I. Controls on CO₂
    A. Photosynthesis
    B. Respiration
    C. Atmosphere
    D. Geologic inputs

II. Forms of carbon --
     A. Forms and examples of each
 
 

	PARTICULATE	DISSOLVED
ORGANIC	Living organisms Dead organic material	Soluble organics:      DOC (dissolved organic carbon)      Amino acids      Sugars
INORGANIC	CaCO3 Carbonates of Mg, K, Na, etc.  (minerals)	DIC (dissolved inorganic carbon)      CO2      H2CO3       HCO3-      CO32-

     B. Carbon cycle – how C moves between these boxes

III. Dissolved inorganic carbon, DIC
         Distribution of DIC as a function of pH – See figure 11-1, Wetzel

IV. pH
     A. Reactions and definitions

(dissociation product constant)
                                                    by definition concentration of water = 1

                              if [H+] = 10^-7 molar, pH = 7

     B. Common pH values
        1. If distilled water reacts with CO₂, get H₂CO₃ and pH ~5.6
        2. Rain with pH less than 5.6 is said to be ‘acid rain’
        3. Most lakes range in pH from 6-9
        4. Low pH lakes
            a. pH < 2; usually due to volcanically produced H₂SO₄ or mine wastes
            b. pH 3.3-4.5 – Sphagnum bogs – exchange of cations for H⁺ by the plants
            c. Acid deposition
        5. High pH in lakes
            a. Carbonates present  (saline lakes, pH>8)
            b. High rates of photosynthesis (decrease CO₂, increase pH)

V. Carbonate buffering system

    A. Reactions
        1.   Hydration reaction 
                                                                [CO₂]_aq

        2.   Dissociation reaction 

        3.   Dissociation reaction 

    B. Solving equations
        1. Basic principles
            (a) What is distribution of C species?
            (b) What happens when one or more species is changed – when the system is perturbed?
            (c) If we know any 2 quantities, then we can determine the others
        2. System of equations

            a. K_w 

            b. K₁
                            Do not put water in the denominator – is 1
            c. K₂

            d. K_sp(CaCO₃) 
                            solids also are 1

VI. Alkalinity

    A. Definition = measure of the buffering capacity of the water; capacity of water to neutralize an acid

        Sum of the anions of weak acids

        In practice for most lakes: 

    B. Changes in and control of alkalinity
CLOSED SYSTEMS – ALKALINITY IS CONSERVED

 
        1. CASE 1
            a. respiration adds CO₂
                Remember, [HCO₃^-] + 2[CO₃^2-] = constant

        2. CASE 2 -- Photosynthesis removes CO₂
                Opposite of CO₂ addition case

        4. Buffering aspects – What happens when acid or bases are added to the system?

            1. add acid – consume alkalinity

            2. add base – increase alkalinity and increase pH
            3. carbonate system prevents pH from changing as much as expected from amount of acid or base added.

VII. Flux to atmosphere (atmospheric controls)
        A. boundary layer model

              where z is the thickness of the boundary layer;
              related to wind speed

VIII. Questions that have been asked about the Carbon cycle in freshwaters
     A. Is carbon a limiting nutrient?

     B. Role of lakes, surface waters in the global C cycle

     C. Acid rain – areas with few carbonates – little alkalinity or buffering capacity
 

 Return to Limnology Lecture homepage

 Return to K.L. Schulz's homepage