Lec 5: Gases (DO & CO 2 ) and pH

1. Lec 5: Gases (DO & CO2) and pH • Factors affecting Oxygen Concentrations • Inorganic & Organic Carbon and the Carbonate Cycle • Wednesday: • Cole, J.J. et al. 1994. Carbon dioxide supersaturation in the surface waters of lakes. • Science 265:1568-1570. 1

2. Dissolved Gases 1. Gases constitute one class of chemical impurities of water: some essential for life, some inert, others toxic 2. Properties of gases governed by both chemical and physical laws 3. Gases tend toward equilibrium between the concentration in the atmosphere and that dissolved in water • 4. Equilibrium (saturation) amount of each gas dissolved in water dependent on: • Pressure • Salinity • c. Temperature 5. Solubility of a gas is independent of the concentrations of other gases in solution 2

3. Nitrogen 78.08 42 1 Oxygen 20.95 35 3 Argon 0.934 Carbon dioxide 0.033 23 2100 Others 0.003 Atmospheric vs. Dissolved Gas Concentrations (% by volume) Dissolved in water Relative Solubility Gas Atmosphere Nitrogen and Phosphorus are important plant nutrients 3

4. 90% of water (by weight) but not biologically available or important in this form Probably the most important single indicator of aquatic conditions for biota Concentration in water generally expressed as PPM (Parts per million) = mg/l, or as percentsaturation: 100%*Amount Present Solubility Oxygen • Determination • DO Probe and meter • Chemically (Winkler method and modifications) 4

5. Oxygen - Forms and Transformations • 21% of atmosphere is O2 • Aerobic/anaerobic - oxic/anoxic (hypoxic) • Saturation concentration of dissolved O2 depends on atmospheric pressure and temperature • Photosynthesis produces oxygen, respiration consumes it • Oxygen drives redox 5

6. Potential Energy and Redox • Which form of N is preferred by primary producers? • How to they convert to the preferred form? Using potential energy Creating potential energy 6

7. 1. Diffusion from atmosphere (Often less important than photosynthesis). Diffusion rate depends on: a. Wave action (rate increases with increasing wave action) b. Atmospheric pressure (rate increases with increasing atmospheric pressure) c. Oxygen saturation of water (rate decreases with increasing saturation) d. Salinity (rate decreases with increasing salinity) e. Moisture content of air (rate decreases with increasing humidity) 2. Photosynthesis (Often more important than atmospheric diffusion). May contribute more than 50% of the oxygen in water. Photosynthesis may contribute 5mg O2/cm2/day Factors affecting Oxygen Conc. 8

8. 10 mg/l O2 at 20OC = 123% saturation at sea level 0 5 10 15 20 25 30 Elev. (m) Pressure (mm Hg) Temperature (degrees C) Factor 0 760 1.00 500 714 1.06 1000 671 1.13 1500 631 1.20 2000 594 1.28 2500 560 1.36 % Saturation 140 120 100 80 60 50 40 30 20 Oxygen (mg./liter) 10 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Oxygen (cc./liter) Nomogram for Determining Saturation of Oxygen at Different Temperatures 10 mg/l O2 at 20OC = 148% (1.20 x 120) saturation at 1500 m (~5000 ft) 7

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10. 1. Photosynthesis and respiration often result in daily fluctuations in the O2 concentration of surface water a. May reach 200% saturation in late afternoon b. May fall to 50% saturation by dawn Oxygen Losses and Fluctuations 2. Oxygen losses due to: a. Respiration b. Decomposition 3. Oxygen distributed in the water column mostly by currents 4. Summer stratification may limit amount of dissolved oxygen in the hypolimnion 10

11. 0 O2 O2 1 2 3 Orthograde 4 • 1. Orthograde • Low productivity Depth (m) 5 Clinograde 6 7 • 2. Clinograde • High productivity T 8 T 9 10 • 3. Positive Heterograde • Increased solubility in the metalimnion due to temperature • Concentrations of algae in the metalimnion * 0 O2 O2 1 2 3 4 Depth (m) 5 Positive Heterograde 6 Negative Heterograde • 4. Negative Heterograde • High metalimnetic respiration and/or decomposition 7 8 T T 9 10 0 5 10 15 0 5 10 15 O2 mg/l O2 mg/l Mid-SummerOxygen Profiles 11

12. Crystal Lake: unproductive, transparent, with deep photosynthesis Other Lakes - range from moderately productive to highly productive All lakes except Adelaide show metalimnetic oxygen maxima 0 2 4 6 8 10 12 14 16 18 20 22 24 26 2 4 6 8 10 12 14 16 18 20 22 24 26 28 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 20 0 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 O2 Profiles for Shallow Dimictic Lakes Temperature OC Akagi Okono, Japan Crystal Lake, Wisc. TOC [O2]S [O2]S TOC [O2] Depth (m) Silver Lake, Wisc. [O2] Note areas of DO deficit TOC Adelaide Lake, Wisc. [O2] [O2]S [O2]S TOC 12 Dissolved Oxygen (mg/l)

13. 0 Lake Mendota, Wisc. 2 4 6 8 Depth (m) 10 IV Aug. 12 14 16 18 III July II June I May 20 22 0 1 2 3 4 5 6 7 8 9 10 11 12 Dissolved Oxygen (mg/l) Development of a Clinograde Oxygen Curve Processes responsible for this pattern? 13

14. Productive and ConsumptiveAspects of Lake Morphology High volume to surface area ratio lakes Low volume to surface area ratio lakes Productive Aspect Consumptive Aspect What other factors might affect this balance? 14

15. Forms of Carbon Transformations of Carbon A General Introduction to Nutrient Cycling and the Carbon Cycle Carbon 15

16. Generally, the most important source of carbon for photosynthesis Carbon Dioxide • Involved in buffering the pH of neutral • and alkaline lakes • The measurement of CO2 in all of its forms is called “Alkalinity” 16

17. 1700 people and many livestock died near Lake Nyos in Cameroon in 1986 Lake Nyos Disaster • A survivor reported a 25m high water • surge and odor of rotten eggs • Caused by catastrophic release of • supersaturated CO2 from the hypolimnion • CO2 probably came from volcanic activity • Landslide or cool weather released the gas • Building up again, using pipes to release • pressurized water 17

18. Carbon dioxide in Solution The Carbon Dioxide Cycle O 2 O 2 (photosynthesis) Plants respiratory CO 2 Plants (respiration) O 2 respiratory CO 2 Animals O 2 respiratory CO 2 dissolved Bacteria organic material CO 2 non-biological oxidation Organic Carbon Inorganic Carbon 18 (mainly CO ) 2

19. Inorganic Carbon-bicarbonate equilibrium Carbon dioxide: CO2 Carbonic acid: H2CO3 Bicarbonate: HCO3- Carbonate: CO32- Organic Carbon Forms of Carbon CO2 + H2O H2CO3HCO3- + H+CO32- + 2H+ -In which direction will PP drive these reactions? 19

20. H2O+CO2<—>H2CO3<—>HCO3– + H+< <—>2HCO3<—>CO3= Carbon Dioxide Cycle in Lakes CO2 + Ca++ Phytoplankton (Euphotic Zone) H2O CaCO3 Sediments 20

21. Proportions of the formsof CO2 in Relation to pH Free Bicarbonate Carbonate pH CO2 HCO3– CO3= 4 0.996 0.004 1.26 x 10-9 5 0.962 0.038 1.20 x 10-7 6 0.725 0.275 0.91 x 10-5 7 0.208 0.792 2.60 x 10-4 8 0.025 0.972 3.20 x 10-3 9 0.003 0.966 0.031 10 0.000 0.757 0.243 21

22. Forms of CO2 in Water in Relation to pH 22

23. CO2 (µm) Daily Fluctuations in Epilimnetic O2 and CO2 60 360 Sunset Sunrise 50 350 40 340 O2 CO2 30 330 O2 (µm) 320 20 10 310 0 300 1800 2400 600 1200 1800 Time 23 18

24. Consider these relationships when we are processing the data from the Hensley Reservoir field trip