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Difficulty: MediumPollution Types, Causes, Effects, and Control

Arrange the following ecological events in the correct chronological sequence as they occur when a major crude oil spill affects a marine aquatic ecosystem.

  1. 1Formation of a dense layer of crude oil across the water surface
  2. 2Restriction of light penetration and inhibition of atmospheric gas exchange
  3. 3Depletion of dissolved oxygen due to reduced photosynthesis and microbial decomposition of hydrocarbons
  4. 4Widespread mortality of fish, marine invertebrates, and benthic organisms

Answer

The correct chronological sequence is: (1) Formation of a dense layer of crude oil across the water surface, (2) Restriction of light penetration and inhibition of atmospheric gas exchange, (3) Depletion of dissolved oxygen due to reduced photosynthesis and microbial decomposition of hydrocarbons, and (4) Widespread mortality of fish, marine invertebrates, and benthic organisms.
The correct sequence begins with the physical discharge of crude oil forming a floating surface slick. This slick immediately blocks sunlight penetration and impedes atmospheric oxygen diffusion into the water. As a result, photosynthetic oxygen production drops while aerobic bacteria rapidly consume available dissolved oxygen to break down the oil. Finally, the severe drop in dissolved oxygen and toxic effects cause widespread asphyxiation and death among aquatic organisms.

Step-by-Step Solution

1
Identify the initial physical impact of the pollutant release
Crude oil forms an insoluble floating slick over the water surface.
Crude oil is less dense than water and hydrophobic, causing it to spread out across the air-water interface.
2
Determine the immediate physical disruption to ecosystem processes
Sunlight penetration is blocked and gaseous exchange between the atmosphere and water is restricted.
The opaque surface film acts as a mechanical barrier preventing solar energy absorption by phytoplankton and gaseous diffusion.
3
Evaluate the chemical changes within the water column
Dissolved oxygen levels drop significantly.
Photosynthesis by primary producers declines due to lack of light, while aerobic decomposers multiply and consume oxygen to break down oil hydrocarbons.
4
Determine the final biological effect on the aquatic community
Aquatic fauna experience widespread suffocation and death.
Prolonged dissolved oxygen depletion (hypoxia) combined with hydrocarbon toxicity causes asphyxiation in fish and marine invertebrates.

Key Concept

Aquatic Degradation from Oil Spills
Estimated Time:1m 30s
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Topics

Pollution Types, Causes, Effects, and Control

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