LAKE ROTORUA

An aerial view of the whole lake with its central island

Photo.
Photo: G.R.Fish


A. LOCATION

  • South Auckland, New Zealand.

  • 38:05S, 176:15E; 280 m above sea level.


B. DESCRIPTION

    The lake basin originated in a caldera about 100,000 years ago. Further volcanic activity changed the shape and drainage from the basin. The present lake size and outflow were established about 13,000 years ago but geothermal activity is still evident near the lake which receives hot water inflows from several springs and geysers near its southern shore, some of which carry white colloidal silica.
    The lake has become highly eutrophic in recent years. The source of about half its N and P income is from the inflow of effluent from sewage derived from Rotorua City and satellite villages situated along its shores. In addition, its modest native flora of aquatic plants has been progressively displaced by successive invasions of exotic species; Elodea canadensis in the 1950's, Lagarosiphon major in the 1960's and Ceratophyllum demersum in the 1970's. These three constitute the extensive stands of lakeweed that now infest the shallow, inshore region of the lake.
    The authorities have to solve both these problems in order to maintain the important recreational and tourist values of Lake Rotorua.
    Some reduction in the lake nutrient income has been effected by the Upper Kaituna Catchment Control Scheme under which, from 1980, the inflows of the lake were surveyed and measures taken to stabilize the river banks for erosion control and the leaching of nutrients from pastures. Plans are also well advanced for major modification of the city sewage treatment and effluent disposal methods which will prevent this large source of nutrients from entering the lake. This project should become operational by 1990.
    There is no feasible solution to the problem of lakeweed infestation but the Department of Conservation in Rotorua maintains a carefully monitored programme of herbicide spraying which keeps navigational hazards to a minimum and reduces excessively heavy stranding of plant material along the lake shores after heavy storms (Q, 1).

C. PHYSICAL DIMENSIONS (Q, 2)

    Surface area [km2] 80
    Volume [km3] 0.87
    Maximum depth [m] 44.8
    Mean depth [m] 11.0
    Water level Regulated
    Normal range of annual water level fluctuation [m] 0.7
    Length of shoreline [km] 40.1
    Residence time [yr] 1.2
    Catchment area [km2] 402

D. PHYSIOGRAPHIC FEATURES

D1 GEOGRAPHICAL
  • Bathymetric map: Fig. OCE-03-01.

  • Names of main islands: Mokoia (1.82 km2).

  • Number of outflowing rivers and channels (name): 1 (Ohau Channel).

D2 CLIMATIC
  • Climatic data at Rotorua, 1951-1980 (3)

    Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann.
    Mean temp. [deg C] 17.6 17.8 16.3 13.5 10.4 8.1 7.3 8.5 10.2 12.1 14.0 16.0 12.7
    Precipitation [mm] 90 116 125 119 137 140 145 146 122 116 97 138 1,491
  • Number of hours of bright sunshine: 2,151 hr yr-1 (3).

  • Solar radiation: 15.3 MJ m-2 day-1 (3).

    Fig. OCE-03-01
    Bathymetric map (Q).

  • Water temperature [deg C](4) (1977)

    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 18.4 19.9 21.0 19.4 14.0 10.0 9.8 9.8 9.9 12.9 14.7 20.8
    8 18.4 19.9 20.9 19.3 14.0 10.0 9.8 9.8 9.8 12.9 14.7 18.0
    14 18.4 19.9 20.6 19.1 13.9 9.9 9.8 9.8 9.7 12.7 14.7 17.4
    17 18.4 19.7 20.4 18.9 13.9 9.8 9.8 9.8 9.6 12.6 13.8 17.2
  • Freezing period: None (Q).

  • Mixing type: Polymictic (Q).

  • Notes on water mixing and thermocline formation
    Lake Rotorua has a large surface area, extensive shallow waters and no high ground to deflect wind from any direction. As a result, its waters are wind mixed for most of the year. Calm weather in summer permits thermal stratification for periods of a few days, rarely extending for longer than 10 days.

E. LAKE WATER QUALITY

E1 TRANSPARENCY [m](4) (1977)
    Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    1.8 3.0 2.0 2.9 3.2 2.2 1.8 2.4 2.1 2.4 2.6 2.1
E2 pH (4) (1977)
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 6.5 6.7 6.9 6.8 6.8 6.7 6.3 6.5 6.3 6.4 6.8 6.8
    8 6.5 6.8 6.9 6.6 6.8 6.7 6.4 6.4 6.4 6.4 6.9 6.6
    14 6.6 6.8 6.7 6.5 6.7 6.7 6.4 6.4 6.4 6.4 6.8 6.5
    17 6.7 6.7 6.5 6.4 6.6 6.6 6.5 6.5 6.4 6.4 6.5 6.4
E4 DO [mg l-1](4) (1977)
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 8.9 8.5 9.1 9.3 9.8 11.5 10.7 11.2 11.5 10.3 10.2 9.0
    8 9.0 8.6 9.3 9.3 9.8 11.5 10.4 11.2 11.9 9.9 10.1 8.3
    14 9.0 8.4 9.3 6.8 9.8 11.4 10.1 11.2 11.3 9.8 10.0 7.5
    17 9.0 8.1 7.2 6.1 9.3 11.3 9.9 11.1 11.2 9.7 8.1 6.6
E6 CHLOROPHYLL CONCENTRATION [micro l-1](4) (1977)
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 23.3 12.0 21.5 20.6 16.0 24.2 22.7 25.7 17.1 5.7 9.4 6.0
    8 23.3 13.3 20.9 20.7 16.0 23.5 25.8 17.8 20.0 8.9 12.1 8.2
    17 26.0 11.3 19.5 19.0 18.5 19.4 32.8 22.8 23.8 12.6 27.7 35.3
E7 NITROGEN CONCENTRATION (4)
  • Total-N [micro l-1] (1977)

    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 310 260 370 300 300 305 320 330 305 268 270 260
    8 300 240 360 280 280 365 320 290 300 248 285 275
    17 305 260 460 445 400 320 300 330 350 264 320 335
E8 PHOSPHORUS CONCENTRATION (4)
  • Total-P [micro l-1] (1977)

    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 23.2 23.6 38.0 22.0 30.6 22.4 27.2 23.8 20.4 26.4 24.4 16.8
    8 24.6 23.6 30.6 21.0 24.0 24.0 25.0 22.8 24.0 29.2 22.4 19.4
    17 23.2 27.0 56.0 34.0 29.2 24.0 25.6 30.2 26.4 31.2 28.4 45.2
E10 PAST TRENDS

    Fig. OCE-03-02
    Past trend of dissolved oxygen [mg l-1](5).

    Fig. OCE-03-03
    Past trend of Total-N [micro l-1](6).

    Fig. OCE-03-04
    Past trend of Total-P [micro l-1](6).


F. BIOLOGICAL FEATURES

F1 FLORA
  • Emerged macrophytes: Negligible.

  • Submerged macrophytes (Q)
    Elodea canadensis, Lagarosiphon major, Egeria densa, Ceratophyllum denersum.
  • Phytoplankton (5)
    Melosira granulata, M. distans, Mougeotia spp., Staurastrum spp., Asterionella spp., Anabaena circinalis, A. flos-aquae, Microcystis flos-aquae.
F2 FAUNA (1)
  • Zooplankton
    Macrocyclops albidus, Bosmina meridionalis, Calamoecia lucasi, Ceriodaphnia dubia.
  • Benthos
    Chironomus zealandicus, Hyridella menziesi, Paranephrops planifrons, Potamopyrgus corolla.
  • Fish
    Gobiomorphus cotidianus, Galaxias brevipinnis, Salmo gairdneri*, S. trutta, Carassius auratus, Gambusia affinis, Retropinna retropinna. * Economically important.
  • Supplementary notes on the biota
    Occasional blooms of Limnomedusae (Craspedacusta sowerbii).
F6 PAST TRENDS
  • Phytoplankton standing crop [g m-2](4)

    Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    1975 - - - - - 181.0 168.0 147.0 60.4 103.8 53.1 116.9
    1976 110.7 60.4 71.8 117.0 83.5 62.7 56.3 81.3 72.9 64.6 71.0 78.1
    1977 107.7 45.0 54.2 52.7 37.6 142.8 - - - - - -
  • Primary production [g C m-2 day-1](Q)

    Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    1974 1.99 2.72 3.87 2.29 1.67 1.36 1.82 2.54 1.31 1.86 1.90 1.82
    1975 1.57 4.21 3.54 2.41 1.41 1.07 1.28 0.99 1.44 1.85 3.06 3.07
    1976 2.57 4.24 2.94 3.54 2.08 1.58 1.52 1.31 1.63 1.83 2.09 4.27
    1977 5.48 3.64 3.25 2.28 1.38 1.76 1.70 2.38 2.76 2.57 2.31 1.61
    1978 1.79 1.81 2.87 2.93 1.57 1.24 - - - - - -
F7 NOTES ON THE REMARKABLE CHANGES OF BIOTA IN THE LAKE IN RECENT YEARS
    Crayfish, common up to the 1960's, has become rare or absent - a change correlated with the advent of very heavy inshore weed growth and severe eutrophication of the lake water. Successive invasions of exotic water plants have attained heavy biomass and excluded all inshore native species. Major growth of the exotic macrophytes occurred for Elodea canadensis over 1955-65, for Lagarosiphon major over 1965-75, for Egeria densa and Ceratophyllum demersum from 1975 to the present time.

G. SOCIO-ECONOMIC CONDITIONS

G1 LAND USE IN THE CATCHMENT AREA
    Area [km2] [%]
    Natural landscape
    - Woody vegetation 134.5 27.9
    - Herbaceous vegetation 21.2 4.4
    - Swamp 1.0 0.2
    Agricultural land
    - Pasture land 212.1 43.9
    Residential area 32.3 6.7
    Lake 81.5 16.9
    Total 482.6 100
  • Main types of woody vegetation (main species): Pinus radiata (plantation),
    Beilschmiedia tawa.
  • Main types of herbaceous vegetation (main species): Cyathea dealbata,
    Leptospermum scoparium.
  • Types of the other important vegetation (main species): Pasture (Lolium
    perenne, Dactylis glomerata).
  • Main kinds of crops: Pasture, timber (mostly Pinus radiata).

  • Levels of fertilizer application on crop fields: Light.

G2 INDUSTRIES IN THE CATCHMENT AREA AND THE LAKE (1988)
    Gross product No. of
    Gross product per year (US$1.00) No. of persons engaged No. of establishments Main products or mSecondary industry
    Primary industry
    - Forestry N.A. N.A. 2 Pine saw logs
    - Horticulture 6 N.A. N.A.
    Secondary industry N.A. 1,100 5 Sawmilling
  • Numbers of domestic animals in the catchment area
    Cattle 25,000, sheep 1,250,000.
G3 POPULATION IN THE CATCHMENT AREA (1988)
    Population Population density [km-2] Major cities (population)
    Urban 54,000 1,700 Rotorua
    Rural 12,000 31
    Total 66,000 164.1

H. LAKE UTILIZATION

H1 LAKE UTILIZATION
    Sightseeing and tourism (no. of visitors in 1987: 919,000) and recreation (swimming, sport-fishing, yachting)

I. DETERIORATION OF LAKE ENVIRONMENTS AND HAZARDS

I1 ENHANCED SILTATION (Q)
  • Extent of damage: Not serious.

  • Supplementary notes
    Changes in lake water level caused by extremes of annual rainfall have caused concern to lakeshore residents in past years. The installation of lake outlet gates in 1980 has now greatly reduced adverse water level changes.
I2 TOXIC CONTAMINATION (6)
  • Present status: Detected but not serious.

  • Main contaminants, their concentrations and sources [micro g-1(wet wt.)] (1975)

    Name of contaminant Fish* Main sources
    Methyl-Hg 0.06-2.80 Natural geothermal geysers
    * Salmo gairdneri tissues included muscle liver.
  • Food safety standards or tolerance limits for toxic contaminant residue
    Acceptable daily intake is set at 30 micro g-1 Hg for adults. Mean methyl Hg in trout fillets of 0.85 micro (wet wt.) g-1 provides an acceptable daily intake of only 35 g.
I3 EUTROPHICATION
  • Nuisance caused by eutrophication (Q)
    Unusual algal bloom: Anabaena spiroides.
  • Nitrogen and phosphorus loadings to the lake [ton yr-1](7) (1984)

    Sources Industrial Domestic Agricultural & Natural Total
    T-N 0 150 685 835
    T-P 0 33.8 69 102.8
  • Supplementary notes (Q, 7)
    New sewage treatment is presently planned that will remove the sewage input from L. Rotorua to be in operation by 1990.

J. WASTEWATER TREATMENTS (Q)

J1 GENERATION OF POLLUTANTS IN THE CATCHMENT AREA
    (d) Measurable pollution with limited wastewater treatment.
J2 APPROXIMATE PERCENTAGE DISTRIBUTION OF POLLUTANT LOADS
    [%]
    Non-point sources 66
    Point sources
    - Municipal 33
    - Industrial 0
    Total 99
J3 SANITARY FACILITIES AND SEWERAGE
  • Percentage of municipal population in the catchment area provided with
    adequate sanitary facilities (on-site treatment systems) or public sewerage: 100%.
  • Percentage of rural population with adequate sanitary facilities (on-site
    treatment systems): 100%.
  • Municipal wastewater treatment systems
    No. of tertiary treatment systems: 1 (activated sludge with simultaneous precipitation of P). No. of secondary treatment systems: 1 (activated sludge).
  • Industrial wastewater treatment systems
    No. of industrial wastewater treatment systems: 1 (oxidation pond).

L. DEVELOPMENT PLANS (Q)

    Catchment work to reduce flooding and to protect stream banks from erosion by the Bay of Plenty Catchment Commission is nearly completed. Redesign of Rotorua City Sewage Treatment and land treatment to remove all pollutants will be completed by City Council. Works are in progress and operation is expected by 1990. Total cost is 51,000,000 NZ$ of which 21,000,000 NZ$ will be subsidized by central government.

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. Water and Soil Conservation Act (1967)
    2. Section 55 of Town and Country Planning Act (1977)
    3. Section 2(b) of Environmental Protection and Enhancement Procedures (1981)
  • Responsible authorities

    1. District Council (incorporating City and County Councils)
    2. Ministry of Works and Development
  • Main items of control

    1. All aspects of natural water use and protection
M2 INSTITUTIONAL MEASURES (the year of establishment)
  1. Dept. of Conservation (1986) Regional Office in Rotorua
  2. Bay of Plenty Catchment Commission and Regional Water Board (1950) in Whakatane
M3 RESEARCH INSTITUTES ENGAGED IN THE LAKE ENVIRONMENT STUDIES
  1. Local office of Ministry of Agriculture and Fisheries
  2. Freshwater Ecology Unit of DSIR Taupo Research Laboratory
  3. Water Quality Centre of Ministry of Works and Development in Hamilton

N. SOURCES OF DATA

  1. Questionnaire filled by Dr. G. R. Fish, Chairman of Guardians of the Rotorua Lakes, Rotorua, New Zealand.
  2. Jolly, V. H. & Brown, J. M. A. (1975) New Zealand Lakes. 388 pp. Auckland University Press/Oxford University Press.
  3. White, E. (1977) Eutrophication of Lake Rotorua N. Z. D. S. I. S. R. Information Series 123, 16 pp.
  4. N. Z. Meteorology Service (Rotorua Branch) Personal communication.
  5. Fish, G. R. Limnological survey of Lakes Rotorua and Rotoiti, April 1975 March 1978. Unpublished data collection.
  6. Fish, G. R. (1975) N. Z. Fisheries Research Bulletin, No. 8, p. 68.
  7. Brooks, R. R., Lewis, J. R. & Reeves, R. D. (1976) N. Z. J. Mar. Freshwat. Res., 10: 233-244.
  8. Rutherford, J. C. & Pridmore, R. D. (1987) The significance of phosphorus and nitrogen in the management of Lake Rotorua. N. Z. Water Supply and Disposal Association Annual Conference, Rotorua, September 1987.