What time of day will dissolved oxygen (DO) levels be highest in un-aerated stabilization ponds?

Dissolved oxygen levels in un-aerated stabilization ponds typically reach their peak during the late afternoon to early evening, but often the highest levels occur around sunset. This is primarily due to the photosynthetic activity of algae and aquatic plants, which produces oxygen during daylight hours. As sunlight reaches its maximum intensity, photosynthesis is at its peak, leading to the greatest production of dissolved oxygen in the water.

In the morning, oxygen levels can be lower as the photosynthetic processes start to ramp up with the increase in sunlight. At midnight, darkness prevails, and without photosynthesis, oxygen production ceases, often leading to decreased DO levels during the night when respiration continues. Just after sunrise, while DO levels begin to rise again with the resumption of photosynthetic activity, they have not yet reached the levels seen later in the day. Therefore, the timing of sunset aligns with the end of strong photosynthetic production, which is why this time shows the highest dissolved oxygen levels in these types of ponds.