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.
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+
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
• Weathering
CaCO3 +H2O + CO2 ↔ Ca2+ + 2HCO3-
• CO2 from atmosphere, H2O from rain, CaCO3 in rocks
• Ca2+ and HCO3- carried to streams, rivers,
lakes, oceans
Where does alkalinity come from?
• The bicarbonate buffer system
Why are pH and alkalinity like cars in a parking lot, not like married couples?
YES! NO
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
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
DIC and pH
The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
• Determines the predominant form of DIC in freshwater systems.
pH
The players: Carbonic Acid
CO2 + H2O ↔ H2CO3
Weak Acid
The players: Bicarbonate
H2CO3 ↔ H+ + HCO3-
• Dissociation declines with decreasing pH
• When substrate rich in carbonates (CO32-):
CaCO3 +H2O + CO2 ↔ Ca2+ + 2HCO3-
The players: Carbonate
HCO3- ↔ 2H+ + CO3
2-
• This only happens when pH very high
• CO32- is relatively insoluble and will precipitate out
when Ca2+ available in water or substrate
The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-***
*** If Ca2+ available, then combines with CO32- to
form CaCO3, which precipitates out.
The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
• Determines the predominant form of DIC in freshwater systems.
pH
The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
Background pH?
CO2 + H2O
H2CO3
H+ + HCO3-
2H+ + CO32-
• Buffers water against rapid changes in pH
• Buffers water against rapid changes in pH…or not.
H+ or CO2
pH
CO2 + H2O ↔ H2CO3
• Buffers water against rapid changes in pH…or not.
No change in pH!
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
H+ or CO2
The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-***
*** If Ca2+ available, then combines with CO32- to
form CaCO3, which precipitates out.
The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
Remember that these are equilibrium reactions!
Add CO2 (e.g., respiration)
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
Remove CO2 (e.g., photosynthesis)
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
• Weathering
CaCO3 +H2O + CO2 ↔ Ca2+ + 2HCO3-
• CO2 from atmosphere, H2O from rain, CaCO3 in rocks
• Ca2+ and HCO3- carried to streams, rivers,
lakes, oceans
Where does alkalinity come from?
• The bicarbonate buffer system
Carbon Sinks
Forests Ocean
Weathering and the Global Carbon Cycle
Export of Alkalinity by the Mississippi
River
Effect of Land Cover on Alkalinity Export by Mississippi Sub-Basins
Carbon Sinks
Forests OceanCropland
Controls on DIC distribution and concentration in freshwaters
Respiration Photosynthesis
How much?
Where?
DIC in Lakes
• Equilibrium with atmospheric CO2…or >
• Bicarbonate buffer system
• External loading (i.e., input from groundwater and rivers)
• Respiration – Photosynthesis balance
Vertical Distribution of DIC in Lakes
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
Origins of Organic C
Autochthonous Allochthonous
Forms of Organic C
DOC: Dissolved organic carbon
POC: Particulate organic carbon (aka, POM)
Function of Source + Stage of Decomposition
Forms of DOC
Methane
CH4
Forms of DOC
Stable Organic Acids
aka
Humic Acids
Blackwater Streams
Headwaters → allochthonous CPOC, low autochthonous OC
POC Patterns
POC PatternsRivers → allochthonous FPOC, higher
autochthonous OC
How much of each source?
Autochthonous Allochthonous
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
Determining C sources with stable isotopes
= Low 13C
= High 13C
Determining C sources with stable isotopes…a big improvement!
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?
Autochthonous
Allochthonous