SUWA-KO (LAKE SUWA)

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

Photo.
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).*

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