LAKE MJOSA

A view from the eastern shore near Stange Photo. Photo: A. Kurata  
 
 

A. LOCATION

  • Hedmark, Oppland and Akershus, Norway.
  • 60:45N, 11:15E; 121 m above sea level.
 

B. DESCRIPTION

    Lake Mjosa, the largest lake of Norway, is a typical Norwegian fjord lake. It has a surface area of 365 km2, maximum depth of 449 m, mean depth of 153 m and maximum length of 117 km. Large parts of the catchment area consist of mountainous regions with gabbroic and granitic bedrock. Around the central zone of the lake there are sedimentary and metamorphic rocks of cambro- silurian age and some calcareous rock types. This district is one of Norway's best agricultural areas. The lake lies well below the highest post-glacial marine limit which in this area is just 200 m above the present ocean level. In the mountain regions there are many glaciers, and the main tributary, River Gudbrandsdalslagen, carries a heavy silt load from the glacier area during the summer. Approximately 75% of the water mass in Mjosa flow through the River Gudbrandsdalslagen (mean annual flow 250 m3 sec-1) which drains a region dominated by high mountains and glaciers. This tributary has been regulated for hydro-electric power generation and the mean winter water flow of the river has thus increased from 64 to 104 m3 sec-1 (65%), while the mean summer flow has decreased from 509 to 451 m3 sec-1 (11%).
    Melting of snow and ice in the mountains causes a high flow during summer which is especially heavy during periods of warm and rainy weather. Approximately 60% of the annual water flow takes place between June and August. Lake Mjosa is used as a reservoir for hydro-electric power generation. The regulation height is 3.61 m which is equivalent to a reservoir approximately 2.3% of the lake volume. Water is drawn from the lake during winter (November-April) and the lake is refilled during the spring flood in May/June. The vernal circulation period normally lasts from the latter half of April to the end of June, while the autumnal circulation down to approximately 200 m lasts from October to January/February. Cooling of the deep water evidently continues until the vernal circulation period begins. The deep water temperature always ranges between 3.5 and 3.7deg C. The central zones of the lake are often ice-free during the winter.
    During the period from the beginning of the 1950's to the midst of the 1970's the lake has developed strongly in an eutrophic direction. However, satisfactory solutions are to be found for waste treatment in rural districts by the "Save Mjosa Campaign" including the rapid development of purification techniques and building of sewage pipelines (1).
 

C. PHYSICAL DIMENSIONS

     
    Surface area [km2] 365
    Volume [km3] 56.2
    Maximum depth [m] 449
    Mean depth [m] 153
    Water level Regulated
    Normal range of annual water level fluctuation [m]* 4
    Residence time [yr] 6
    Catchment area [km2] 16,420
    *

    Fig. EUR-32-03.
     
 

D. PHYSIOGRAPHIC FEATURES

D1 GEOGRAPHICAL
  • Bathymetric map:

    Fig. EUR-32-01.
  • Names of main islands: Helgoya (ca. 25 km2)(2).
  • Number of outflowing rivers and channels (name): 1 (R. Vorma).
D2 CLIMATIC
  • Climatic data at Oslo, 1866-1966 (3)
     
    Mean temp. [deg C]
    Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann.
    -4.7 -4.0 -0.5 4.8 10.7 14.7 17.3 15.9 11.3 5.9 1.1 -2.0 5.9
    Precipitation [mm]
    49 35 26 44 44 71 84 96 83 76 69 63 740
     
  • Number of hours of bright sunshine (1956-1965): 1,632 hr yr-1.
  • Solar radiation: 8.7 MJ m-2 day-1.
 
  • Water temperature [deg C](4)

    • Lake centre, May 1977 Range: 3.30-3.70. Mean: 3.38.

    Fig. EUR-32-02
    Water temperature at the central zone.
 
  • Freezing period: December-March.

    • The central zones of the lake are often ice-free during the winter (1).
  • Mixing type: Dimictic (1).

    Fig. EUR-32-03
    Variation in surface water level and water flow in tributaries and outlet, 1977 (4).
 

E. LAKE WATER QUALITY

E1 TRANSPARENCY:

Fig. EUR-32-04.

    Fig. EUR-32-04

    Transparency [m] at 4 stations, 1976 (5).

 

    Fig. EUR-32-05

    Relationship between water flow in the River Gudbrandsdalslagen and transparency at the central zone of the lake (4).

E2 pH (4)

    Central zone, May 1977: 6.9 (6.8-7.0).
E4 DO [mg l-1](4)
    Central zone, May 1977: 11.2 (10.3-11.6).
    Saturation degree [%]: 86.9 (80.3-90.1).

    Fig. EUR-32-06

    Vertical distribution of DO at the central zone of the lake, 1966, 1973, 1975 and 1976 (5).

E6 CHLOROPHYLL CONCENTRATION [micro g l-1]:

Fig. EUR-32-07.

    Fig. EUR-32-07

    Chlorophyll concentration of integrated samples from 0-10 m at 4 stations, 1976 (1).

E7 NITROGEN CONCENTRATION (4)

  • NO3-N [mg l-1]

    • Central zone, May 1977: 0.355 (0.340-0.390).
  • Total-N [mg l-1]

    • Central zone, May 1977: 0.455 (0.420-0.540).
E8 PHOSPHORUS CONCENTRATION (4)
  • PO4-P [micro g l-1]

    • Central zone, May 1977: 2.8 (2.0-5.0).
  • Total-P [micro g l-1]

    • Central zone, May 1977: 5.6 (4.0-8.0).

    Fig. EUR-32-08

    Seasonal variations of T-P and PO4-P in the surface layers (0-8 m) at the central zone of the lake, 1974 (6).

E10 PAST TRENDS:

Fig. EUR-32-09.

    Fig. EUR-32-09

    Increasing of T-N and NO3-N concentration at the central zone of the lake (4).
 
  • chlorophyll a concentration [micro g l-1]*
     
    1976 1977 1978 1979 1980
    Mean 4.8 4.3 4.6 3.5 3.6
    Maximum 12.5 6.7 6.8 8.5 4.1
    * In composite samples 0-10 m from the central zone of the lake for the period 1976-1980.
     
 

F. BIOLOGICAL FEATURES

F1 FLORA (4)
  • Phytoplankton

    • Asterionella formosa, Fragilaria crotonensis, Diatoma elongatum, Tabellaria fenestrata, Rhizosolenia eriensis, Melosira italica, Oscillatoria bornetii f. tenuis.
F2 FAUNA (4)
  • Zooplankton

    • Calanoid (Limnocalanus macrurus, Eudiaptomus gracilis), cyclopoid (Cyclops lacustris, Mesocyclops oitbonoides), cladoceran (Daphnia galeata, D. cristata, Bosmina longispina).
  • Benthos

    • Oligochaeta (Stylodrilus beringianus, Peloscolex ferox, Tubifex tubifex), chironomid (Heterotrissocladius subpilosus, Paracladophelma obscura, Micropsectra spp.)
  • Fish

    • Vendace, pike, perch, roach, bream, brown trout.
  • Supplementary notes on the biota

    • The total fish production has increased, particularly with respect to such species as vendace, pike, perch, roach and bream, while the more noble salmon species such as whitefish and brown trout have decreased.
F3 PRIMARY PRODUCTION (6)
    Primary production of phytoplankton at the central zone of the lake for the period 1973-1980
    1973 1974 1975 1976 1977 1978 1979 1980
    Daily mean values [mg C m-2] 270 320 500 570 370 140 110 110
    Daily maximum values [mg C m-2] 643 651 1,355 1,425 750 362 279 218
    Yearly values [g C m-2] 50 55 100 100 69 25 20 20
     
F4 BIOMASS:

Fig. EUR-32-10 (6).

    Fig. EUR-32-10

    Phytoplankton biomass (a) and composition in percentages (b) in composite samples (0-10 m) from the central zone of the lake, 1980

F5 FISHERY PRODUCTS (4)

  • Annual fish catch [metric tons]

    • 1980: 219. F6 PAST TRENDS (6) Mean values for algal biomass and percentages of blue-green algae at the central zone of the lake during the production period May-October for the period 1976-1980.
     
    1976 1977 1978 1979 1980
    Mean values [g m-3] 1.67 1.42 1.05 1.01 0.82
    Blue-green algae [%] 37.9 4.7 17.0 0.6 2.7

    Fig. EUR-32-11

    Past trend of algal biomass and its composition (5).
 

G. SOCIO-ECONOMIC CONDITIONS (6)

G2 INDUSTRIES IN THE CATCHMENT AREA AND THE LAKE 1980
  • Main products of agriculture

    • Wheat, potato, vegetables and dairy products.
  • Main kind of manufacturing

    • Paper mill, cannery, brewery, metallurgical factory, mineral water factory.
  • Main kinds of manufacturing products

    • Wood pulp, flour, canned food, brewing products, metal, mineral water.
G3 POPULATION IN THE CATCHMENT AREA
     
    1980
    Population Population density [km-2] Major cities (population)
    Total 200,000 12.2 Lillehammer, Gjovik, Hamar
     
 

H. LAKE UTILIZATION

H1 LAKE UTILIZATION (1, 6)
    Source of water, sight-seeing and tourism, recreation (sport-fishing and yachting), fisheries and power generation.
 

I. DETERIORATION OF LAKE ENVIRONMENTS AND HAZARDS

I1 ENHANCED SILTATION (6)
  • Extent of damage: Not serious.
I3 EUTROPHICATION (6)
  • Nuisance caused by eutrophication

    • Algal bloom of Oscillatoria bornetii f. tenuis caused obnoxious and abnormal taste and odour of drinking water.
  • Nitrogen and phosphorus loadings to the lake [t yr-1](1)
     
    1976
    Sources Industrial Domestic Agricultural Forest Natural Total
    T-N 200 700 2,200 2,000 400 5,500
    T-P 95 105 45 90 58 393
     
  • Supplementary notes (6)

    • Past trend of total loading of phosphorus [t yr-1] to the lake.
     
    Loadings 1973/1975 1976 1977 1978 1979 1980
    Via tributaries 129 120 83 106 165 -
    From nearby areas 188 188 147 113 87 -
    Total 317 308 230 219 252 225

    Fig. EUR-32-12

    (6) Yearly loading of phosphorus to the lake via tributaries and non-point sources from nearby areas and points sources (points of discharge).

 

    Fig. EUR-32-13

    (4) Long-term changes of agricultural area, number of cattle and consumption of fertilizer in the catchment area.
 

    Fig. EUR-32-14

    (5) Long-term changes of population and municipal phosphorus load to the lake.

I4 ACIDIFICATION (4)

  • Extent of damage: Not serious.
 

J. WASTEWATER TREATMENTS

J1 GENERATION OF POLLUTANTS IN THE CATCHMENT AREA
    d) Measurable pollution with limited wastewater treatment (6).
J2 APPROXIMATE PERCENTAGE DISTRIBUTION OF NUTRIENT LOADS (1, 6)
     
    1976
    N [%] P [%]
    Natural 7.3 14.7
    Forest 36.4 22.9
    Agricultural 40.0 11.5
    Domestic 12.7 26.7
    Industrial 3.6 24.2
    Total 100 100
     
  • Percentage of municipal population in the catchment area provided with

    • adequate sanitary facilities (on-site treatment systems) and public sewerage: 90%.
  • Percentage of rural population with adequate sanitary facilities (on-site

    • treatment systems): 10%.
  • Municipal and industrial wastewater treatment systems

    • No. of municipal and industrial wastewater treatment systems: 42.
 

K. IMPROVEMENT WORKS IN THE LAKE

K1 RESTORATION (6)
    When in late summer 1976 the drinking water for about 200,000 people suffered from heavy bloom of blue-green algae, the Norwegian Parliament adopted a new plan of action called "Mjosa Campaign" with the objective of reducing the phosphorus load to 175 t by the end of the campaign (1979-1980). As a result of this campaign, 42 new sewage treatment plants for retention of phosphorus have been built and 280 km of new sewer pipe have been installed. The purification efficiency at the new sewage treatment plans is fully satisfactory as over 90% of phosphorus is eliminated. In the densely built-up areas phosphorus in the sewage discharge has been reduced from about 92 to 25 t per year.
 

L. DEVELOPMENT PLANS (6)

    None.
 

M. LEGISLATIVE AND INSTITUTIONAL MEASURES FOR UPGRADING LAKE ENVIRONMENTS

M3 RESEARCH INSTITUTES ENGAGED IN THE LAKE ENVIRONMENT STUDIES (1)
  1. The Norwegian Institute for Water Research (NIVA)
 

N. SOURCES OF DATA

  1. Holtan, H. (1979) The Lake Mjosa story. Arch. Hydrobiol. Beih. Ergebn. Limnol., 13: 242-258.
  2. Royal Norwegian Embassy. Personal communication.
  3. Johannessen, T. W. (1970) The Climate of Scandinavia. "World Survey of Climatology, Volume 5, Climates of Northern and Western Europe" (ed. Wallen, C. C.), pp. 23-79. Elsevier Scientific Publishing Company, Amsterdam-London-New York.
  4. Holtan, H. (1980) The case of Lake Mjosa. Prog. Wat. Tech., Vol. 12: 103-120. IAWPRC Pergamon Press, Great Britain.
  5. Holtan, H. (1978) Eutrophication of Lake Mjosa in relation to the pollutional load. Verh. Internat. Verein. Limnol., 20: 734-742.
  6. Holtan, H. (1981) Eutrophication of Lake Mjosa and its recovery. WHO Water Quality Bulletin, 6(4): 99-103, 156.