pH
Carbon Dioxide in the water
Photosynthesis in the water
Cellular Respiration
Decomposition

pH is a measure of how acidic or basic water is. It is a measure of the balance of positive hydrogen ions (H⁺) and negative hydroxide ions (OH^-) in the water. The pH range goes from 0 - 14, with 7 being neutral. Water that has more hydrogen ions is acidic (pH < 7), whereas water that has more hydroxide ions is basic (pH > 7).

pH Scale

Acids produce hydrogen ions (H⁺). Strong acids like hydrochloric acid produce lots of hydrogen ions; this is because in water the molecules of hydrogen chloride dissociate completely into hydrogen and chloride ions.

HCl (aq) —> H⁺ + Cl^-

Water also dissociates to produces ions as follows:

H₂O —> H⁺ + OH^-

The concentration of a molecule or ion is represented by square brackets. The symbol for the concentration of hydrogen ions is [H+].

pH is defined as follows:

pH = -log [H⁺]

As defined, pH values are on a logarithmic scale, where each successive number represents a 10-fold change in the acidity (or basicness) of the water. For example, water with a pH of 4 is ten times more acidic than water having a pH of 5. The following table shows some chemical species in water and their hydrogen concentrations and pH values.

Chemical Species	[H+]	pH
HCl   (hydrochloric acid)	1	0
H2O    (water)	0.0000001	7
NaOH  (sodium hydroxide)	0.00000000000001	14

Aquatic organisms are very sensitive to the pH of the aquatic environment. Most freshwater organisms in Western Oregon thrive best in a pH range between 6.5 and 8.5.  pH levels > 9 begin to be harmful to salmonids (salmon and trout) and perch.

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Carbon dioxide is important in the chemistry of natural waters. Aquatic plants depend on CO₂ in water for growth and respiration. Carbon dioxide mixes between the air and the water.

CO₂ (air) <=> CO₂ (water)

CO₂ reacts with the water to form carbonic acid, a weak acid:

CO₂ + H₂0 <=> H₂CO₃ (carbonic acid)

Carbonic acid goes to equilbrium in the water according to the following reactions:

H₂CO₃ <=> H⁺ + HCO₃^-

HCO₃^- <=> H⁺ + CO₃^2-

Since we also have H₂0 <=> H⁺ + OH^-, we now have the following species in the water:

CO₂   H₂0   H⁺   OH^-   H₂CO₃   HCO₃^-   CO₃^2-

If there were no other constituents or processes in the water, the pH would be 5.65.  This is the pH that we find in rain water.

"Acid rain" has a pH < 5.65, the acidity of normal rain water. Sulfur dioxide (SO2) and various nitrogen oxides (NOx) are the main causes of acid rain. Acid rain occurs when these gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds. Sulphur dioxide is generally a byproduct of industrial processes and the burning of fossil fuels. Ore smelting, coal-fired power generators and natural gas processing are the main contributors. Automobile exhausts also contribute to acid rain.

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Photosynthesis in the water

Photosynthesis is the biochemical process in which plants and algae harness the energy of sunlight to produce food. Photosynthesis of aquatic plants and algae in the water occurs when sunlight acts on the chlorophyll in the plants. Here is the general equation:

6 H₂0 +  6 CO₂ + light energy —>  C₆H₁₂O₆ +  6 O₂

Note that photosynthesis consumes dissolved CO₂ and produces dissolved oxygen (DO). Refering to the section on Carbon Dioxide in the water, we can see that a decrease in dissolved CO₂ results in a lower concentration of carbonic acid (H₂CO₃), according to:

CO₂ + H₂0 <=> H₂CO₃ (carbonic acid)

As the concentration of H₂CO₃ decreases so does the concentration of H⁺, and thus the pH increases.

In summary, rapidly growing algae or submerged aquatic plants remove CO₂ from the water during photosynthesis, which increases pH.  When photosynthesis occurs during the daylight hours, we can expect both dissolved oxygen and pH to increase. During the non-daylight hours, these parameters decrease.

The following graphs were taken from the Gene Pool, a marsh at the Preserve. The graphs show the diurnal cycle caused by photosynthesis during the daylight hours. Note the relationships between the daily maxima and minima for both dissolved oxygen and pH.

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Cellular Respiration

Cellular respiration is the process in which organisms, including plants, convert the chemical bonds of energy-rich molecules such as glucose into energy usable for life processes. The equation for the oxidation of glucose is:

C₆H₁₂O₆ +  6 O₂ —> 6 H₂0 +  6 CO₂ + energy

Cellular respiration occurs in plants and algae during the day and night, whereas photosynthesis occurs only during daylight.

Decomposition

Decomposition is the reduction of formerly living organisms into simpler forms of matter. When plants and algae die in the water, decomposition occurs as microbes, such as bacteria, fungi and protzoa, break down the organic matter. During aerobic decomposition microbes consume dissolved oxygen.

Decomposition completes the biochemical cycle that starts with photosynthesis. Decomposition releases the mineral nutrients (e.g., N, P, K) bound up in dead organic matter in an inorganic form that is available for use as reactants for photosynthesis.

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Background Information

Water Quality Concepts
Water Quality Monitoring at the Preserve

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