During an educational trip, a student of botany saw a beautiful lake in a village. She collected many plants from that area. She noticed that villagers were washing clothes around the lake and at some places waste material from houses was destroying its beauty. After few years, she visited the same lake again. She was surprised to find that the lake was covered with algae, stinking smell was coming out and its water had become unusable. Can you explain the reason for this condition of the lake?
The process of eutrophication is responsible for this condition of lake. The domestic waste and organic compounds such as detergents can provide plant nutrients which can enhance the growth of algae and aquatic plants.
These are decomposed by the bacterial population giving disagreeable odour and spoiling the beauty of the lake.
What are biodegradable and non-biodegradable pollutants?
Biodegradable pollutants are those which are decomposed by bacteria e.g., sewage, cow dung, fruit, vegetable etc.
Non-biodegradable pollutants are those which cannot be decomposed by bacteria e.g., mercury, aluminium, lead, copper, DDT etc.
What are the sources of dissolved oxygen in water?
The following are the sources of dissolved oxygen in water
(i) Photosynthesis by aquatic plants
(ii) Due to direct contact of the water surface with air i.e., natural aeration
(iii) Mechanical aeration
What is the importance of measuring BOD of a water body?
Biological Oxygen Demand $(\mathrm{BOD})$ is the measure of level of pollution caused by organic biodegradable material. These biodegradable materials are decomposed by microorganism (bacteria) consuming dissolved oxygen.
Low value of BOD indicates that water contains less biodegradable material.
Why does water covered with excessive algae growth become polluted?
Presence of excessive algae growth shows that water contains a lot of phosphate due to inflow of fertilizers, etc., from the surroundings. The decomposition of algae growth produces bad smell and unattractive appearance making it unfit for recreational use like swimming, boating etc. Further, decrease in dissolved oxygen may be harmful for aquatic species like fishes etc.