Florida Wastewater Practice Test

What is the effect on pH in a properly operating facultative pond during the day?

1. pH decreases

2. pH increases during the day and decreases at night

3. pH remains constant

4. pH increases at night

The correct answer is: pH increases during the day and decreases at night

In a properly operating facultative pond, the pH increases during the day and decreases at night due to the processes of photosynthesis and respiration. During daylight hours, the presence of sunlight enables photosynthetic organisms, such as algae and aquatic plants, to convert carbon dioxide and water into oxygen and organic matter. This process consumes carbon dioxide, which reduces its concentration in the water, leading to an increase in pH; the water becomes less acidic and more alkaline. At night, the absence of sunlight halts photosynthesis, while respiration continues. Aquatic organisms consume oxygen and release carbon dioxide, leading to an increase in carbon dioxide concentration in the water. This higher concentration of carbon dioxide reacts with water to form carbonic acid, which contributes to a decrease in pH, making the water more acidic. Thus, the natural cycle of photosynthesis during the day and respiration at night causes the pH levels in a facultative pond to fluctuate, which is accurately captured by the choice that states pH increases during the day and decreases at night.