What caused the decrease in dissolved O2 concentration during the night in the pond?

The decrease in dissolved oxygen concentration during the night in a pond is primarily attributed to decreased photosynthesis. During daylight hours, aquatic plants and algae perform photosynthesis, a process that converts carbon dioxide and water into glucose and oxygen, thereby increasing the levels of dissolved oxygen in the water. However, at night, photosynthesis ceases due to the absence of sunlight.

At this time, the organisms in the pond, including plants and animals, continue to respire, consuming oxygen and releasing carbon dioxide. With no replenishment of oxygen from photosynthesis, the overall oxygen levels in the water decrease.

This understanding indicates that the shift from light to dark conditions leads to a significant reduction in the oxygen supplied by photosynthesis, while respiration continues unabated, causing a net loss of dissolved oxygen.