When is dissolved oxygen concentration typically at its lowest in aquatic systems?

Dissolved oxygen concentration in aquatic systems is typically at its lowest just before sunrise due to a combination of biological and physical factors. Throughout the day, photosynthetic organisms such as algae and aquatic plants produce oxygen, with the highest levels generally occurring in the afternoon when sunlight is most intense. However, at night, these organisms stop photosynthesis but continue to respire, consuming oxygen. This leads to a depletion of dissolved oxygen levels overnight.

As the sun rises, photosynthesis resumes, gradually increasing oxygen levels throughout the morning. Therefore, the period just before sunrise is commonly when oxygen levels are at their nadir, thereby making it the correct choice.

In contrast, during summer afternoons, heavy rain, or early afternoons, various factors may contribute to elevated oxygen levels or fluctuations in concentration caused by temperature changes or aeration. However, none of these conditions typically coincide with the lowest oxygen levels that are observed before dawn.