Aquatic Biology
BIO321
Cabon in the Aquatic System - CO2 as a "dissolved gas"
Horne and Goldman - Chapter 7
I. The Importance of CO2 in Aquatic systems
A. The global increase in CO2
1960 = .0315% by volume, 1975 = .033%, 1985 = .034%, 1995 = .037%
B. CO2 is 200x more soluble in water than O2
C. CO2 is a major buffer system in aquatic ecosystems, where HCO3- regulates pH
D. CO2 regulates germination and spoulation of plants and respiration rates and growth in animals
E. CO2 is a major "nutrient" in photosynthesis : CO2 + H2O --> C6H12O6 + O2
II. Three major pools of carbon play important roles in aquatic systems
A. Equilibrium products of Carbonic Acid
CO2 is the main form used by plants
HCO3- can be used by some plants but usually must be converted to CO2
HCO3- +carbonic anhydrase --> CO2
CO3= is not utilizable by plants
B. Organic particulate mater- organic carbon from decomposition
DOM
Particulate
CPOM
FPOM
C. "living" Carbon - the carbon in living organisms
III. Sources of carbon in aquatic ecosystems
A. cellular respiration (photosynthesis conversely removes CO2 )
B. Hardwater lakes from CaCO3 -->an infinite supply of CO2 - used by heterotrophic blue green algae
C. Atmospheric CO2 and "acid" rain pH = 5.6
D. Decompositional byproduct of bacteria in the tropholytic zone
E. Groundwater - soils, bedrock, the watershed
IV. The CO2 Equation
carbonic acid bicarbonate
carbonate
CO2(air)  CO2(dissolved) +H2O  H2CO3  H+ + HCO3-  2H+ + CO3=
rapid
slow
fast
fast
hydration
A. Components
1. H2CO3 (carbonic acid) dissociates rapidly into bicarbonate, but is formed very slowly from dissolved
CO2 in a rate limiting hydration reaction. H2CO3 is found in rain and in groundwater and as this
weak acid percolates through limestone etc, Ca is dissolved out:
Ca -->Ca(HCO3)2 (Calcium bicarbonate, which is relatively soluble in water)
HCO
2.
3 is the buffer form
3. CO2 is of course the form used in photosynthesis
4. CO3= (carbonate)
B. Strong photosynthesis shifts the equation left as CO2 is used, which increases pH
1. keep in mind that pH = -log [H+] and is thus
an inverse relationship of pH with hydrogen ion
concentration: pH is a measure of the # or
activity of H+ ions
C. During the summer, when photosynthesis
predominates, the pH of systems increases,
while in winter system pH typically decreases
pH
[ H+]
D. At night pH declines when CO2 is produced via cellular respiration of plants and animals
V. Equilibrium CO2
A. Alkalinity conversely is the ability to accept of use H+ ions and is a measure of "buffering capacity" - of
how much acid can be added to the system without a corresponding change in pH - the quality and
quantity of buffering compounds present.
B. Low alkalinity occurs when low levels of buffers are present (CO3=, OH-,SiO3)
C. Ca+2 + 2HCO3-  Ca(HCO3)2(soluble)  CaCO3(precipitates) + H2O + CO2
This equation shows that a certain amount of CO2 (Equilibrium CO2 ) is needed to keep the equation
balanced and to prevent an insoluble calcium carbonate precipitate from forming - this equilibrium CO2
is the amount of CO2 needed to hold Ca in solution at a given alkalinity.
Alternately Agressive CO2 is that amount of CO2 in excess of equilibrium CO2
When CO2 levels drop a white precipitation (marl) can be seen on the leaves of plants during the day in
lake valentine.
Equilibrium CO2
0.15 ppm
2.5 ppm
64.1
Total Alkalinity
25
100
300
D. Thus during active photosynthesis not only is pH altered, but other side reactions occur.
1. carbonate precipitates - epilimnetic decalcification
2. as T increases, CO2 declines, CaCO3
3. Alkalinity declines in the summer (decreased buffering capacity)
4. Epilimnetic coprecipitation of PO4-, Mn, Mg, Na
VI. CO2 thus regulates the BIOTIC components of lakes
A. Autoregulation of phytoplankton
pulses of algae in the system as CO2 useage decreases system CO2 causing
precipitation of CaCO3 and PO4, leading to decreased nutrients and
decreased growth.
po pn #
B. Species composition may also be regulated by the form of Carbon present.
1. Blue green algae are found in summer because they are poor
competitors for free CO2 and thus do well in high pH systems where they
time
compete well for the more abundant HCO32. Green algae and diatoms (golden algae) are less efficient at using bicarbonate and thus do best at lower
pH's where CO2 predominates
pH4-6 favors greens and diatoms
pH>7 favors blue greens (Lake Valentine)
VII. CO2 may be a limiting nutrient in aquatic ecosystems
A. C/P ratio in living organisms is typically 106:1 so we need much more carbon than P, while there is almost
always abundant P (+bottom recharge)
- a bloom of bluegreens can occur at 0.01mg/l P and since this level is most likely always present in
aquatic systems, why should we worry about pollution??
B. Conversely Carbon is limiting (only 0.a4 to 1.1mg/l C from atmosphere) and so carbon must be coming
from some other source - Allochthonous inputs of C may determine the productivity of the system.
Allochthonous organic Carbon + bacteria -->CO2 -->HCO3- -->CO3=
C
Biomass Production
1.
Thus the availability of carbon that is the root cause of eutrophication and not excess P. The excess
cycles until an equilibrium is reached
[algal bloom -->dieoff-->decomposition-->carbon recharge-->algal bloom]
2. Species composition and density is regulated by carbon
>40mg/l alkalinity systems are usually productive
<40 mg/l alkalinity systems are usually carbon limited oligotrophic systems
Soda lakes with very high alkalinity (3320mg/l) often have only Spirulina , an alkaline loving
bluegreen algae species.