What effect does sunset have on DO levels in stabilization ponds?

In stabilization ponds, dissolved oxygen (DO) levels are influenced by several factors, including sunlight and the activities of aquatic plants and microorganisms. At sunset, the presence of sunlight diminishes, which significantly affects photosynthesis. During daylight hours, aquatic plants and phytoplankton produce oxygen through photosynthesis, which contributes to higher DO levels.

As sunset occurs and light diminishes, the rate of photosynthesis slows down and eventually stops, leading to a decrease in the production of oxygen. In addition, the respiration processes of microorganisms and aquatic life continue, consuming the available oxygen. This combination of reduced oxygen production and continued consumption leads to a decline in DO levels over time.

Thus, the correct understanding is that DO decreases linearly as night progresses, as the influence of photosynthesis is lost, and respiration continues unabated. This dynamic illustrates the interplay between light availability and biological activity in the stabilization pond environment.