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Appendix 4: Lake Rotorua nutrient inputs and water quality - loads and targets 1965-2002

 

1965

1976-77

1981-82

1984-85

2002

Target

Reduction from 2002 to meet target

Population

25,000

50,000

52,600

54,000

  

-

  

Phosphorus input

             

Raw sewage t/y

5

18

30

47

  

-

  

Treated sewage t/y

5

7.8

20.6

33.8

1

3

  

Stream t/ya

34

34

34

34

34

34

  

Internal t/y

ND

0

20

35

  

0b

  

Total t/y

39

42

75

103

35

37

nil

Nitrogen input

             

Raw sewage t/y

34

100

170

260

  

-

  

Treated sewage t/y

20

73

134

150

32

30

-2

Stream t/ya

455

485

420

415

660

405

-255

Septic tanks t/y

50

80

15

10

  

0

  

Internal t/y

ND

0

140

>260

  

0a

  

Total t/y

475

558

694

>825

692

435

-257

Average lake quality

             

Total phosphorus mg/m3

  

23.8

47.9

72.6

  

20

  

Total nitrogen mg/m3

  

310

519

530

  

300

  

Chlorophyll mg/m3

  

5.5

37.8

22.6

  

10

  

Peak chlorophyll mg/m3

  

28

62

58

  

17-24

  

Secchi disc m

2.5-3

2.3

1.9

1.7

  

2.5-3

  

Deoxygenation g/m3/d

  

0.4

0.7

0.9

  

0.25

  

a Flood flow particulate P and N are excluded because they probably settle on the lakebed and are not 'available' to phytoplankton.

b Internal loads may be non-zero even when external loads are reduced.

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