Dissolved oxygen refers to the amount of oxygen that is present in water, which is essential for the survival of aquatic life such as fish, invertebrates, and microorganisms. This oxygen comes from the atmosphere and is also produced by photosynthetic organisms like algae and aquatic plants. The level of dissolved oxygen can vary based on factors like temperature, salinity, and water movement. High levels of dissolved oxygen are crucial for maintaining healthy ecosystems, while low levels can lead to problems such as hypoxia, which can harm or kill aquatic organisms.