What is the importance of dissolved oxygen levels in water?

Dissolved oxygen levels in water are crucial for the survival of aquatic life because most fish and other aquatic organisms rely on oxygen dissolved in water for respiration. Aquatic animals extract oxygen from water using their gills, and without sufficient levels of dissolved oxygen, these organisms can suffer from stress or even die. Healthy populations of aquatic species, such as fish and invertebrates, require adequate dissolved oxygen to thrive, grow, and reproduce. Therefore, monitoring dissolved oxygen levels is essential for assessing the health of aquatic ecosystems.

The other options touch on related concepts but do not accurately reflect the primary role of dissolved oxygen. For instance, while dissolved oxygen can be affected by temperature, it does not serve as a direct indicator of water temperature itself. Similarly, while it can influence metabolic processes, it does not determine pH levels nor specifically signal the presence of harmful chemicals in the same way that contaminants might; rather, its levels can decrease in polluted waters but are more a symptom than a direct indicator of chemical presence.