the cartridge filters
used to trap dissolved and particulate 137Cs, and in theplankton-enriched samples were determined using
GEM30-70, GEM60-83, GMX45P4-76 coaxial highpurity germanium detectors (Canberra Japan, Tokyo,
Japan) and Spectrum Explorer software (Canberra Japan,
Tokyo, Japan). The efficiency of the detector wascalibrated using a standard volume radioactivity source
MX033U8PP (Japan Radioisotope Association, Tokyo,
Japan). Background correction was conducted every two
weeks for 200,000 seconds. The measurement time wasdetermined for each sample with a counting error of less137
than 5%, and the measured Cs activities were corrected
for decay on the day of sample collection based on thephysical half-life of 137Cs (30.1 years).
To evaluate the amount of mineral particles
remaining in the plankton-enriched samples, taking
advantage of the fact that very little Al can transfer toplankton bodies (Pempkowiak et al., 2006; Ho et al.,2007), in all, 28 of the freeze-dried plankton-enrichedsamples collected during May 2016 to May 2017 were
decomposed with hydrogen fluoride, and the Alconcentration in each sample was measured using ICPMS (Agilent 8800, Agilent Technologies, Inc., Santa
Clara , CA, USA). Every two samples of bottom sedimentcollected at Stns.1–3 in the same period were also driedat 105°C and decomposed with hydrogen fluoride in thesame way, and the Al concentration was measured.
2.4  Cs Concentration Factor of Plankton and the
Occupancy of Planktonic 137Cs in Reservoir Water
The 137Cs concentration factors for phytoplankton
and zooplankton were calculated using the following
equation:
137

ρ･Cs

CF = Cs pl 
dis

(1)

where CF is the wet weight-based 137Cs concentration
factor for phyto- and zooplankton, ρ is the density ofreservoir water (1.0×103 kg m−3), Cspl is the 137Csconcentration per wet weight of plankton [Bq kg−1], andCsdis is the dissolved 137Cs concentration [Bq m−3]. In theevaluation of 137Cs concentration factors, the purity(weight percentage of plankton) in the plankton-enrichedsamples was calculated from the detected Al
concentration according to the following equation:
P = 1−
pl

Alsample
Alsed (2)where Ppl is the purity of plankton in the sample,
Alsample is the Al concentration in the plankton-enrichedsample, and Alsed is the Al concentration in the bottom
sediment to which the geometric mean value of the two
samples was applied. In this equation, the Al concentrationin pure plankton was assumed to be 0 mg kg−1. The
occupancy of 137Cs in plankton versus reservoir water was
calculated using the following equation:
rpl =Cspl･mpl

α･n･A･l (Csdis+Cspar )(3)

where rpl is the ratio of planktonic 137Cs to 137Cs in the
total reservoir water, mpl is the wet weight of therecovered plankton [kg], α is the correction factor for the
proportion of plankton recovered from the total plankton
in all layers, n is the number of times the plankton net
was hauled vertically, A is the cross-sectional area at the
top of the plankton net (0.071 m2), l is the depth within
which the plankton net was hauled [m], and Cspar is theparticulate 137Cs concentration [Bq m−3]. The correction
factor α was set at α = 1 for zooplankton, as it wasrecovered in almost all layers. While the value of α for
phytoplankton was determined based on the vertical
distribution of chlorophyll-a concentration, by thefollowing equation:α=∫ 0lCChl.a dz 
∫ 0LCChl.a dz(4)

where l is the depth from the reservoir surface to theend of the euphotic layer [m], L is water depth [m], andCChl.a is the chlorophyll-a concentration.
In Eq. 3, it is assumed that the concentrations of
dissolved and particulate 137Cs in the reservoir water are
vertically homogeneous and the same as those in thesurface layer. As to the validity of this assumption, thedissolved 137Cs concentration between surface and bottom
water in Yokokawa Reservoir showed almost no
differences except at the center of the reservoir in summer(Tsuji et al., 2017), and the turbidity was almost uniform
except just above the reservoir bottom (unpublisheddata). Therefore, the total 137Cs concentration in thesurface reservoir water would be a little lower than the
averaged 137Cs concentration in the reservoir water
overall, but the underestimation would not be very large.

3. Results and Discussions
3.1 Dynamics of Dissolved and Particulate 137Cs in
Reservoir Water
The concentration of dissolved 137Cs in surface waterat Stns.1– 3 in the reservoir was in the range of
110 –330 Bq m−3, with concentrations high in summer andlow in winter (Fig. 3(a)), similar to the seasonal
fluctuation observed in the inflowing river (Tsuji et al.,2016), the downstream part of the reservoir (Nakanishi &Sakuma, 2019), and also a natural lake (Matsuzaki et al.,120

H TSUJI et al.

2020). No decreases in dissolved 137Cs concentrations
with time or differences among the stations wereobserved. A uniquely high dissolved 137Cs concentration
was observed at Stn. 3 in January 2017 compared to theother stations, but the dissolved 137Cs concentrations at
Stn. 3 at similar dates in 2015 and 2016 were not so high,suggesting that unexpected 137Cs contamination may haveoccurred during this survey. The particulate 137Csconcentrations were lower than those of dissolved forms
in all the data and ranged from 13 to 185 Bq m−3, tending
to decrease in the downstream flow direction (Fig. 3(a)).
This trend was due to concentration of suspended solids
decreasing toward downstream (Fig.3(b)), which strongly
indicates precipitation of suspended solids to the bottomof the reservoir during the downstream flow process. In
this catchment, a tremendous rainfall event with a total
rainfall of 572 mm was observed from September 6 to 11,

137Cs concentration (Bq m-3)

(a)

400

SS concentration (mg L-1)

dissolved: St.1
particulate: St.1

St.2
St.2

St.3
St.3

1 Aug 10 Nov
2016
2016

12 Jan2017

25 May
2017

300

200

100

0

(b)

2015, but no remarkable change in either form of 137Csconcentration was observed before or after this period.
During the two months from this rainfall event to thesurvey in November 2015, most of the suspended solids
in the influent water from the runoff event would haveprecipitated to the reservoir bottom.
The 137Cs concentrations per unit weight of suspended
solids collected using a 1 μm pore size filter ranged from
3.2×102 to 4.3×104 Bq kg−1 with a geometric mean of
1.6×104 Bq kg−1 (Fig. 3(c)), and no significant decreasing
trend with time was observed at the three stations. As forthe trend in flow direction, there was a period during
which the 137Cs concentration in the suspended solids
increased in the flow direction, especially in theNovember survey in all three years. The distribution
coefficient of 137Cs (Kd), calculated by dividing the 137Csconcentration per unit weight of suspended solids by the

7 Aug
2014

13 Nov
2014

3 Mar
2015

19 May
2015

4 Aug
2015

17 Nov 10 Mar 17 May
2016
2015
2016

40

St.1

St.2

St.3

1 Aug
2016

10 Nov
2016

12 Jan2017

25 May
2017

St.2
St.2

St.3
St.3

12 Jan2017

25 May
2017

30

20

10

0

7 Aug
2014

13 Nov
2014

3 Mar
2015

19 May
2015

4 Aug
2015

17 Nov 10 Mar 17 May
2015
2016
2016

(c)

137Cs in SS (Bq kg-1)

Kd of 137Cs (L kg-1)

106

137Cs in SS :St.1
Kd of 137Cs : St.1

105

104

103

7 Aug
2014

13 Nov
2014

3 Mar
2015

19 May
2015

4 Aug
2015

17 Nov 10 Mar 17 May
2015
2016
2016

1 Aug
2016

10 Nov
2016

Fig. 3 (a) Dissolved and particulate 137Cs concentrations, (b) suspended solid (SS) concentrations, and (c) 137Cs concentrations per unit
weight of suspended solids and distribution coefficient of 137Cs (Kd) in the surface water at Stn.1 (upstream), Stn.2 (midstream),
and Stn.3 (downstream) of Yokokawa Reservoir.

Dynamics of 137Cs in water and phyto- and zooplankton in a reservoir affected by the Fukushima Daiichi nuclear power plant accident 121
dissolved 137Cs concentration, ranged from 1.0×104 to
8.0×105 L kg−1 with a geometric mean of 8.7×104 Bq kg−1
(Fig. 3(c)), which differed little from the reported values
in the upstream or downstream parts of the river (Tsuji
et al., 2016; Nakanishi & Sakuma, 2019). The trend of
the 137Cs distribution coefficient in the flow direction was
similar to that of the 137Cs concentration in the suspended
solids.
During a period when 137Cs concentrations in thesuspended solids increased in the flow direction, a clear
decrease in suspended solid concentrations in the flow
direction was also observed (Fig. 3(b)). In this period, it
is inferred that mineral particles derived from forest soil
runoff were abundant in the reservoir water due to somelarge rainfall events. Therefore, the increase in 137Csconcentration in the suspended solids in the flow
direction could be explained by the sedimentation
mechanism of coarse particles being deposited relatively
fast in the upstream area during the flow process(Walling & Moorehead, 1989), and finer particles withhigher 137Cs concentration per unit weight (He & Walling,

3.2 Dynamics of Planktonic 137Cs and its Concentration
Factor
As a species component of phytoplankton in thereservoir, dinoflagellates (Peridinium cinctum) were mostdominant, with their occupancy decreasing as thereservoir water flowed from upstream (Fig.4(a)) to
midstream (Fig.4(b)), to downstream (Fig.4(c)). Amongother species, diatoms (Fragilaria crotonensis,
Asterionella formosa) were representative phytoplankton.

Phytoplankton
100%

100%

80%

80%

Occupancy

Occupancy(a)1996) being transported further downstream. On the other
hand, in periods when the inflow of suspended particles
from upstream is relatively sparse, especially during
winter, the seston particles produced inside the reservoir,
specifically plankton, would dominate as suspended
solids in the reservoir water, and 137Cs concentration insuch particles may obscure the trend of 137Csconcentration in suspended solids in the flow direction.
Accordingly, it has been suggested that 137Cs dynamics inplankton affect the formation of the spatial distribution ofparticulate 137Cs in lakes in seasons with little rain.

60%
40%

0%

0%

2015

2016

2014

2017

Phytoplankton

100%

2015

2016

2017

Zooplankton

100%

80%

80%

Occupancy

Occupancy

40%
20%

2014

60%
40%

60%
40%

20%

20%

0%

0%

2014
(c)

2015

2016

2014

2017

2015

Phytoplankton
100%

100%

80%

80%

Occupancy

Occupancy

60%

20%

(b)

Zooplankton

60%
40%

40%
20%

0%

0%

Peridinium

2015

2016

Fragilaria

Asterionella

2017
Others

2017

60%

20%

2014

2016

Zooplankton

2014

2015

Calanoida
Bosmina2016
Daphnia
Cyclopoida

2017
OthersFig. 4 Components of plankton at (a) Stn. 1 (upstream), (b) Stn. 2 (midstream), and (c) Stn.3 (downstream) in Yokokawa Reservoir.

122

H TSUJI et al.
Chlorophyll-a (mg m-3)
0

0

50

100

0

10

20

30

0

0

5

10

10

2

Depth (m)

0

5

20

4
10

30

6

15

8

Stn.1 (upstream)

1040

Stn.2 (midstream)20

Stn.3 (downstream)50

7 Aug 2014

4 Aug 2015

1 Aug 2016

3 Mar 2015

10 Mar 201612 Jan 2017

Fig. 5 Vertical distribution of chlorophyll-a concentrations in summer and winter at Yokokawa Reservoir (Stns.1–3).
Chlorophyll-a was not measured at Stn.1 in August 2016 because there was very little water depth.

137Cs per wet weight of plankton(Bq kg-1)

104

PP : St.1
ZP : St.1

St.2
St.2

St.3
St.3

103

102

101

100

7 Aug
2014

13 Nov
2014

3 Mar
2015

19 May
2015

4 Aug
2015

17 Nov 10 Mar 17 May
2015
2016
2016

1 Aug
2016

10 Nov
2016

12 Jan2016

25 May2016

Fig. 6 Concentration of 137Cs in the phytoplankton (PP) and zooplankton (ZP) per unit weight of wet sample.

Among zooplankton, copepods (Calanoida copepodid,
Cyclopoida copepodid) and water fleas (Daphnia galeata,
Bosmina spp.) were particularly abundant but nodistinctive seasonal or horizontal trends were observed.
The seasonal trends, however, were unclear for both
phytoplankton and zooplankton.
The peak chlorophyll-a concentration in the reservoir
water was observed at depths of 2 to 3 m at Stns. 1–3,
with higher chlorophyll-a concentrations in summer andin the upper reaches (Fig. 5). Some degree of correlation
appeared between the dry weight of the collected
phytoplankton and the average concentration of2chlorophyll-a in the euphotic layer, (Stn. 1 : R = 0.68,
22Stn. 2 : R = 0.55, Stn. 3 : R = 0.43), suggesting that thechlorophyll-a concentration was an effective indicator ofphytoplankton population density. The vertical distributionof chlorophyll-a concentrations confirmed that most
phytoplankton in the reservoir water lived in the range ofthe euphotic layer.
The 137Cs concentration per wet weight of plankton(converted to 90% moisture content) was 0.86–560
Bq kg−1 (geometric mean: 30 Bq kg−1) (Fig. 6), and the137
Cs concentration factor of phytoplankton calculated
from Eq. 1 was 5.3×101–3.3×104 (geometric mean:
1.3×103). The 137Cs concentrations of zooplankton ranged
from 3.2 to 430 Bq kg−1 (geometric mean: 45 Bq kg−1),
and its 137Cs concentration factor was 1.1×102 to 1.6×104(geometric mean: 2.2×103). These values, however, havea possibility of overestimation due to adulteration bymineral particles with high 137Cs concentrations. The Al

Dynamics of 137Cs in water and phyto- and zooplankton in a reservoir affected by the Fukushima Daiichi nuclear power plant accident 123
Table 2 Al concentrations per dry weight in plankton and sediment samples. The percentage of plankton in the samples estimated from the
Al concentrations, the concentration factors (CF) of 137Cs of plankton, and the ratio of planktonic 137Cs to total lake water forplankton-enriched samples collected from May 2016 to May 2017. Samples with planktonic content greater than 95% are shownin bold, and these are considered representative data on the dynamics of planktonic 137Cs.
Sample typeplanktonsediment

phyto
phyto
zoo
phyto
zoo
phyto
phyto
zoo
phyto
zoo
phyto
zoo
phyto
zoo
phyto
zoo
phyto
zoo
phyto
zoo
phyto
zoo
phyto
zoo
phyto
zoo
phyto
zoo

Collected date
17 May 2016

Station No.
〔Stn.〕
1
2
3

1 Aug 2016

1
2
3

10 Nov 2016

1
2
3

12 Jan 2017

1
2
3

25 May 2017

1
2
3

13 Nov 2014
26 Sep 2018
10 Mar 2016
10 Nov 2016
10 Mar 2016
25 Jul 2016

1
2
3

Al content(mg kg−1)
8,000
2,600
9,700
3,700
6,900
2,300
1,800
4,400
1,300
11,000
710
540
3,000
4,300
3,900
3,500
300
2,500
690
3,500
1,200
1,800
43
2,900
1,100
2,600
1,100
1,400
15,00024,00034,00039,00046,00035,000concentration, which indicates impurities, in 28 plankton
samples collected after May 2016 was 43–11,000
mg kg−1, and the Al concentration in the bottom sediment
samples collected during the same period ranged from15,000 to 46,000 mg kg−1 (Table 2). Accordingly, theplankton content in the 28 samples estimated by Eq. 2
was 58%–100% and nine samples of phytoplankton andthree samples of zooplankton contained plankton with apurity of >95%. In many of the samples, the influence ofinclusion of mineral particles on the 137Cs concentration
was considered not to be negligible. Therefore, the 137Csconcentration factor in this reservoir was evaluated only
for samples with planktonic content of >95%. By thisrefinement, the geometric mean values of the 137Csconcentration factor for phytoplankton and zooplankton
were 340 and 1,700, respectively, which were of thesame order of magnitude as previously reported values(Table 3). Even for these samples, no dependenceof the concentration factor on season or on station
was observed.

Estimation weightpercentage of plankton
58%
93%
73%
91%
81%
88%
95%
88%
97%
70%
96%
97%
92%
88%
90%
90%
98%
87%
98%
90%
97%
95%
100%
85%
97%
93%
97%
96%

CF
18,000
980
5,000
770
2,600
260
650
1,200
280
1,900
360
400
950
1,500
1,100
1,800
250
480
330
7,100
470
5,700
53
1,400
580
1,800
700
2,200

Ratio of planktonic 137Cs
to total lake water (%)
0.06
0.04
0.018
0.016
0.073
0.00540.039
0.0038
0.38
0.011
0.052
0.00130.00440.00140.31
0.0010
0.108
0.036
0.105
0.011
0.83
0.0087
0.028
0.096
0.026
0.0080The percentage of planktonic 137Cs to the whole 137Cs
in the water estimated by equations 3 and 4 ranged from0.0044% (Nov 2016) to 1.4% (Aug 2015) for
phytoplankton and 0.0010% to 0.17% for zooplankton for
all samples. While limited to samples with planktonic
content of >95%, the percentage ranged from 0.012% to0.83% for phytoplankton and 0.00054% (Mar 2016) to
0.096% (May 2017) for zooplankton. This result indicatesthat the effect of 137Cs in plankton larger than 41 μm on
the overall 137Cs dynamics in the reservoir is very limited.
In past laboratory experiments (Twiss & Campbell, 1998;
Thomas et al., 2018), the upper limit of the 137Cs uptake
ratio to zooplankton living in freshwater was reported tobe less than 1%. Therefore, the amount of 137Cs that canbe taken up by zooplankton may be limited to about 1%of the total amount of 137Cs in the water. On the other
hand, Thomas et al. (2018) reported that the amount of137
Cs transferred to freshwater diatomic phytoplankton,
including with cell diameter of 6.5 μm, could exceed 10%of the total 137Cs in the water according to their laboratory

124

H TSUJI et al.

Table 3 137
 Cs concentration factor (CF) of phyto- and zooplankton (wet condition) in the freshwater environment. CFs with a dagger mark(†) were calculated against the total 137Cs concentration, and those without the mark were calculated against the dissolved 137Csconcentration.
Reference

Station/Country

Collected
year

Mesh size ofplankton net

Range ofcollection

PlanktonThis study

Yokokawa dam
Japan2014 – 2017

41 μm

euphotic zone

phyto

〃

〃

2014 – 2017

200 μmwhole depthzooMatsuda et al.
(2015)

2012 – 2013

335 μm1 m depth

phyto and zoo

-

2400

2012 – 2013

335 μm〃phyto and zoo-

3802012 – 2013

335 μm〃phyto and zoo-

1302011 – 2016

100 μm1 m depth

phyto

Aulacoseira sp.

720†

〃

Lake Hayama
Japan
Lake AkimotoJapan
Lake Tagokura
Japan
Lake Onuma
Japan
Lake Onuma
Japan

2011 – 2016

200 μm〃zoo

Bosmina longirostri,
Holopedium gibberum

40†

Knapinska-Skiba et al.
(2003)

Świna River
Poland470†〃〃
Suzuki et al.
(2018)

Ravera and Giannoni(1995)

1997

Lake Monate
1986
Italy
Lake Comabbio
1986
〃
Italy
Saxén et al.
Lake Iso Valkjärvi
1992(2009)
Finland
Lake Valkea Mustajärvi
1987–1990〃
Finland
Lake Halsjärvi
1987–1988〃
Finland
Lake Rahtijärvi
1987–1990〃
Finland
Lake Savijärvi
1987–1990〃
Finland
Lake Vähä Valkjärvi
1988–1989〃
Finland
Lake Mekkojärvi
1988〃
Finland*concentration factor of 137Cs (geometric mean value)

Dominant species
Peridinium cinctum,
Fragilaria crotonensis,
Asterionella formosa
Calanoida copepodid,
Daphnia galeata,
Bosmina spp.,
Cyclopoida copepodid

CF*
340

1700

60 μm

surfacephyto and zoo

phyto:Melosira granulata,
Microcystis aeruginosa
zoo:Daphnia cucullata,
Bosmina coregoni,
Cyclops sternuus

88 μm

whole depth

phyto and zoo

-

220

88 μm

whole depth

phyto and zoo

-

100

-

-

zoo

-

170†

-

-

zoo

-

43†

-

-

zoo

-

110†

-

-

zoo

-

130†

-

-

zoo

-

37†

-

-

zoo

-

38†

-

-

zoo

-

86†

experiment. The difference between the report and thisstudy suggested the possibility that the fraction ofphytoplankton not recovered by the 41-μm mesh size ofthe plankton net held non-negligible amounts of 137Cs.
Accordingly, it would be difficult to discuss the whole
dynamics of planktonic 137Cs in the reservoir by focusing
only on plankton larger than 41 μm in cell length.
Even if the amount of planktonic 137Cs is limited
compared to the total 137Cs in lake water, the dynamics ofplanktonic 137Cs are expected to have a significant impact
on the uptake of 137Cs by aquatic biota as a pathway of137
Cs bioaccumulation through predation. Therefore, it isnecessary to position and consider planktonic 137Cs
dynamics in relation to 137Cs transfer to freshwater fish in
the future.
To summarize the remaining issues in this study, no
seasonal or site-dependent characteristics of planktonic
137
Cs in the reservoir could be observed because it wasdifficult to exclude the effect of adulteration from mineralparticles in the plankton samples. It was also suggested

that uncollected plankton less than 41 μm in length may
make a non-negligible contribution to 137Cs dynamics inlakes. Both of these issues are pertinent to theunderstanding of the dynamics of 137Cs in freshwater
ecosystems, so advances in recovery methods of highpurity plankton in the future are expected to clarify thesedetailed dynamics.

4. Conclusions
The present study of 137Cs dynamics in a reservoir
revealed the following.
1. 	Dissolved 137Cs concentrations in the reservoir’s
surface water at stations in the upstream part, central
part and near the embankment fluctuated seasonally:
they were high in summer and low in winter. No
differences in dissolved 137Cs concentrations between
the stations were observed.
2. 	The particulate 137Cs concentration in the reservoir’s
surface water was lower than that of dissolved 137Cs.

Dynamics of 137Cs in water and phyto- and zooplankton in a reservoir affected by the Fukushima Daiichi nuclear power plant accident 125
The 137Cs concentration per unit weight of suspended
solids and the distribution coefficient of 137Cs tended
to be higher toward the downstream direction mainlyin autumn, probably because coarse mineral particles
with low 137Cs concentration predominated assuspended solids in this season.
3. 	No seasonal regularity in composition of plankton
species was observed. No seasonal variations of 137Cs
in phyto- and zooplankton could be observed,
because the effect of mineral particle contamination
was not negligible in many of the samples based on
Al concentrations in the plankton-enriched samples.
4. 	The concentration factors of 137Cs in phytoplankton
and zooplankton were 340 L kg−1 and 1,700 L kg−1,
respectively, which were on the same order as inprevious reports.
5. 	The occupancy of planktonic 137Cs in plankton larger
than 41 μm to total 137Cs in the reservoir water wasmuch less than 1.4%.
Acknowledgements
Hydrological data on the reservoir were provided by
the Yokokawa Dam Management Office. We thank Dr.
Ito and Messrs. Iri, Harada and Narita for their help withthe field survey, and Prof. Kenji Nanba of Fukushima
University for his advice on measuring Al in plankton
samples. We would also like to thank the two reviewers
for their many helpful suggestions. This study was
supported by a Grant-in-Aid for Scientific Research(KAKENHI 15K16129) from the Japan Society for the
Promotion of Science.
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Hideki TSUJI
Hideki Tsuji is a senior researcher at the National
Institute for Environmental Studies. He is
conducting field research on the transport andaccumulation of radiocesium in rivers and lakes.
He is also carrying out chemical analysis,
constructing a numerical model and developing
equipment to be used for environmental
measurements of bioavailable radiocesium.

Megumi NAKAGAWA
Megumi Nakagawa is a specialist working at theCenter for Environmental Biology and Ecosystem
Studies, National Institute for Environmental
Studies. She has identified and counted
phytoplankton and zooplankton in Lake
Kasumigaura, the second largest lake in Japan,
for 25 years. She is also one of the staff at theJapan Focal Point of UNEP's GEMS/Water program.

Kazuya YOSHIMURA
Dr. Kazuya Yoshimura is a researcher at the
Sector of Fukushima Research and Development,
Japan Atomic Energy Agency (JAEA). His
research interests lie in environmental radiation,
especially in the hydrological migration ofradiocesium and its relationship with dose rates interrestrial environments. Recently he hasaddressed evaluation of exposure and radiation protection.

Kazuki IIJIMA
Kazuki Iijima heads the Fukushima Environmental
Evaluation Research Division at the Collaborative
Laboratories for Advanced Decommissioning
Science, Japan Atomic Energy Agency (JAEA).
He is interested in how radionuclides behave inthe environment, especially the interactions oftrace amounts of radionuclides with minerals andorganic matter of natural origins. Previously, he investigated the behaviorof radionuclides under conditions deep underground (stable and static forlong periods). After the Fukushima Daiichi nuclear power plant accident,
he started studying their behavior at the earth’s surface. (He says it is so
unstable and too dynamic!)Kazuyuki SAKUMA

rivers, and reservoirs.

Kazuyuki Sakuma, Ph.D., is a researcher at the
Sector of Fukushima Research and Development,
Japan Atomic Energy Agency (JAEA). He isinterested in the environmental modeling ofradionuclide behavior, watershed modeling,radiation transport simulation in the field, andinvestigation of radionuclide behavior in forests,

Seiji HAYASHI
Hironori FUNAKI
Hironori Funaki is an assistant principal scientist
at the Fukushima Environmental Monitoring
Division, Japan Atomic Energy Agency (JAEA).
He performs research in hydrogeology,
geochemistry and environmental radioactivity.
His current research interests include the transportof radioactive materials in lakes and reservoirs.

Seiji Hayashi is a research group manager at the
Fukushima Branch of the National Institute forEnvironmental Studies. He serves as project
leader for research on radioactive substancebehavior in multimedia environments. His
energetic research activities are contributing to
the environmental recovery of Fukushima
Prefecture and its surrounding region.
(Received 16 October 2020, Accepted 21 December 2020)