SUWA-KO (LAKE SUWA)

A look down view from the northeast with Suwa City in the foreground


Photo: Nagano Prefectual Government

A. LOCATION

• Nagano Prefecture, Japan.


• 36:03N, 138:05E; 759 m above sea level.

B. DESCRIPTION

Lake Suwa is a tectonic lake located on the central highland of Honshu at an altitude of about 760 m above sea level. A single stream, Tenryu River, drains the lake water into the Pacific Ocean. The lake surface remains frozen for 2-3 months during winter. Long straight rows of ice ridge locally called 'Omiwatari' sometimes traverse the frozen lake surface in the winter morning after a cold night.
The lake has been filled with sediments transported by rivers from its relatively wide drainage basin to become a typical eutrophic lake with an average depth of about 5 m. Nearly 30% of the drainage basin are covered by forest vegetation, while the greater part of the population is concentrated in a few cities near the lake shore. The spectacular growth of industrial activity around the lake since the 1960's caused a very rapid hypertrophication of the lake as indicated by heavy blooms of Microcystis (blue-green algae) that take place every summer.

C. PHYSICAL DIMENSIONS (Q)

Surface area [km2]	13
Volume [km3]	0.063
Maximum depth [m]	7.2
Mean depth [m]	4.7
Water level	Regulated
Length of shoreline [km]	15.9
Residence time [yr]	0.1
Catchment area [km2]	515

D. PHYSIOGRAPHIC FEATURES

D1 GEOGRAPHICAL

• Bathymetric map: Fig. ASI-31-01.


• Names of main islands: None (1).


• Number of outflowing rivers and channels (name): 1 (Tenryu R.)(Q).

D2 CLIMATIC

• Climatic data at Suwa, 1951-1980 (2, 3)

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Ann.
Mean temp. [deg C]	-2.0	-1.2	2.6	9.3	14.3	18.3	22.2	22.9	18.6	12.1	6.4	1.3	10.4
Precipitation [mm]	47	55	84	120	117	205	219	141	179	112	67	41	1,386

• Number of hours of bright sunshine: 2,246 hr yr-1.


• Solar radiation (Matsumoto, 1973-1982): 14.1 MJ m-2 day-1.


• Snowfall (1954-1980): 0.2 m.

Fig. ASI-31-01
Bathymetric map (1).

• Water temperature [deg C](8)

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar*	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.5	-	-	3.7	11.4	16.6	19.7	23.2	26.7	22.5	15.8	10.9	3.6
5	-	-	3.5	9.5	14.5	18.5	19.8	21.7	21.4	14.8	7.7	3.6

* 1984-1985.

• Freezing period: January-February (8).


• Mixing type: Dimictic.


• Notes on water mixing and thermocline formation
  The thermocline is formed from May to August and from January to February (8).

E. LAKE WATER QUALITY

E1 TRANSPARENCY [m](8)

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar*	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
	-	-	1.5	1.6	1.5	1.4	0.80	0.60	0.52	0.74	1.2	1.5

* 1984-1985.

E2 pH (8)

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar*	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.5	-	-	8.3	8.1	8.2	7.4	9.6	9.4	8.6	8.4	8.0	7.9
5	-	-	8.2	7.7	7.7	7.1	8.0	6.7	6.8	7.6	7.5	7.9

* 1984-1985.

E3 SS [mg l-1](8)

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar*	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.5	-	-	11	7	5	7	21	19	25	15	10	8
5	-	-	7	5	5	8	9	12	17	14	11	9

* 1984-1985.

E4 DO [mg l-1](8)

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar*	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.5	-	-	12	11	9.7	9.4	13.7	12	8.7	9.0	9.5	11
5	-	-	13	9.7	7.0	5.2	6.1	2.6	4.5	7.6	9.4	11

* 1984-1985.

E5 COD [mg l-1](8)

Determined by KMnO4 method.

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar*	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.5	-	-	5.6	3.6	3.8	4.4	8.4	8.1	11.9	6.5	4.2	4.4
5	-	-	3.6	2.7	3.6	3.7	3.7	4.8	6.9	4.5	4.2	4.4

* 1984-1985.

E6 CHLOROPHYLL CONCENTRATION [micro l-1](8)

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar*	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.5	-	-	59	33	24	23	90	193	101	39	36	54
5	-	-	41	17	32	14	26	18	28	25	38	45

* 1984-1985.

E7 NITROGEN CONCENTRATION (8)

• Total-N [mg l-1]

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar*	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.5	-	-	0.69	1.5	1.0	0.80	1.8	2.8	1.4	1.5	0.94	0.59
5	-	-	1.2	1.4	1.2	0.94	0.98	1.4	0.97	1.3	1.0	0.76

* 1984-1985.

E8 PHOSPHORUS CONCENTRATION (8)

• Total-P [mg l-1]

Station 1, 1983-1985
Depth [m]	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
0.5	-	-	.097*	.047	.053	.062	.10	.19	.18	.090	.059	.059
5	-	-	.010	.050	.066	.070	.064	.091	.15	.089	.068	.059

* 1984-1985.

E10 PAST TRENDS: Fig. ASI-31-02, 03, 04, 05 and 06.

Fig. ASI-31-02
Trend of transparency (4).

Fig. ASI-31-03
Trend of DO (4).

Fig. ASI-31-04
Trend of chlorophyll a (5).

Fig. ASI-31-05
Trend of nitrogen concentration

Fig. ASI-31-06
Trend of phosphorus concentration (5). PP: Particulate P. RP: Reactive P. DOP: Dissolved organic P.

F. BIOLOGICAL FEATURES

F1 FLORA (4)

• Emerged macrophytes
  Phragmites communis, Zizania latifolia, Nuphur japonicum.
• Floating macrophytes
  Trapa japonica, Hydrocharis asiatica, Nymphoides peltata.
• Submerged macrophytes
  Potamogeton maackianus, P. crispus, Vallisneria gigantea, Hydrilla verticillata.
• Phytoplankton
  Melosira japonica, asterionella formosa, Microcystis aeruginosa, anabaena spiroides.

F2 FAUNA (4)

• Zooplankton
  Brachiorus calyciflorus, Keratella cochlearis, Synchaeta stylata, Filinia longiseta.
• Benthos
  Chipangopaludia japonica, Chipronomus plumosus, Tokunagayusurika akamushi, Limnodrilus gotoi, Branchiura sowerbyi.
• Fish: Carassius carassius, Cyprinus carpio, Hypomesus olidus.

F3 PRIMARY PRODUCTION RATE: Fig. ASI-31-07.

Fig. ASI-31-07
Gross primary production rate (4).

F4 BIOMASS (4)

Whole lake, July-August 1971

	Biomass [t (dry wt.)]
Bacteria	16
Phytoplankton	438
Macrophytes	121
Zooplankton	39
Benthos	69
Fish	6.2

Cf. Fig. ASI-31-08, 09 and 10.

Fig. ASI-31-08
Seasonal change of phytoplankton biomass, 1970 (4).

Fig. ASI-31-09
Seasonal change of zooplankton biomass, 1970 (4).

Fig. ASI-31-10
Seasonal change of benthos biomass, December 1969-October 1971 (4).

F5 FISHERY PRODUCTS (Q)

• Annual fish catch [metric tons]
  1985: 201.
• Fishery products other than fish [metric tons]
  1985: 21.

F6 PAST TRENDS: Fig. ASI-31-11 (Q).

Fig. ASI-31-11
Trend of annual fish catch.

G. SOCIO-ECONOMIC CONDITIONS

G1 LAND USE IN THE CATCHMENT AREA (8)

1985
	Area [km2]	[%]
Natural landscape
- Woody vegetation and	370.73	71.9
- Herbaceous vegetation
Agricultural land	65.96	12.8
Residential area	25.68	5.0
Others	52.96	10.2
Total	515.33	100.0

• Main types of woody vegetation (7)
  Deciduous broadleaf forest (Quercus mongolica var. grosseserrata, Q. serrata, Betula platyphylla var. japonica, B. ermanii); conifer forest (abies veitchii, A. mariesii, A, homolepis, Tsuga diversifolia, Larix Kaempferi, Pinus densiflora); conifer plantation (Larix kaempferi, Pinus densiflora).
• Main types of herbaceous vegetation (7)
  Grasslands, weeds and alipine plant communities.
• Main kinds of crops: Rice, vegetables, beans and fodder crops (Q).

G2 INDUSTRIES IN THE CATCHMENT AREA AND THE LAKE (Q)

1978-1982
	Gross product Per year [mill. yen]	No. of persons engaged	No. of establishments
Primary industry
- Crop production	18,200	11,200	N.A.
- Fisheries	460	N.A.	N.A.
Secondary industry	631,900	51,400	2,935
Tertiary industry	438,900	8,500	4,956

• Numbers of domestic animals in the catchment area
  Cattle 4,447, swine 13,252, poultry 695.

G3 POPULATION IN THE CATCHMENT AREA (8)

1985
	Population	Population density [km-2]	Major cities (population)
Rural	32,227	-
Total	180,280	339.4

H. LAKE UTILIZATION

H1 LAKE UTILIZATION (Q)

Source of water, sightseeing and tourism (no. of visitors in 1985: 5,240,000), fisheries, recreation (sport-fishing, yachting).

H2 THE LAKE AS WATER RESOURCE (Q)

1985
	Use rate [m3 sec-1]
Industrial	0.002
Others
- Fish culture	0.23

I. DETERIORATION OF LAKE ENVIRONMENTS AND HAZARDS

I2 TOXIC CONTAMINATION (Q)

• Present status: None.

I3 EUTROPHICATION (Q)

• Nuisance caused by eutrophication
  Unusual bloom of Microcystis (Jul.-Sep.). Harms on fisheries due to the depletion of dissolved oxygen.
• Nitrogen and phosphorus loadings to the lake [t yr-1]

1984
Sources	Industrial	Domestic	Agricultural	Natural	Total
T-N	181.8	200.6	208.8	240.5	831.7
T-P	52.3	25.7	21.1	11.9	111.0

I4 ACIDIFICATION (Q)

• Extent of damage: None.

J. WASTEWATER TREATMENTS (Q)

J1 GENERATION OF POLLUTANTS IN THE CATCHMENT AREA

f) Significant development of the catchment area with some provision for municipal and industrial wastewater treatments, resulting in measurable discharge of pollution load into the lake.

J3 SANITARY FACILITIES AND SEWERAGE

• Municipal wastewater treatment systems
  No. of secondary treatment system: 1. No. of other types (anaerobic digestion): 3.

K. IMPROVEMENT WORKS IN THE LAKE (Q)

K1 RESTORATION

Suction dredging of bottom mud. Amounts dredged: 1,510,000 m3 (1969-1980) and 500,000 m3 (1981-1985).

M. LEGISLATIVE AND INSTITUTIONAL MEASURES FOR UPGRADING LAKE ENVIRONMENTS (Q)

M1 NATIONAL AND LOCAL LAWS CONCERNED

• Names of the laws (the year of legislation)


1. For national laws see "Biwa-ko"
2. Prefectural Government Code for Prevention of Public Nuisances
• Responsible authorities


1. Nagano Prefectural Government

M3 RESEARCH INSTITUTES ENGAGED IN THE LAKE ENVIRONMENT STUDIES

1. Suwa Hydrobiological Station, Faculty of Science, Shinshu University
2. Nagano Prefecture Research Institute for Hygienic and Environmental Studies

N. SOURCES OF DATA

1. Japan Map Centre (ed.)(1982) Collection of Maps of Japanese Lakes (Nihon no Kosho-zushu)(Technical Data of National Geographical Institute, D 1 No. 221). National Geographical Institute, Tsukuba.*
2. Meteorological Agency of Japan (1982) Meteorological Tables of Japan; Monthly Means by Locations for 1951-1980 (Nihon Kikohyo, Sono 2; Chiten betsu, Tsuki-betsu Heinenchi, 1951-1980). 302 pp. Japan Meteorological Association, Tokyo.*
3. Uchida, E., Asada, T. & Kawamura, T. (ed.) Handbook of Japanese Meteorology, Vol. II (Nihon Kisho-soran, Gekan). 1,060 pp. Toyo keizaishinpo-sha, Tokyo.*
4. Kurasawa, H. & Okino, T. (1983) Lake Suwa. Natural History and Limnology of Suwa District (Suwa no Shizen-shi, Rikusui-hen), pp. 113-176. Suwa Education Society (Inland Water Committee), Suwa.*
5. Nakamoto, N. & Routine Observation Group (1983) Evaluation of available nutrients in the water of Lake Suwa by use of the MBOD bioassay (Suwa-ko ni okeru MBOD-ho ni yoru seibutsu-riyo-kano eiyo-busshitsu-ryo no hyoka no kokoromi). Interim Report of the Lake Suwa Catchment Area Ecosystem Studies (Suwa-ko Shusui-iki Seitaikei Kenkyu Keika-hokoku): Kankyo-kagaku Kenkyu-hokokushu B168-R12-8, 9: 15-24.*
6. Takeuchi, K., Masaki, F., Hiraide, T. & Okino, T. (1983) Materials of the regular observation in Lake Suwa (5). Seston, chlorophyll a, primary production and community respiration (Suwako teiten-kansoku siryo (5). Sesuton, kurorofiru-ryo, kiso-seisanryo oyobi biseibutsu-gunshu no so kokyu). Ibid., 9: 69-78.*
7. Nagano Prefecture Vegetation Map Study Group (1973) Vegetation Map and Phytosociological Study of Nagano Prefecture, Vol. 1 (Nagano-ken no Shokusei-zu, Shokubutsushakaigaku-teki Kenkyu, 1). Nagano Prefectural Government, Nagano.*
8. Nagano Prefectural Government (1986) Results of Water Quality Monitoring in the Public Water Bodies for 1983-1985 (Kokyo-suiiki Suishitsu Sokutei Kekka, 1983-1985).*

* Printed in Japanese. The titles are tentatively translated into English with original titles in romanized Japanese in parentheses.