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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
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