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Difficulty: Very hardCausal Detail Extraction

Read the following passage and arrange the ecological events in the exact causal sequence described in the text, from the initial trigger to the final outcome.

The formation of marine dead zones is an ecological cascade initiated when agricultural runoff introduces excessive nitrogen and phosphorus into coastal waters. This nutrient loading precipitates a sudden and explosive proliferation of phytoplankton. Once these sprawling algal blooms exhaust the available surface nutrients, they perish en masse and precipitate to the benthic layer. This massive accumulation of organic detritus catalyzes a rapid expansion of localized bacterial populations. During the ensuing decomposition process, the intense cellular respiration of these multiplying microbes actively strips the surrounding water column of dissolved oxygen. The resulting hypoxic environment severely disrupts the local ecosystem; mobile organisms are forced to evacuate the region, while sessile species suffer widespread asphyxiation, ultimately rendering the habitat barren.

  1. 1Agricultural nutrient runoff triggers an explosive surge in coastal phytoplankton populations.
  2. 2Sprawling algal blooms exhaust available surface nutrients, causing them to perish and sink.
  3. 3Localized bacterial populations expand rapidly to decompose the accumulating organic detritus.
  4. 4Intense cellular respiration from multiplying microbes strips the water of dissolved oxygen.
  5. 5Hypoxic conditions severely disrupt the ecosystem, causing widespread asphyxiation of sessile species.

Answer

The causal sequence begins with nutrient runoff causing phytoplankton proliferation, followed by the exhaustion of nutrients causing the algae to perish and sink. This accumulation of dead matter leads to rapid bacterial expansion, which then depletes dissolved oxygen through respiration. Finally, the resulting hypoxia causes the asphyxiation of marine life.
The passage outlines a clear, unbroken five-step ecological cascade: nutrient loading causes explosive algal blooms; these blooms exhaust their nutrients and die; the resulting dead organic matter fuels rapid bacterial growth; bacterial respiration during decomposition severely depletes oxygen; and this oxygen depletion causes widespread asphyxiation and ecosystem disruption. The correct order perfectly tracks this explicitly stated chain reaction.

Step-by-Step Solution

1
Identify the absolute initial cause stated in the passage.
Agricultural runoff introducing excessive nutrients is identified as precipitating a sudden proliferation of phytoplankton.
Establishing the starting point of the ecological cascade is necessary to sequence subsequent events correctly.
2
Locate the direct consequence of the massive phytoplankton proliferation.
The passage states that once the sprawling blooms exhaust the available nutrients, they perish and precipitate to the benthic layer.
This establishes the second link in the chain, moving from algal life to mass death.
3
Determine the effect of the dead algae accumulating on the ocean floor.
The massive accumulation of organic detritus catalyzes a rapid expansion of localized bacterial populations.
This connects the death of the phytoplankton directly to the biological response of the decomposing bacteria.
4
Trace the environmental consequence of the expanding bacterial populations.
The passage explains that intense cellular respiration from these multiplying microbes actively strips the surrounding water of dissolved oxygen.
This links the microbial decomposition process to the physical change in the water's chemistry.
5
Identify the final ecological outcome of the oxygen depletion.
The resulting hypoxic environment severely disrupts the ecosystem, causing sessile species to suffer widespread asphyxiation.
This completes the multi-step causal sequence leading to the formation of a barren dead zone.

Key Concept

Causal Detail Extraction in complex texts requires mapping an extended, multi-step chain of events where the effect of one sentence serves as the explicit cause for the next.
Estimated Time:2m 30s
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Causal Detail Extraction

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