Question

What is the significance of dissolved oxygen to aquatic organisms? How does it get depleted?

Answer 1

Hint: Oxygen is an essential gas required for the survival of terrestrial life. Although major aquatic organisms that fish do not have lungs to breathe but they do require oxygen for specific metabolisms.

Complete step by step answer: It is known that oxygen is one of the major gases required for the survival of most living organisms. Along with animals, many plant species acquire oxygen from the atmosphere surrounding. But the oxygen present in water bodies is called dissolved oxygen as it does not acquire much percentage in water. This dissolved oxygen comes from some of the underwater plants or algae that use sunlight for photosynthesis. Also, oxygen from the environment dissolves in the surface layers of water bodies. The amount of dissolved oxygen present in water bodies is measured in parts per million. Fishes are the major aquatic organisms found in water bodies. They do not have lungs to breathe atmospheric air but have gills. Despite the absence of lungs, the fishes acquire oxygen through gills for breathing and other important metabolic functions. It is said that the larger fishes need more oxygen levels as compared to smaller fishes. Other organisms like zooplanktons also need oxygen for their survival. Now, let’s see the various causes of the decrease in DO levels. The most common cause is algal blooms. The algal blooms are large spreads of algae that cover the water bodies and use all the oxygen present in water causing the fishes to suffocate by blocking the dissolving of oxygen from the environment. This is called eutrophication of water bodies. The other reason is the overpopulation of zooplankton species that feed on phytoplankton that produce oxygen. The dissolved oxygen levels are also affected by changes in acidity or alkalinity of water. In the case of aquacultures, the most prevalent cause of depleted oxygen levels is overstocking.

Note: the increased trend of disposing of chemical waste from factories directly into streams. These contaminated lethal discharges cause drastic changes in acidity levels of water bodies. Also, they carry the risk of eutrophication.