Carbon in Lakes

1
Carbon in Lakes
Lakes Carbon Lakes Carbon
Sinks Sources
Cole, 1994
Houghton, 2001
2
Dissolved Particulate
Inorganic DIC CO2, H2CO3, HCO3-, CO32- CaCO3
Organic Diss. organic carbon (DOC) Large refractory compounds Small, reactive compounds Living organisms Dead particulate organic material
3
Chemical Species of Dissolved Inorganic Carbon

• CO2(gas) lt--gt CO2(aq) Free Carbon
  Dioxide
• CO2(aq) H2O lt--gt H2CO3 Carbonic Acid
• H2CO3 lt--gt HCO3- H
  Bicarbonate
• HCO3- lt--gt CO32- H Carbonate

CO2(aq) lt--gt H2CO3 lt--gt HCO3- H
CO32- H
4
8.3
6.35 10.32

• Rain in eqm with CO2 5.6
• Lakes 6-9
• - pH lt 6 low weathering rates, acid rain,
  DOC-rich, oligotrophic
• - pH gt 8 high weathering rates, eutrophic
• closed basin or calcareous lakes

5
DIC, pH, and buffering
CO2(aq) lt--gt H2CO3 lt--gt HCO3- H
CO32- H

• Add acidity Drive to CO2 and H2CO3, consume
  HCO3-, CO32-
• Remove acidity/add base drive to HCO3-, CO32-
• If DIC low, acid/base addition greatly alters pH

6
Alkalinity

• Acid neutralizing capacity
• Alkalinity HCO3- 2CO3-- OH- (other bases)
  - H
• Expressed as equivalents or Eq- moles of
  acidity (H) to get to pH 4.5
• 1 Eq 0.5 g/L CaCO3
• Typical values 50-300 mg/L CaCO3 or 1-6 mEq/L

7
CO2 lt--gt H2CO3 lt--gt HCO3- lt--gt CO32-
8
DIC in Stratified Lakes
9
Calcite precipitation
Lake Zurich, Kelts and Hsü, 1978
10
Conclusions

• Reactions between CO2, H2CO3, HCO3, and CO32-
  buffer the pH of most aquatic systems
• External sources of DIC include atmospheric
  diffusion and rock weathering.
• Rock weathering and evaporative balance control
  lake alkalinity.
• Photosynthetic and respiratory cycling of CO2
  creates gradients in DIC concentrations and can
  affect pH
• Coupled changes in pH and CO2 concentration
  induce precipitation of calcite
• DIC inputs and internal cycling create strong
  temporal and spatial patterns in lake DIC