Which condition leads to lower levels of dissolved oxygen in water?

High salinity and warm temperatures lead to lower levels of dissolved oxygen in water due to several interrelated factors. As water temperature rises, its capacity to hold oxygen decreases—warm water is less efficient at dissolving gases compared to cooler water. This natural phenomenon means that organisms relying on dissolved oxygen may face challenges in warmer conditions.

Additionally, high salinity can also have a detrimental effect on dissolved oxygen levels. When seawater or water with elevated salinity concentrations is present, the solubility of oxygen reduces further, exacerbating the issue. Aquatic life, especially those requiring higher levels of oxygen, can suffer when both temperature and salinity are elevated, leading to hypoxic conditions where oxygen levels fall below what is necessary for survival.

In contrast, rapid water flow generally enhances oxygen levels, as moving water absorbs more oxygen from the atmosphere. Low nutrient levels would not typically decrease dissolved oxygen; in fact, they may promote better oxygen levels since excessive nutrients can lead to algal blooms and subsequent oxygen depletion, a process known as eutrophication. Similarly, cooler water temperatures support higher oxygen solubility, contrary to the conditions that lead to lower levels.