Laguna (2017) 24: 19-25

ISSN 2185-2995

短報

Salinity fluctuation factor in the brackish Lake Shinji
Ryukichi Sugai1, Shogo Sugahara2 and Yasushi Seike2,3

Abstract: The aim of this work is the identification of the main factor affecting the monthly

salinity variation in Lake Shinji, Japan. A strong correlation was observed between the mean daily
flow rate of the Hii River, measured over 80 days at Kamishima, and the chlorinity in the central

part of the lake. The fluctuation of chlorinity in Lake Shinji could be reproduced based on the flowrate at Kamishima. This result demonstrates that the salinity fluctuation of Lake Shinji significantly
depends on the flow rate of the Hii River.

Key words: Chlorinity, Estuary, Flow rate, Hii River, Lake Shinji, Lake Nakaumi, Ohashi RiverThe Ohashi River is characterized by reciprocating

Introduction

currents that correspond to the tidal variations of theLake Shinji is a lagoon between the Shimane

Sea of Japan. This implies that when the water level at

Peninsula and the Japanese mainland. It is a brackish

the eastern end of the river (downstream) is higher thanlake, characterized by a salinity range from oligohalinethat at the western end (upstream), saline water flows

to, rarely, mesohaline. The lake is located downstream

upstream from Lake Nakaumi to Lake Shinji. Theseof the Hii River, a Class A river flowing from Mountintrusions of saline water through the Ohashi River haveSentsu, on the border with the Hiroshima Prefecture.

been reported in several studies (Fukuoka et al. 2004,

The main stream of the Hii River flows into the western

2005; Ishitobi et al. 1989, 1999; Mizoyama et al. 2011).

side of Lake Shinji and supplies about 80% of the total

The intruded saline water forms a saline bottom stratum

freshwater inflow. Lake Nakaumi (polyhaline) is situatedthat plays an important role in the cycling of matterto the east of Lake Shinji and the two lakes are connected

within the lake. This saline bottom stratum is found toby the Ohashi River. Both lakes are managed as part of

be closely related to the return of inorganic nutrients

the Class A Hii River system.

(Kamiya et al. 1996, 2001; Sugai et al. 1986, 2012, 2015).

1

Shimane Environment & Health Public Corporation, 1-4-6 Koshibara, Matsue, Shimane 690-0012, Japan.

2

Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan.

3

Estuary Research Center, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan.

受付日：2016 年 9 月 1 日，受理日：2017 年 7 月 18 日

19

10

Ryukichi Sugai・Shogo Sugahara・Yasushi SeikeFig．１ R. Sugai

1

Japan Sea

Sakai Channel

Miho
Bay

Shimane Peninsula
Matsue City

Izumo Plain
Izumo City

YahataKoshin,S3
L.Nakaumi
MatsueL.Shinji
Ohashi River
Main stream of Hii River

Otsu
Kamishima

Shinigaya

M.Sentsu

Fig. 1 Study area.

In brackish lakes such as Lake Shinji and Lake

Methods

Nakaumi, salinity is the key element controlling not only

the advection between the two bodies of water but also

Study area

“Aoko”. It follows that the identification of the factors

study were those of Lake Shinji, the Ohashi River, andfor understanding the chemical composition of its water.

Miho Bay. The Hii River water system is a Class A river

As shown in Fig. 1, the water areas analyzed for thisthe occurrence of phytoplankton blooms such as theLake Nakaumi, from the main stream of the Hii River toaffecting the salinity variation in the lake is essential

with a catchment area of 2,070 km² and a watercourse

Somura et al. (2009) and Sugai (1988) suggested theof 153 km from Mount Sentsu to Miho Bay. The main

existence of close relationships between the flow ratestream of the Hii River flows into the western side ofin the Hii River measured at Otsu and the chlorinity in

Lake Shinji.

Lake Shinji.

This research was conducted to identify the main factor

affecting the monthly salinity variation in Lake Shinji,

Chlorinity and water level

In this research, we used flow rate data at three different(Cl – concent rat ion) d at a base d on a n automat ic

from the upper layer at the center of Lake Shinji. Data

Koshin Observatory), and at the outlet of the lake

Chlorinity was used in place of salinity. The chlorinity

using recent hydrological balance and water quality data.

monitoring system at the center of Lake Shinji (hereafter

locations along the Hii River and the data for chlorinity(hereafter Matsue Observatory) were retrieved fromfrom 2004 to 2013 were considered in the analysis.

a website maintained by the Izumo Office of River,
Chugoku Regional Development Bureau, Ministry ofLand, Infrastructure, Transport and Tourism (MLIT)(http://www1.river.go.jp/cgi-bin/SelectMapSite.exe). In
20

11

Salinity fluctuation factor in the brackish Lake Shinji
Fig．2

(a)Cl- (mg L-1)

14,000

MatsueR. Sugai

Koshin(U)12,00010,000

8,000
6,000

4,000

1

2

3

(c)

4

Matsue

5

6

7

8

9

YahataKoshin1… 1… 1… 1… 1… 1… 1… 1… 1… 1… 2… 2… 2… 2… 2… 2… 2… 2… 2… 2… 3… 3…

flowrate
rateofofHiiHiiRiver
River(Kamishima）(Kamishima）
flow

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

9

10

8

7

6

5

4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
3

400
350
300
250
200
150
100
50
0

(b)
(b)

2

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0

1

Flow rate (m3 s -1)

Water level (m)

0

1
20
39
58
77
96
115
134
153
172
191
210
229
248
267
286
305
324
343
362
381
400
419
438
457
476
495
514
533
552
571
590
609
628
647
666
685
704
723
742

2,000

Day

Fig. 2 Changes in (a) Cl – concentrations at Matsue Observatory and Koshin

Observatory, (b) the water levels at Koshin Observatory, Matsue Observatory and
Yahata Observatory, and (c) the flow rate of the Hii River (Kamishima Observatory) in
July 2011.

addition, the chlorinity data in the upper layer (1m fromupstream of the Otsu observatory, and the chlorinity atmonthly water quality survey located at the center ofrate for 70, 80 and 90 days before the day of the monthly

the water surface) at point S3 (sampling point in the

St.S3 was also examined. We calculated the average flow

Lake Shinji; hereafter St.S3) were also used.

water quality survey in Lake Shinji. Then we investigated

Data for the water levels at the Koshin Observatory, atthe relationship between each average flow rate and thethe Matsue Observatory, and at the outlet of the Ohashi

measured chlorinity. These data were retrieved from theRiver (hereafter Yahata Observatory) were also retrieved

website.

from the website.

Results and Discussion

Hii River flow rate

Salinity fluctuation factor

The primary factor affecting chlorinity variation in

Lake Shinji is inferred to be the flow rate of the Hii

We focused on the differences in salinity variations

River. In this study, we analyzed the relationship between

between July 2011 and July 2013 at two different

in the upper layer at St.S3, located near the Koshin

In the upper layer at the Koshin Observatory, a low levelthe flow rate at the Otsu observatory and the chlorinity

locations (Koshin Observatory and Matsue Observatory).

Observatory, using recent data (2004 – 2013). In addition,of chlorinity of about 1,000 mg L –1

the relationship between the flow rates at the Shinigaya

was observed inJuly 2011 (Fig. 2a), while a high level of chlorinity ofobservatory and the Kamishima observatory, locatedaround 4,000 mg L –1 was observed in July 2013 (Fig.
21

12

Ryukichi Sugai・Shogo Sugahara・Yasushi SeikeFig．3

(a)Cl- (mg L-1)

14,000

MatsueR. Sugai

Koshin(U)12,00010,000
8,000

6,000
4,000
2,000
1
20
39
58
77
96
115
134
153
172
191
210
229
248
267
286
305
324
343
362
381
400
419
438
457
476
495
514
533
552
571
590
609
628
647
666
685
704
723
742

MatsueYahataKoshin

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

9

10

8

7

6

flowrate
rateofofHiiHiiRiver
River(Kamishima）(Kamishima）
flow

5

4

1
20
39
58
77
96
115
134
153
172
191
210
229
248
267
286
305
324
343
362
381
400
419
438
457
476
495
514
533
552
571
590
609
628
647
666
685
704
723
742

(c)

3

400
350
300
250
200
150
100
50
0

(b)

2

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0

1

Flow rate (m3 s -1)

Water level (m)0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Day

Fig. 3 Changes in (a) Cl – concentration at Matsue Observatory and Koshin

Observatory, (b) the water levels at Koshin Observatory, Matsue Observatory and
Yahata Observatory, and (c) the flow rate of the Hii River (Kamishima Observatory) in
July 2013.

3a). In contrast, at the Matsue Observatory, chlorinity

Matsue Observatory peaked with the upstream flow ofJuly 2011 and from 3,000 to 12,000 mg L (Fig. 3a) inwater is provided to Lake Shinji by this upstream flow ofvaried widely; from 1,000 to 12,000 mg L (Fig. 2a) insaline water. It is suggested that a regular supply of saline–1–1

July 2013. As shown in Fig. 2, when the flow rate of thesaline water from the Ohashi River, occurring when the

Hii River at Kamishima Observatory increased rapidly

water levels at Lake Nakaumi, in combination with the

because of a flooding (July 7 to 8, 2011, see Fig. 2c), thetidal variations in the Sea of Japan (Miho Bay), exceed

water level at the Matsue Observatory, in the upstream

those at Lake Shinji.

portion of the Ohashi River, also rose rapidly (Fig. 2b),

As shown in the results for July 2011 (Fig. 2), during

along with the water level of Lake Shinji at the Koshin

periods characterized by high inflow of freshwater andrelatively strong downstream current, the upstream flow

was rarely higher than the water level at the Matsue

Observatory. In this case, because the Ohashi River had alow salinity, the water level at the Yahata Observatory

was suppressed. The corresponding chlorinity variation

Observatory (148/744 hours). These periods was also

at Matsue Observatory shows that saline water had notcharacterized by a relatively low chlorinity in theindeed reached the observatory during July 8 to 12, 2011

upstream flow. Results for July 2013 (Fig. 3) indicatedas a result of the flooding (Fig. 2a). When the water levels

that during periods characterized by low inflow ofat the Koshin Observatory and the Matsue Observatory

freshwater and high salinity, the water level at the Yahata

went back to normal, the Ohashi River regained its

Observatory was often higher than the water level at thereciprocating current regime, and the chlorinity at the

Matsue Observatory (306/744 hours) and in these period
22Fig．4

R. Sugai

Salinity fluctuation factor in the brackish Lake Shinji

4,000

a1 4,000

3,000

3,000

3,000

2,000

0
6,000㻝㻜㻚㻜㻜

2,000
1,000

for 90days

3,000

3,000

2,000
for 80days

06,000㻝㻜㻚㻜㻜

2,000
1,000

a34,000

3,000

3,000

Cl(mg/l)

4,000

2,000

1,000
0

for 70days

for 80days

1,000

㻝㻜㻜

c2

2,000
1,000

for 80days

0㻝㻜㻜㻚㻜㻜
6,000㻝㻜㻚㻜㻜y=-2210ln(x)+10400
流量(m3/s)
R² = 0.802
5,000

b34,000

2,000

0
㻝㻜

㻝㻜㻜㻚㻜㻜
y = -1960ln(x) + 9680
流量(m3/s)
R² = 0.556

3,000

0
㻝㻜㻜㻚㻜㻜
6,000㻝㻜㻚㻜㻜y = -2040ln(x) + 9250
流量(m3/s)
R² = 0.757
5,000

5,000

for 90days

b24,000

Cl(mg/l)

1,000

2,000

0
㻝㻜㻜㻚㻜㻜
6,000㻝㻜㻚㻜㻜y = -2330ln(x) + 10900
流量(m3/s)
R² = 0.829
5,000

Cl(mg/l)a2 4,000
Cl(mg/l)

4,000

c1

1,000

for 90days

0㻝㻜㻜㻚㻜㻜
6,000㻝㻜㻚㻜㻜y=-2150ln(x)+9630
流量(m3/s)
R² = 0.778
5,000

5,000

y = -2000ln(x) + 9820
R² = 0.562

b14,000

Cl(mg/l)Cl(mg/l)Cl- (mg L-1)(c)Otsuy = -2370ln(x) + 110006,000
R² = 0.823
5,000

1,000

Cl- (mg L-1)

6,000

y = -2180ln(x) + 9750
R² = 0.774
5,000

5,000

Cl- (mg L-1)(b)Kamishima(a)Shinigaya

6,000

for 70days㻝㻜㻝㻜㻜㻚㻜㻜
y = -1820ln(x) + 9120
流量(m3/s)
R² = 0.521

c3

3,000
2,000
1,000
㻝㻜㻜

0

for 70days

㻝㻜

㻝㻜㻜

3 3-1 -1
Mean
dailyflow
flow rate
rate (ms s) )
Mean
daily
(m

Fig. 4 Relationship between Cl– concentration (2004~2013) at St.S3 (sampling point of the monthly

water quality survey) and the mean daily flow rate in the Hii River at (a) Shinigaya Observatory, (b)Kamishima Observatory and (c) Otsu Observatory for 70, 80, and 90 days before the day of monthly
water quality survey (2004~2013) in Lake Shinji.

chlorinity in the upstream flow was relatively high.

4b2). In contrast, the correlation was weak for the Otsu

Shinji essentially depends on the flow rate of the Hii

the former cases. These results suggest that if interested

We thus hypothesized that the chlorinity in Lake

Observatory (Fig. 4c, R 2 = 0.521-0.562) compared toRiver, which indirectly determines the volume of saline

in salinity variations, the flow rate data at the Otsu

water flowing upstream from the Ohashi River. In recent

Observatory is less reliable compared to those at the(2004–2013) observational data, as shown in Fig. 4a,

Kamishima Observatory and the Shinigaya Observatory.

close relationships were found between each of the mean

Somura et al. (2009), investigating the impact of climate

daily flow rates for 70, 80 and 90 days at the Shinigayachange on the Hii River basin and salinity in Lake Shinji,

Observatory in the Hii River and the chlorinity at St.S3

have examined the relationship between the monthly

in Lake Shinji. The strongest correlation (coefficient

averaged flow at Otsu Observatory and the monthlyof determination: R = 0.829) was found for an 80-day

averaged salinity (calculated from chlorinity) in bottom

2mean daily flow rate at Kamishima Observatory (Fig.

water at the center in Lake Shinji measured by the MLIT.
23

14

Ryukichi Sugai・Shogo Sugahara・Yasushi SeikeFig．5 R. Sugai

Observed valueCl- (mg L-1)

6,000

St.S3

Calculated value

5,000
4,000
3,000
2,000
1,000
0

2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Fig. 5 Comparison of the measured Cl– concentration at St. S3 (sampling pointof the monthly water quality survey) and the Cl– concentration calculated using
the Cl–Q equation (Fig.4 b2).

They used chlorinity data from the monthly survey andReferences

river flow data of the previous month. Their result (R2= 0.532) is close to our result (R 2 = 0.521-0.562) at the

Fukuoka, S., Matsushita, T., Okamura, S., Imai, S. andif compared to the results obtained with data from thesaline water advancing into a brackish lake. Annu. J.

Otsu Observatory, which, however, is less significant

Funabashi, S. (2004) Behavior and characteristics ofShinigaya Observatory (R = 0.757-0.778) and from the2of Hydr Eng, JSCE, 48:1405-1410. (in Japanese with

Kamishima Observatory (R2 = 0.802-0.829).

English abstract)

The chlorinity at St.S3 in Lake Shinji was thenFukuoka, S., Yamamoto, A., Okamura, S. and Mizoyama,

From the results, we confirmed that the estimatedsalinity in Lake Shinji. Annu. J. of Hydr. Eng., JSCE,

calculated from the obtained Cl–Q formula (Fig. 4b2).

I. (2005) Estimation of salinity flux and variation ofvariations correspond to the measured values, and that49:1249-1254. (in Japanese with English abstract)

the reproducibility is high. Fig. 5 shows the measured

Ishitobi, Y., Kamiya, H., Hayashi, K. and Gomyoda, M.

chlorinity at St. S3 from 2004 to 2013 and the changes(1989) The tidal exchange in Lake Shinji under low

in chlorinity with flow rate calculated from the Cl–Q

discharge conditions. Jpn J Limnol 50:105-113.

formula (Fig. 4b2) obtained in this research. As shown

Ishitobi, Y., Kamiya, H., Yokoyama, K., Kumagai, M.

reproduced using the flow rate. We thus conclude that thewater intrusion into a coastal lagoon, Lake Shinji,

in Fig. 5, the chlorinity variations could be successfully

and Okuda, S. (1999) Physical conditions of saline

variations in chlorinity in Lake Shinji depend on the flow

Japan. Jpn J Limnol 60:439-452.

rate of the Hii River as the primary factor.

Kamiya, H., Ishitobi, Y., Inoue, T., Nakamura, Y. andAfter the completion of the Ohashi River Improvement

Yamamuro, M. (1996) Origin of nutrients accumulated

Project, agreed upon by the governors of Tottori andin the bottom water of Lake Shinji in summer. Jpn J

Shimane Prefectures in 2009, we plan to verify whether

Limnol 57:313-326. (in Japanese with English abstract)a significant difference exists between the slope of the

Kamiya, H., Ishitobi, Y., Inoue, T., Nakamura, Y. andthe formula that will be obtained in an analogous study

phosphorus (DOP) and phosphate from the sediment

Cl–Q formula obtained in this research and the slope of

Yamamuro, M. (2001) Effluxes of dissolved organic

using updated data sets.

to the overlying water at high temperature and low

dissolved oxygen concentration conditions in aeutrophic brackish lake. Jpn J Limnol 62:11-21. (inJapanese with English abstract)

Mizoyama, I., Ohya, Y. and Fukuoka, S. (2011) Flow
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Salinity fluctuation factor in the brackish Lake Shinjimecha n ism i n a con nected brack ish la ke a ndnumerical flow computation over a long period．
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Fluxes of inorganic nutrients from bottom sedimentsin the brackish Lake Shinji. Jpn J Limnol 73:179-186.
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and Seike, Y. (2015) Evaluation of water area based on
fluxes of nitrogen, phosphorus, and dissolved organiccarbon from bottom sediments in two connectedbrackish lakes. Jpn J Limnol 73:179-186. (in Japanesewith English abstract)25