lecture goals to review how ph and alkalinity work. to discuss the forms and transformations of...
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![Page 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](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d625503460f94a451aa/html5/thumbnails/1.jpg)
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.
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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+
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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
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• 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
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Why are pH and alkalinity like cars in a parking lot, not like married couples?
YES! NO
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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
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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
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DIC and pH
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The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
• Determines the predominant form of DIC in freshwater systems.
pH
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The players: Carbonic Acid
CO2 + H2O ↔ H2CO3
Weak Acid
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The players: Bicarbonate
H2CO3 ↔ H+ + HCO3-
• Dissociation declines with decreasing pH
• When substrate rich in carbonates (CO32-):
CaCO3 +H2O + CO2 ↔ Ca2+ + 2HCO3-
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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
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The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-***
*** If Ca2+ available, then combines with CO32- to
form CaCO3, which precipitates out.
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The bicarbonate buffer system
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
• Determines the predominant form of DIC in freshwater systems.
pH
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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
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• Buffers water against rapid changes in pH…or not.
H+ or CO2
pH
CO2 + H2O ↔ H2CO3
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• Buffers water against rapid changes in pH…or not.
No change in pH!
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
H+ or CO2
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The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-***
*** If Ca2+ available, then combines with CO32- to
form CaCO3, which precipitates out.
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The Whole Cycle
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
Remember that these are equilibrium reactions!
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Add CO2 (e.g., respiration)
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
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Remove CO2 (e.g., photosynthesis)
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3
2-
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• 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
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Carbon Sinks
Forests Ocean
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Weathering and the Global Carbon Cycle
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Export of Alkalinity by the Mississippi
River
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Effect of Land Cover on Alkalinity Export by Mississippi Sub-Basins
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Carbon Sinks
Forests OceanCropland
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Controls on DIC distribution and concentration in freshwaters
Respiration Photosynthesis
How much?
Where?
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DIC in Lakes
• Equilibrium with atmospheric CO2…or >
• Bicarbonate buffer system
• External loading (i.e., input from groundwater and rivers)
• Respiration – Photosynthesis balance
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Vertical Distribution of DIC in Lakes
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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
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Origins of Organic C
Autochthonous Allochthonous
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Forms of Organic C
DOC: Dissolved organic carbon
POC: Particulate organic carbon (aka, POM)
Function of Source + Stage of Decomposition
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Forms of DOC
Methane
CH4
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Forms of DOC
Stable Organic Acids
aka
Humic Acids
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Blackwater Streams
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Headwaters → allochthonous CPOC, low autochthonous OC
POC Patterns
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POC PatternsRivers → allochthonous FPOC, higher
autochthonous OC
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How much of each source?
Autochthonous Allochthonous
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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
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Determining C sources with stable isotopes
= Low 13C
= High 13C
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Determining C sources with stable isotopes…a big improvement!
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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?
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Autochthonous
Allochthonous