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Difficulty: MediumPollution Types, Causes, Effects, and Control

Excessive runoff of nitrate and phosphate fertilizers from agricultural lands into a nearby river results in rapid algal proliferation. Which of the following sequence of events directly leads to the mass mortality of fish in this aquatic ecosystem?

  1. Decomposition of dead algae by aerobic bacteria depletes dissolved oxygen levels in the water.Answer
  2. B
    Toxins accumulate in increasing concentrations at higher trophic levels due to thermodynamic energy loss.
  3. C
    Denitrifying soil bacteria convert the excess aquatic nitrates into toxic ammonia gas.
  4. D
    Increased water turbidity permanently denatures the photosynthetic enzymes of aquatic plants.

Answer

Decomposition of dead algae by aerobic bacteria depletes dissolved oxygen levels in the water.
The correct answer correctly describes eutrophication: excess agricultural fertilizer runoff leads to rapid algal blooms. When these algae die, aerobic decomposing bacteria consume the available dissolved oxygen through cellular respiration, leading to severe hypoxia and widespread fish mortality.

Step-by-Step Solution

1
Identify the primary pollutant and initial biological reaction.
Nitrate and phosphate runoff cause nutrient enrichment (eutrophication), triggering an algal bloom.
Nitrates and phosphates act as limiting nutrients for algae in freshwater habitats.
2
Trace the ecological progression as algae reach the end of their life cycle.
Algal mats die off rapidly, providing abundant organic matter for saprophytic aerobic bacteria.
High mortality rates among dense algal blooms increase organic substrate available for decomposers.
3
Determine the impact of bacterial activity on aquatic life.
Aerobic respiration by decomposers rapidly increases Biological Oxygen Demand (BOD) and depletes dissolved oxygen, suffocating fish.
Fish require adequate dissolved oxygen in water for cellular respiration and survival.

Key Concept

Eutrophication and Biological Oxygen Demand (BOD)
Estimated Time:1m 15s
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Topics

Pollution Types, Causes, Effects, and Control

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