1. Lecture Goals To review how pH and alkalinity work.
To discuss the forms and transformations of inorganic and organic carbon in freshwaters, and the broader patterns of distribution of these forms.

2. What is pH? “Puissance d’hydrogene”, where hydrogen = H+
Low pH = acidic = high concentration of H+
pH ranges from < 1 to 14 on logarithmic scale, so unit change represents 10x change in concentration of H+

3. What is alkalinity?
Acid-neutralizing capacity (ANC) of water, or the ability to offset the positive charges of H+ cations with negatively charged anions
Determined by the concentration of bases: HCO3-, CO32-, OH-
High ANC = small change in pH with addition of a strong acid (i.e., well-buffered)
At neutrality (pH = 7), then activity of H+ and HCO3-, CO32-, OH- are equal

4. Where does alkalinity come from?
The bicarbonate buffer system
Weathering
CaCO3 +H2O + CO2 ↔ Ca2+ + 2HCO3-
CO2 from atmosphere, H2O from rain, CaCO3 in rocks
Ca2+ and HCO3- carried to streams, rivers, lakes, oceans

5. Why are pH and alkalinity like cars in a parking lot, not like married couples?
YES! NO

6. Inorganic C in freshwaters
Buffers water against rapid changes in pH via bicarbonate buffer system
Determines how much C available for photosynthesis and generation of organic substances (i.e., foundation of organic productivity)
Contributes to overall conductivity of water = concentration of ions that influence physiological processes in biota

7. Carbon Dioxide
CO2
Expected to be at equilibrium with atmosphere – 200x more soluble than O2
0.037% of atmosphere, and low partial pressure, but increasing
Many lakes are supersaturated with CO2

8. DIC and pH

9. The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32- pH
Determines the predominant form of DIC in freshwater systems.

10. The players: Carbonic Acid
CO2 + H2O ↔ H2CO3
Weak Acid

11. The players: Bicarbonate
H2CO3 ↔ H+ + HCO3-
Dissociation declines with decreasing pH
When substrate rich in carbonates (CO32-):
CaCO3 +H2O + CO2 ↔ Ca2+ + 2HCO3-

12. The players: Carbonate
HCO3- ↔ 2H+ + CO32-
This only happens when pH very high
CO32- is relatively insoluble and will precipitate out when Ca2+ available in water or substrate

13. CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-***
The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-***
*** If Ca2+ available, then combines with CO32- to form CaCO3, which precipitates out.

14. The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32- pH
Determines the predominant form of DIC in freshwater systems.

15. The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
CO2 + H2O
H2CO3
Background pH?
H+ + HCO3-
2H+ + CO32-
Buffers water against rapid changes in pH

16. H+ or CO2
CO2 + H2O ↔ H2CO3
 pH
Buffers water against rapid changes in pH…or not.

17. CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
H+ or CO2
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
No change in pH!
Buffers water against rapid changes in pH…or not.

18. CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-***
The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-***
*** If Ca2+ available, then combines with CO32- to form CaCO3, which precipitates out.

19. Remember that these are equilibrium reactions!
The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
Remember that these are equilibrium reactions!

20. Add CO2 (e.g., respiration)
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
Add CO2 (e.g., respiration)

21. Remove CO2 (e.g., photosynthesis)
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
Remove CO2 (e.g., photosynthesis)

22. Where does alkalinity come from?
The bicarbonate buffer system
Weathering
CaCO3 +H2O + CO2 ↔ Ca2+ + 2HCO3-
CO2 from atmosphere, H2O from rain, CaCO3 in rocks
Ca2+ and HCO3- carried to streams, rivers, lakes, oceans

23. Carbon Sinks
Forests
Ocean

24. Weathering and the Global Carbon Cycle

25. Export of Alkalinity by the Mississippi River

26. Effect of Land Cover on Alkalinity Export by Mississippi Sub-Basins

27. Carbon Sinks
Cropland
Forests
Ocean

28. Controls on DIC distribution and concentration in freshwaters
Respiration
Photosynthesis
How much?
Where?

29. DIC in Lakes Equilibrium with atmospheric CO2…or >
Bicarbonate buffer system
External loading (i.e., input from groundwater and rivers)
Respiration – Photosynthesis balance

30. Vertical Distribution of DIC in Lakes

31. DIC in Rivers
Decomposition dominates over photosynthesis, so tend to produce CO2 rather than consuming
- Respiration can be so high that CO2 is maintained above equilibrium
Inflowing water high in CO2 from bacterial respiration
High turbulence causes CO2 to be lost quickly, but can see high CO2 in non-turbulent areas and during low flows
Rivers and streams also act to move alkalinity (i.e., HCO3- and CO32-) to lakes or to the ocean

32. Origins of Organic C
Autochthonous
Allochthonous

33. Function of Source + Stage of Decomposition
Forms of Organic C
DOC: Dissolved organic carbon
POC: Particulate organic carbon (aka, POM)
Function of Source Stage of Decomposition

34. Forms of DOC
Methane
CH4

35. Forms of DOC
Stable Organic Acids
aka
Humic Acids

36. Blackwater Streams

37. Headwaters → allochthonous CPOC, low autochthonous OC
POC Patterns
Headwaters → allochthonous CPOC, low autochthonous OC

38. Rivers → allochthonous FPOC, higher autochthonous OC
POC Patterns
Rivers → allochthonous FPOC, higher autochthonous OC

39. How much of each source?
Autochthonous
Allochthonous

40. Determining C sources with stable isotopes
Isotopes: forms of elements with different numbers of neutrons
13C / 12C = 13C
13C values often differ between aquatic and terrestrial primary producers:
13C Algae > 13C Terrestrial Plants
Therefore, 13C signal in consumers can tell you where they are getting their C

41. Determining C sources with stable isotopes
= Low 13C
= High 13C

42. Determining C sources with stable isotopes…a big improvement!

43. McCutchan and Lewis 2002
In Colorado headwaters, autochthonous C accounted for <2-40% of total organic matter.
However, autochthonous C accounted 40-80% of invertebrate biomass…WHY?

44. Autochthonous
Allochthonous