Water Quality and Pollution

The chemicals released in water during industrial processes include arsenic, lead and fluorides. They contaminate the water and cause toxicity.

Statement 1 is incorrect. BOD - Standard method for indirect measurement of the amount of organic pollution in a sample of water. Thus, high BOD means high pollution. Statement 2 is also incorrect.             Eutrophication - The process by which a water body acquires high concentration of nutrients, especially phosphates and nitrates. It promotes growth of algae and the decomposing organisms deplete the available oxygen in water. Thus, there is a decrease in concentration of oxygen and increase in concentration of carbon dioxide.

 Aquatic animals use dissolved oxygen in water for respiration.

This is the correct option. The contamination of river water can be found by the presence of coliform bacteria in the river water and by measuring the pH of the river water using universal indicator paper.

Excessive growth of algae gives a green appearance to ponds. It is caused by excessive quantities of chemicals which act as a source of nutrients.

Eutrophication is when the environment becomes enriched with nutrients. It causes algal blooms and reduced O₂ content in water bodies.

Cyclo morphosis is the phenomenon in which morphological alterations in the structure of organisms take place in response to variations in temperature.

Primary productivity is high in eutrophic water bodies. Due to excessive nutrients, especially nitrogen and phosphorus, these water bodies are able to support an abundance of aquatic plants, thereby leading to a high primary productivity. Because of abundance of aquatic plants and algae, the oxygen availability decreases. When organisms (like aquatic plants and algae) die, they consume the oxygen in the body of water, thereby creating the state of hypoxia.

Eutrophication is the increase in addition of nutrients to freshwater or marine systems. It leads to increase in plant and algae growth (biological production) and often to undesirable changes in ecosystem structure and function.

Eutrophication is the process where water bodies become overly enriched with nutrients (especially nitrogen and phosphorus), leading to excessive algae growth and oxygen depletion. None of the given options A, B, or C correctly describe this process.

Eutrophic lakes are characterized by high nutrient content (especially nitrogen and phosphorus), leading to excessive algal and plant growth. This results in rich accumulation of organic matter in the sediments from dead organisms. Option B describes oligotrophic lakes, C describes dystrophic lakes, and D refers to artificial reservoirs rather than natural eutrophic lakes.

The correct answer is 'Nitrates and Phosphates'. Eutrophication is the process where water bodies become overly enriched with minerals and nutrients, leading to excessive plant growth. Nitrates and phosphates from agricultural runoff and sewage are the primary nutrients causing this. 'Silt load' causes turbidity, 'sewage' contains nutrients but the question asks what accumulation causes eutrophication (the nutrients themselves), and 'vegetation' is the result not the cause.

Polishing ponds are similar to oxidation ponds but receive effluent from an oxidation pond or from a plant with an extended mechanical treatment. These ponds are dominated by zooplankton.

Chlorella is very popular in Japan. It is also used as a nutritional supplement with possible effects on metabolic rate. It can reduce mercury levels in humans by chelation of the mercury to the cell wall of the organism.

Chlamydomonas, Scenedesmus, Chlorella, Pondorhina, Euridina, etc., are living in sewage water. They are mainly useful to clean the water by releasing oxygen.