The time of day in which testing is done can make a difference in the DO levels. If testing is conducted early in the morning, when the water temperature is cooler, and then is conducted, later in the afternoon on a sunny day, the increase in water temperature may result in changed DO levels. Temperature also changes with the different depths of the water. Fish and other organisms need more oxygen to survive, thus they probably live more toward the bottom where DO levels are higher. Carp and dragonfly nymphs thrive under warmer conditions.
Fish can sense very slight temperature changes.
When the temperature increases, cool water species will be replaced by warm water species. Cold-blooded animals cannot survive in temperatures below 0 degrees Celsius, and warm water animals cannot tolerate anything warmer than 36 degrees Celsius. In warmer waters, the DO levels may go down, and the BOD levels may go up because of the increased decomposition of plant matter. Some organisms may have metabolic rates that increase to a point at which it is impossible for them to survive. Water temperature increases causing the rate of photosynthesis by algae to increase.