Water Pollution (BOD, Algal Blooms, Eutrophication, and Biomagnification)

Attempt the quiz above first, then review every question with its correct answer and a short explanation below.

1. Q1.According to NCERT Class 12 Biology, domestic sewage contains what percentage of impurities, making it completely unfit for human use?

   • A.10 percent
   • B.1 percent
   • C.0.1 percent✓ Correct Answer
   • D.0.01 percent

   Explanation: NCERT Class 12 Biology Chapter 16 states: 'A mere 0.1 per cent impurities make domestic sewage unfit for human use.' These impurities consist of suspended solids, colloidal materials, and dissolved organic/inorganic nutrients.

2. Q2.Which of the following parameters is directly proportional to the amount of biodegradable organic matter present in sewage water?

   • A.Dissolved Oxygen (DO)
   • B.Biochemical Oxygen Demand (BOD)✓ Correct Answer
   • C.Salinity
   • D.pH level

   Explanation: BOD measures the rate of oxygen uptake by microbes in a water sample. It is directly proportional to the amount of biodegradable organic matter; the more organic waste present, the more oxygen is consumed by decomposers, resulting in a higher BOD (NCERT).

3. Q3.When domestic sewage is discharged into a river, what happens immediately to the Dissolved Oxygen (DO) levels and Biochemical Oxygen Demand (BOD) at the discharge point?

   • A.BOD decreases sharply, while DO increases.
   • B.BOD rises sharply, while DO plummets, causing fish mortality.✓ Correct Answer
   • C.Both DO and BOD remain unchanged.
   • D.BOD and DO both increase simultaneously.

   Explanation: NCERT Class 12 Biology details: 'At the point of sewage discharge, there is a sharp decline in dissolved oxygen because microbes consuming organic matter use up the oxygen. Consequently, BOD increases sharply at this point, leading to fish kills.'

4. Q4.The excessive growth of planktonic (free-floating) algae in nutrient-rich water bodies, imparting a distinct color to the water and deteriorating its quality, is known as:

   • A.Biomagnification
   • B.Algal Bloom✓ Correct Answer
   • C.Eutrophication
   • D.Bioaccumulation

   Explanation: NCERT Chapter 16 notes: 'Presence of large amounts of nutrients in waters also causes excessive growth of planktonic (free-floating) algae, called an algal bloom. This imparts a distinct colour to the water bodies and causes deterioration of the water quality and fish mortality.'

5. Q5.Which invasive aquatic weed, introduced in India for its beautiful purple flowers, is famously called the 'Terror of Bengal' because it drains oxygen from water bodies and kills fish?

   • A.Pistia
   • B.Eichhornia crassipes (Water Hyacinth)✓ Correct Answer
   • C.Hydrilla
   • D.Vallisneria

   Explanation: NCERT Chapter 16 states: 'You may have heard about the scourge of the water hyacinth (Eichhornia crassipes), the world’s most problematic aquatic weed, also called "Terror of Bengal". It grows abundantly in eutrophic water bodies, clogging them and draining oxygen, leading to death of fish.'

6. Q6.The phenomenon where the concentration of a non-biodegradable toxicant increases at successive higher trophic levels in a food chain is defined as:

   • A.Eutrophication
   • B.Biomagnification (Biological Magnification)✓ Correct Answer
   • C.Bioaccumulation
   • D.Siltation

   Explanation: Biomagnification refers to increase in concentration of the toxicant at successive trophic levels. This happens because a toxic substance accumulated by an organism cannot be metabolized or excreted, and is thus passed on to the next higher trophic level (NCERT).

7. Q7.Which of the following chemical substances are well-known to undergo biomagnification in aquatic food chains?

   • A.Nitrates and Phosphates
   • B.DDT and Mercury✓ Correct Answer
   • C.Sodium and Calcium
   • D.Glucose and Starch

   Explanation: NCERT Class 12 Biology explicitly mentions DDT and Mercury as classic examples of toxic substances that undergo biomagnification in aquatic ecosystems because they cannot be metabolized or excreted by organisms.

8. Q8.In an aquatic food chain showing DDT biomagnification, if the water has a DDT concentration of 0.003 ppb (parts per billion), what concentration can it reach in fish-eating birds at the top of the food chain?

   • A.0.5 ppm
   • B.2 ppm
   • C.25 ppm✓ Correct Answer
   • D.0.04 ppm

   Explanation: NCERT Chapter 16 lists the biomagnification values of DDT: Water (0.003 ppb) -> Zooplankton (0.04 ppm) -> Small fish (0.5 ppm) -> Large fish (2 ppm) -> Fish-eating birds (25 ppm). The concentration increases by nearly 10 million times.

9. Q9.How does high concentration of DDT accumulated in fish-eating birds due to biomagnification lead to a decline in their population?

   • A.By directly poisoning adult birds and making them blind.
   • B.By disturbing calcium metabolism, causing thinning of eggshells and their premature breaking, leading to decline in bird populations.✓ Correct Answer
   • C.By turning birds into herbivores.
   • D.By preventing birds from flying.

   Explanation: NCERT Class 12 Biology states: 'High concentrations of DDT disturb calcium metabolism in birds, which causes thinning of egg shell and their premature breaking, eventually causing decline in bird populations.'

10. Q10.The natural aging of a lake by biological enrichment of its nutrients (like nitrogen and phosphorus) over geological time is known as:

    • A.Biomagnification
    • B.Eutrophication✓ Correct Answer
    • C.Siltation
    • D.Bioaccumulation

    Explanation: NCERT Chapter 16 defines Eutrophication: 'Eutrophication is the natural aging of a lake by nutrient enrichment of its water.' Natural runoff brings nitrogen and phosphorus, promoting plant and animal growth, leading to organic accumulation on the lake bed.

11. Q11.When human activities (like sewage discharge and agricultural runoff containing fertilizers) dramatically accelerate the natural aging process of a lake, it is called:

    • A.Natural Eutrophication
    • B.Cultural or Accelerated Eutrophication✓ Correct Answer
    • C.Reverse Eutrophication
    • D.Heavy metal biomagnification

    Explanation: NCERT states: 'Pollutants from man's activities like effluents from the industries and homes can radically accelerate the aging process. This phenomenon is called Cultural or Accelerated Eutrophication.' Silt and organic matter pile up, shallowing and heating the lake.

12. Q12.Which of the following heavy metals in groundwater is a major cause of 'Black Foot Disease' and is highly toxic in states like West Bengal and Bihar?

    • A.Cadmium
    • B.Arsenic✓ Correct Answer
    • C.Lead
    • D.Mercury

    Explanation: Arsenic contamination in groundwater is a severe problem in the Ganges delta (West Bengal and Bangladesh). Chronic exposure to arsenic causes skin lesions, peripheral vascular disease leading to gangrene ('black foot disease'), and cancers.

13. Q13.Which famous neurological disease was caused in Japan by eating fish contaminated with methylmercury released from industrial wastewater?

    • A.Itai-itai disease
    • B.Minamata disease✓ Correct Answer
    • C.Black Foot disease
    • D.Blue Baby syndrome

    Explanation: Minamata disease is a neurological syndrome caused by severe mercury poisoning. It was first discovered in Minamata City, Japan, in 1956, due to release of methylmercury in industrial wastewater which bioaccumulated in local shellfish and fish consumed by the population.

14. Q14.The painful bone and joint disease 'Itai-Itai' (ouch-ouch disease) was first documented in Japan due to environmental poisoning by which heavy metal?

    • A.Mercury
    • B.Cadmium✓ Correct Answer
    • C.Lead
    • D.Arsenic

    Explanation: Itai-itai disease was caused by Cadmium poisoning due to mining activities in Toyama Prefecture, Japan. Cadmium accumulation in bones causes osteomalacia (softening of bones) and kidney failure, making bones extremely fragile and painful when broken.

15. Q15.Which of the following legislative Acts was passed by the Government of India in 1974 to safeguard our water resources from pollution?

    • A.The Air Act
    • B.The Water (Prevention and Control of Pollution) Act✓ Correct Answer
    • C.The Environmental Protection Act
    • D.The Wildlife Protection Act

    Explanation: NCERT Class 12 Biology Chapter 16 states: 'The Government of India has passed the Water (Prevention and Control of Pollution) Act, 1974 to safeguard our water resources.'

16. Q16.Why does the accumulation of organic waste in a lake lead to oxygen depletion, even if no toxic chemicals are discharged?

    • A.Organic waste converts oxygen to nitrogen directly.
    • B.Aerobic decomposers (bacteria and fungi) consume dissolved oxygen rapidly as they multiply to break down the organic matter.✓ Correct Answer
    • C.Organic waste blocks sunlight from dissolving oxygen.
    • D.Fish stop producing oxygen.

    Explanation: Aerobic bacteria require oxygen to digest organic matter. When organic wastes pile up, bacterial populations increase dramatically, consuming dissolved oxygen (DO) faster than it can dissolve from the air, causing hypoxia (lack of oxygen).

17. Q17.Biomagnification differs from Bioaccumulation because biomagnification involves:

    • A.Only a single organism storing toxins over its lifetime.
    • B.The progressive increase in toxin concentration at higher levels of the food chain across trophic levels.✓ Correct Answer
    • C.The excretion of toxins by the organism.
    • D.No trophic interactions.

    Explanation: Bioaccumulation is the buildup of a toxicant in an individual organism's tissue over time (rate of absorption exceeds rate of excretion). Biomagnification is the cumulative build-up across trophic levels, where predators get higher doses by eating pre-concentrated prey.

18. Q18.What is the biological term for the visible clumps of aerobic bacteria associated with fungal filaments to form mesh-like structures, used in secondary sewage treatment to reduce BOD?

    • A.Lichens
    • B.Flocs✓ Correct Answer
    • C.Mycorrhizae
    • D.Sludge

    Explanation: During secondary sewage treatment, the primary effluent is passed into large aeration tanks where it is constantly agitated. This allows vigorous growth of useful aerobic microbes into 'flocs' – masses of bacteria associated with fungal filaments to form mesh-like structures (NCERT Chapter 10).

19. Q19.Methemoglobinemia, commonly known as 'Blue Baby Syndrome' in infants, is caused by drinking groundwater contaminated with high levels of which chemical?

    • A.Fluoride
    • B.Nitrate✓ Correct Answer
    • C.Arsenic
    • D.Lead

    Explanation: Blue baby syndrome is caused by high nitrate levels in drinking water. In the infant gut, nitrates are converted to nitrites, which bind to hemoglobin to form methemoglobin. Methemoglobin cannot transport oxygen, causing the baby's skin to turn blue due to oxygen starvation.

20. Q20.High concentration of Fluoride in drinking water causes which of the following physiological disorders in humans?

    • A.Black foot disease
    • B.Skeletal fluorosis (joint deformation) and mottling of teeth✓ Correct Answer
    • C.Corneal inflammation
    • D.Minamata syndrome

    Explanation: Fluoride is beneficial in trace amounts (1 ppm) for dental health. However, excess fluoride (>1.5 ppm) causes dental fluorosis (yellow/brown mottling of teeth) and chronic exposure leads to skeletal fluorosis, characterized by calcium deposition in bones, joint stiffness, and severe pain.