You've probably all seen a solid like sugar or salt dissolve in water. Warmer water can hold less dissolved oxygen than cooler water, so temperature has a big effect on dissolved oxygen concentations.
Algae produce oxygen during the day through photosynthesis.
When there is an overabundance of organic matter like dead algae, for example, aquatic bacteria grow rapidly. They consume oxygen during respiration, and can lower dissolved oxygen concentrations. Mixing that occurs in swiftly moving creek water can increase dissolved oxygen levels. Fish and other aquatic organisms can be harmed when dissolved oxygen levels get too low. Low dissolved oxygen can change the chemistry and biology of water.