Inorganic and Organic Carbon

1
Carbon In Lake Waters

• Inorganic and Organic Carbon
• ecological significance
• forms
• equilibrium
• alkalinity and hardness
• sources

2
Ecological Significance of Inorganic Carbon

• Carbon source for primary production
• nCO2 H2O ? (CH2O)n O2
• A chemical buffer for aquatic systems

Photosynthesis!
3
Forms of Inorganic Carbon in Water

• free CO2 and carbonic acid (H2CO3)
• Carbon Dioxide is very soluble in water
• bicarbonate (HCO3-)
• carbonate (CO3-2)

4
CO2-Bicarbonate-CarbonateEquilibrium

• fraction of each carbon complex is pH dependant
• the major pH buffering system for lakes and
  streams
• pH is important to the availability of nutrients

5
CO2-Bicarbonate-Carbonate Equilibrium
from Wetzel, 1983
6
Marl Formation

• high rates of CO2 use for photosynthesis
• solid calcium carbonate (CaCO3) precipitates
• suspended CaCO3 particles can cause lake
  whitening, reducing light penetration

7
Lake Michigan
Figure 2-10 in Horne and Goldman
8
Sources of Inorganic Carbon

• atmosphere
• H2O CO2 ? H2CO3 ? H HCO3-
• decomposition
• C(H2O)n O2 ? CO2 H2O energy
• respiration (plants and animals)
• (CH2O)n O2 ? nCO2 H2O

9
Sources of Inorganic Carbon (continued)

• groundwater
• chemical weathering
• H2CO3 CaCO3 (H2O) ?
• Ca(HCO3)2 ? Ca2 2HCO3-

10
Alkalinity

• the measure of a waters capacity to neutralize a
  strong acid
• Types
• (all often expressed in terms of mg CaCO3/L)
• total alkalinity
• carbonate alkalinity
• hydroxide alkalinity
• bicarbonate alkalinity

11

• Alkalinity is important because it reflects the
  buffering capacity of the lake
• ex Acid Rain impacts-

12
Hardness

• the concentration of multivalent cations, mainly
  Ca2 and Mg2 but sometimes also Fe3 and Al3
• Waters that have high alkalinity will also be
  hard (but not necessarily the other way around)

13
Organic Carbon

• Distribution
• dissolved organic carbon
• labile
• refractory
• particulate organic detritus
• living organisms

14
Sources of Organic Carbon

• allochthonous
• autochthonous

15
Key Points

• Carbon is necessary for photosynthesis and
  important for pH buffering
• The CO2 Equilibrium provides the buffering
  capacity
• how much buffering capacity a lake has is
  reflected by alkalinity
• Marl formation
• Sources of organic and inorganic carbon