Earth Sciences Seminar 1 May 2003

David HamiltonBiological Sciences

University of Waikato

Water quality trends in the Rotorua lakes

- Eloise Ryan (UoW)- David Burger (UoW)- Warwick Silvester (UoW)

- Environment Bay of Plenty

- Eastern Fish & Game

- Andrew Lang

Acknowledgements

Rotorua lakes location map

- Anabaena planktonica in Lake Rotoiti

- Nitrogen fixation by Anabaena planktonica in Lake Rotoiti

- Lake nutrient budget for Lake Rotoiti: internal vs external processes

- Hypolimnetic dissolved oxygen: a flag for the declining state of the Rotorua lakes

Talk outline

Photo: Rotorua Post

Photo: Rotorua Post

Photo: D. Burger

Photo: D. Burger

Photo: NZ Herald

Algal blooms: an outcome of nutrient enrichment

Lots of buoyantor motile cells

Calm conditions/light winds

‘Telescoping’

Chla fluorescence (mg/m3) or Temperature (oC)

Dep

th (

m)

Chlorophyll a and temperature in Lake Rotoiti

2/2/03

FT

10

20

30

40

50

60

70

Lake Tikitapu02468

101214161820222426

0 2 4 6 8 10 12 14 16Fluorescence

Dep

th (

m)

Lake Okareka

02468101214161820

3 4 5 6 7 8 9 10 11Fluorescence

Dep

th (

m)

Eloise Ryan’s Ph.D. research on deep vs surface

chlorophyll maxima

Anabaena planktonica

Photo: B. O’Brien

CO2

NH4

NO3PO4

N2

Nitrogen fixation

No oxygen

azoferredoxin

molybdoferredoxin

N2 + 3H2 2NH3 + H2

Acetylene reduction: CH=CH H2C= H2C

Dep

th (

m)

Chla fluorescence (mg/m3) or Temperature (oC)

Nitrogen fixation in Lake Rotoiti

Chlorophyll a and temperature 2/2/03

0 1 2 3 4 5

1

2

3

4

5

6

Dep

th (m

)

N-fixation rate (nmol/L/hr)

Main lake 2/13/03

Okawa Bay 2/13/03

Main lake 2/2/03

Okawa Bay 2/2/03

Dep

th (

m)

N-fixation (nmol/L/hr)

FT

2/2/03

N-fixation Temperature/fluorescence0

1

2

3

4

5

0

1

2

3

4

5

Rate of N fixation (whole lake):

= 1 nmol C2H4 /L/hr * 12 hr/day * 28 ng N 3 nmol C2H4

rate in bottle unit and light conversion stoichiometric conversion

* 1.77 x 1011L * 10-12 kg/ng

lake volume (top 5m) unit conversion

~ 60 kg N/day

N-fixation: whole-lake calculations for Lake Rotoiti

Amount of chlorophyll a

= 10 ug/L * 1.77 x 1011L * 10-9 kg/ug

concentration lake volume (top 5m) units conversion

= 1770 kg Chla

Equivalent amount of nitrogen in phytoplankton= 1770 kg Chla * 50 g C/g Chla * 1g N/7g C concentration carbon to chla nitrogen to carbon= 12642 kg N

= 12.6 tonnes

N-fixation: whole-lake calculations for Lake Rotoiti

N supplied by fixation per day as fraction of total N pool= 60 kg N/day 12642 kg N

= 0.4 % per day

Load over three months= 120 days * 60 kg N/day= 7200 kg N

= 7.2 tonnes

Annual load of N = 56 tonnes …Bioresearches 1991

Possible contribution of 13% of total N load

197 228 258 289 319 350 380 411 441 472 502 533

-1

3

7

11

14

18

22

Dep

th (m

)

10 12 14 16 18 20

J A S O N D J F M A M J

Current state of Lake Rotoiti - Station 2

195 226 256 287 317 348 378 409 439 470 500 531

-1

3

7

11

14

18

22

Dep

th (m

)

0 2 4 6 8 10

J A S O N D J F M A M J

20

30

40

50

60

Dissolved oxygen (mg/L)

0

10

0

10

20

30

40

50

60

Temperature (oC)

* Data from EBOP monitoring program

91 92 93 94 95 96 97 98 99 100 101 102

20

40

60

Years

Dep

th (

m)

Nutrient sampling in Lake Rotoiti

00 01 02

92 93 94

0.0

5.0

10.0

15.0

Dis

solv

ed o

xyge

n (m

g/L

)

92 93 94

0

50

100

150

Nit

rate

(m

g/L

)

Lake Rotoiti, DO and nutrients, 40-m depth, ‘91-’94

Dis

solv

ed o

xyge

n (m

g/L

)N

itra

te (

ug/L

)

92 93 94

0

50

100

150

Am

mon

ium

(ug

/L)

Am

mon

ium

(ug

/L)

Transformations of N:Nitrification and denitrification

Organic N NH4+ NO2

- NO3-

N2O, N2

Mineralisation Nitrification

Denitrification

Environmentally important redox reactions

Reaction Eh (V) DG

Reduction of O2

O2 + 4H + +4e- --> 2H2O +0.812 -29.9

Reduction of NO3-

2NO3- + 6H+ + 6e- --> N2 + 3H2O +0.747 -28.4

Reduction of Mn4+

MnO2 + 4H+ + 2e- --> Mn2+ +2H2O +0.526 -23.3

Reduction of Fe3+

Fe(OH)3 + 3H+ + e- --> Fe2+ +3H2O -0.047 -10.1

Reduction of SO42-

SO42- + 10H+ + 8e- --> H2S + 4H2O -0.221 -5.9

Reduction of CO2

CO2 + 8H+ + 8e- --> CH4 + 2H2O -0.244 -5.6

DE

CR

EA

SIN

G E

NE

RG

Y Y

IEL

D

92 93 94

0

50

100

150

Nit

rate

(m

g/L

)N

itra

te (

ug/L

)

80

60

40

20

Total phosphorus (m

g/L)

Redox sequence - Lake Rotoiti

Volume below 30m depth = =280,360,000 m3

Concentration of NO3 denitrified = 0.15 mg/L = 0.15 g m-3

Amount denitrified = 280,360,000*0.15/1000 kg

= 42000 kg = 42 tonnes

Annual TN load = 56 tonnes

How much denitrification in Lake Rotoiti?

Fate of incoming nitrogen: 75% is denitrified

Volume below 30m depth = 280,360,000 m3

Rapid increase in PO4 = 0.07 mg/L = 0.07 g m-3

Sediment P-release = 280,360,000*0.07/1000 kg

= 19600 kg = 20 tonnes

Annual TP load = 4.5 tonnes

How much sediment P-release in Lake Rotoiti?

Sediment P-release overwhelms incoming P by > 400%

92 93 94

0

2

4

6

8

10

12

Year

Dis

solv

ed o

xyge

n at

38m

(m

g/L

)

92 93 94

0

20

40

60

80

Year

Tota

l pho

spho

rus

at 3

8m (

mg/

m3)

Total phosphorus and oxygen in Lake Rotoiti at 40m depth

y = -10.725Ln(x) + 50.572

R2 = 0.7212

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8 10 12 14

Relationship between dissolved oxygen and total phosphorus in Lake Rotoiti at 40m depth

Tot

al p

hosp

horu

s (m

g/m

3 )

Dissolved oxygen (mg/L)

102.0 102.5

0

40

80

120

Year

Tota

l pho

spho

rus

at 5

0m (

mg/

m3)

Tot

al p

hosp

horu

s (m

g m

-3)

S O N D J F M A M J J

2001 2002

Comparison of total phosphorus (surface and 50m)and dissolved oxygen (50m), Lake Rotoiti, 2001-02

102 102 102

1

2

3

4

5

Time

Dep

th (

m)

0 5 10 15

Dissolved oxygen (mg/L)

D J F M

102 102 102

1

2

3

4

5

Time

Dep

th (

m)

20 22 24

Temperature (C)

Temperature, dissolved oxygen and nutrientson Okawa Bay, 2002

D J F M

TP = 175 mg/m3

NH4 = 500 mg/m3

The Lake Rotoiti bubbles!Methanogenesis?

Photo: N. Miller

Progressive loss of oxygen from depth in Lake Rotoiti

0 5 10 15 20

0

20

40

60

Distance (km)

Dep

th (

m)

0 2 4 6 8

Dissolved oxygen (mg/L)

0 5 10 15 20

0

20

40

60

Dep

th (

m)

0 5 10 15 20

0

20

40

60

Dep

th (

m)

EBOP2001

Fish1968

Jolly1955

* data in March each year

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1955-56

1970-71

2001-02

% S

atur

atio

n

M A M J J A S O N D J F M A M

Dissolved oxygen comparison, Lake Tikitapu

1955-56 Jolly (1968)1970-71 McColl (1972)2001-02 Environment Bay of Plenty (unpubl.)

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1955-56

1970-71

2001-02

% S

atur

atio

n

Dissolved oxygen comparison, Lake Okataina

1955-56 Jolly (1968) Depth 50-55m1970-71 McColl (1971) Depth 79m2001-02 Environment Bay of Plenty (unpubl.) Depth 62m

J J A S O N D J F M A M J

100

90

80

70

60

50

40

30

20

10

0

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1955-56

1970-71

2001-02

% S

atur

atio

n

Dissolved oxygen comparison, Lake Okareka

1955-56 data from Jolly (1956)1970-71 data from McColl (1972)2001-02 data from Environment Bay of Plenty (unpubl.)

A M J J A S O N D J F M A M J

Ammonia S3

0

0.1

0.2

0.3

0.4

0.5

mg/

l

0-8m12m15.5m19m

Oxygen S3

0

2

4

6

8

10

12

mg/

l 2.2m6.2m12.2m15.4m19m

Phosphate S3

0

0.02

0.04

0.06

0.08

0.1

350 5 25 45 65 85Julian Day

mg/

l

0-8m12m15.5m19m

David Burger’s Ph.D. research:Nutrient pulses during stratification events in Lake Rotorua

A ‘stable system’

Nutrients in

Algae

Mixing

High O2

Moderate O2

Sedimentation

An ‘enriched’ system

Nutrients in

Algal blooms

Mixing

High O2

Little or no O2

Nutrient release

Sedimentation

N fixation

19702003

1992

Stable

1965

State of the Rotorua lakes