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
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
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+ + CO32-
pH
• Determines the predominant form of DIC in freshwater
systems.
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
+
2CO3
• 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+ + CO32-***
*** If Ca2+ available, then combines with CO32- to
form CaCO3, which precipitates out.
The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
pH
• Determines the predominant form of DIC in freshwater
systems.
The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32CO2 + H2O
Background
pH?
H2CO3
H+ + HCO32H+ + CO32-
• Buffers water against rapid changes in pH
H+ or CO2
CO2 + H2O ↔ H2CO3
 pH
• Buffers water against rapid changes in pH…or not.
H+ or CO2
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
No change
in pH!
• Buffers water against rapid changes in pH…or not.
The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-***
*** If Ca2+ available, then combines with CO32- to
form CaCO3, which precipitates out.
The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32Remember that these are equilibrium reactions!
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
Add CO2 (e.g., respiration)
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
Remove CO2 (e.g., photosynthesis)
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
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
Cropland
Forests
Ocean
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
POC Patterns
Headwaters → allochthonous CPOC,
low autochthonous OC
POC Patterns
Rivers → 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