Most aquatic plants and animals need oxygen to survive. Waters of consistently high dissolved oxygen are usually considered healthy and stable ecosystems capable of supporting many different kinds of aquatic organisms Sources of Dissolved Oxygen Much of the dissolved oxygen in water comes from the atmosphere. Algae and rooted aquatic plants also deliver oxygen to water through photosynthesis. In general, rooted aquatic plants are more abundant in lakes and impounded rivers than in rivers with significant current. Large daily fluctuations in dissolved oxygen are characteristic of bodies of water with extensive plant growth.
Photosynthesis stops at night, but plants and animals continue to respire and consume oxygen.
As a result, dissolved oxygen levels fall to a low point just before dawn. Gases, like oxygen, dissolve more easily in cooler water than in warmer water. In temperate areas, rivers respond to changes in air temperature by cooling or warming. River discharge is related to the climate of an area - during dry periods, flow may be severely reduced, and air and water temps are often higher. Both of these factors tend to reduce dissolved oxygen levels.