If the concentration of dissolved oxygen decreased what might happen to the entire aquatic ecosystem

Dissolved oxygen refers to the oxygen present in water which is necessary for many forms of aquatic life. Any change in the amount of dissolved oxygen affects the aquatic life.

Fishes, invertebrates and the aquatic plants need dissolved oxygen for respiration. Microbes like bacteria and fungi also require it for the breakdown of organic matter. A decrease in dissolved oxygen affects the growth, reproduction of fishes and mortality of fishes rises. This results in the accumulation of dead fishes called the fish kill leading to water pollution. Decrease in dissolved oxygen also affects the growth of other aquatic forms and results in the accumulation of organic material in the water body. This is because it results in increased BOD (biochemical oxygen demand) which is the amount of oxygen required by the aerobic microorganisms to breakdown the organic matter.