What affects the pH levels in water during the night due to algae presence?

The correct answer, which pertains to the decrease in photosynthesis, highlights a critical relationship between algae photosynthesis and pH levels in water. During the daytime, algae are actively engaged in photosynthesis, a process that uses sunlight to convert carbon dioxide and water into glucose and oxygen. This process absorbs carbon dioxide from the water, leading to an increase in pH levels, making the water more alkaline.

However, at night, the absence of sunlight results in a cessation of photosynthesis. Without this process, the carbon dioxide that is naturally produced through respiration by the algae and other aquatic organisms remains in the water without being utilized. As carbon dioxide levels increase, they combine with water to form carbonic acid, resulting in a decrease in pH levels, making the water more acidic.

This dynamic demonstrates how algal populations significantly influence the water's pH, especially with the transition from day to night. Other factors like increased or decreased oxygen levels can be influenced by photosynthesis and respiration but do not directly explain the change in pH during the night, nor do they encapsulate the process at the core of this specific question.