What effect does algae have on pH and dissolved oxygen (DO) levels at night?

Algae play a significant role in aquatic ecosystems, particularly concerning the levels of pH and dissolved oxygen (DO) in water bodies. At night, when photosynthesis ceases, algae continue to respire, consuming oxygen and producing carbon dioxide. This process can lead to a decrease in DO levels as the oxygen that is available in the water is used up by the respiration of algae and other aquatic organisms. Concurrently, the increased carbon dioxide in the water can result in a decrease in pH, as carbon dioxide reacts chemically with water to form carbonic acid, thereby lowering pH levels.

In summary, at night, the respiration activities of algae cause a decrease in both pH and dissolved oxygen. Thus, the correct answer reflects the ecological interaction between algae and the chemical properties of the water during nighttime conditions.