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Earth Sciences Seminar 1 May 2003 David Hamilton Biological Sciences University of Waikato Water quality trends in the Rotorua lakes.

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Presentation on theme: "Earth Sciences Seminar 1 May 2003 David Hamilton Biological Sciences University of Waikato Water quality trends in the Rotorua lakes."— Presentation transcript:

1 Earth Sciences Seminar 1 May 2003 David Hamilton Biological Sciences University of Waikato Water quality trends in the Rotorua lakes

2 - Eloise Ryan (UoW) - David Burger (UoW) - Warwick Silvester (UoW) - Environment Bay of Plenty - Eastern Fish & Game - Andrew Lang Acknowledgements

3 Rotorua lakes location map

4 - 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

5 Photo: Rotorua Post

6 Photo: D. Burger

7

8 Photo: NZ Herald

9 Algal blooms: an outcome of nutrient enrichment Lots of buoyant or motile cells Calm conditions/ light winds ‘Telescoping’

10 Chla fluorescence (mg/m 3 ) or Temperature ( o C) Depth (m) Chlorophyll a and temperature in Lake Rotoiti 2/2/03 F T 10 20 30 40 50 60 70

11 Lake Tikitapu 0 2 4 6 8 10 12 14 16 18 20 22 24 26 0 2468 10 121416 Fluorescence Depth (m) Lake Okareka 0 2 4 6 8 10 12 14 16 18 20 34567891011 Fluorescence Depth (m) Eloise Ryan’s Ph.D. research on deep vs surface chlorophyll maxima

12 Anabaena planktonica Photo: B. O’Brien CO 2 NH 4 NO 3 PO 4 N2N2

13 Nitrogen fixation No oxygen azoferredoxin molybdoferredoxin N 2 + 3H 2  2NH 3 + H 2 Acetylene reduction: CH=CH  H 2 C= H 2 C

14 Depth (m) Chla fluorescence (mg/m 3 ) or Temperature ( o C) Nitrogen fixation in Lake Rotoiti Chlorophyll a and temperature 2/2/03 Depth (m) N-fixation (nmol/L/hr) F T 2/2/03 N-fixationTemperature/fluorescence 012345012345 012345012345

15 Rate of N fixation (whole lake): = 1 nmol C 2 H 4 /L/hr * 12 hr/day * 28 ng N  3 nmol C 2 H 4 rate in bottle unit and light conversion stoichiometric conversion * 1.77 x 10 11 L * 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 10 11 L * 10 -9 kg/ug concentration lake volume (top 5m) units conversion = 1770 kg Chla

16 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

17 J A S O N D J F M A M J Current state of Lake Rotoiti - Station 2 J A S O N D J F M A M J 20 30 40 50 60 Dissolved oxygen (mg/L) 0 10 0 20 30 40 50 60 Temperature ( o C) * Data from EBOP monitoring program

18 Nutrient sampling in Lake Rotoiti 00 01 02

19 Lake Rotoiti, DO and nutrients, 40-m depth, ‘91-’94 Dissolved oxygen (mg/L) Nitrate (ug/L) Ammonium (ug/L)

20 Transformations of N: Nitrification and denitrification Organic N NH 4 + NO 2 - NO 3 - N 2 O, N 2 Mineralisation Nitrification Denitrification

21 Environmentally important redox reactions DECREASING ENERGY YIELD

22 Nitrate (ug/L) 80 60 40 20 Total phosphorus (mg/L) Redox sequence - Lake Rotoiti

23 Volume below 30m depth = =280,360,000 m 3 Concentration of NO 3 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

24 Volume below 30m depth = 280,360,000 m 3 Rapid increase in PO 4 = 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%

25 Total phosphorus and oxygen in Lake Rotoiti at 40m depth

26 Relationship between dissolved oxygen and total phosphorus in Lake Rotoiti at 40m depth Total phosphorus (mg/m 3 ) Dissolved oxygen (mg/L)

27 Total phosphorus (mg 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

28 D J F M Temperature, dissolved oxygen and nutrients on Okawa Bay, 2002 D J F M TP = 175 mg/m 3 NH 4 = 500 mg/m 3

29 The Lake Rotoiti bubbles! Methanogenesis? Photo: N. Miller

30 Progressive loss of oxygen from depth in Lake Rotoiti EBOP 2001 Fish 1968 Jolly 1955 * data in March each year

31 % Saturation 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.)

32 % Saturation Dissolved oxygen comparison, Lake Okataina 1955-56 Jolly (1968) Depth 50-55m 1970-71 McColl (1971) Depth 79m 2001-02 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

33 % Saturation 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

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

35 A ‘stable system’ Nutrients in Algae Mixing High O 2 Moderate O 2 Sedimentation

36 An ‘enriched’ system Nutrients in Algal blooms Mixing High O 2 Little or no O 2 Nutrient release Sedimentation N fixation

37 1970 2003 1992 Stable 1965 State of the Rotorua lakes

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