Which of the following is absent in polluted water?

The polluted water is rich in phosphates and nitrates. Due to the availability of excess nutrients and reduced oxygen, the algae like blue green algae use the nutrients and grow in the water. This results in formation of algal blooms. Larva of stone fly is a sensitive insect that grows only in well aerated water and does not grow in polluted water. Thus, it acts as a bio-indicator of water pollution. Related Theory When the environment becomes rich in nutrients, the aquatic environment faces a problem of increase in the algae. Some of the algae produce toxins which are harmful for other organisms. These toxins can enter the food chain and can be fatal.