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