Correct option is C
The correct answer is (c).
The excessive influx of essential limiting nutrients, specifically nitrates and phosphates, into aquatic ecosystems triggers a primary biological process known as eutrophication.
This massive nutrient enrichment disrupts the ecological balance, resulting in the rapid and uncontrolled proliferation of phytoplankton and algae on the water surface.
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- Cultural eutrophication is dramatically accelerated by human activities, predominantly through agricultural fertilizer runoff, untreated sewage discharge, and industrial waste.
- The dense algal blooms block vital sunlight penetration, destroying submerged aquatic vegetation (SAV) which serve as critical habitats for marine life.
- When these massive algal populations inevitably die, their subsequent microbial decomposition consumes enormous quantities of dissolved oxygen (BOD spikes).
- This depletion creates vast aquatic "dead zones" where the survival of aerobic organisms, like fish and benthic invertebrates, becomes impossible.
- Toxic algal blooms (cyanobacteria) associated with eutrophication also release potent hepatotoxins and neurotoxins, posing severe risks to human and livestock health.
- Phosphorus is generally considered the primary limiting nutrient in freshwater lakes, whereas nitrogen limits biological productivity in marine and estuarine ecosystems.
- Mitigation strategies heavily emphasize constructing riparian buffer zones, upgrading wastewater treatment infrastructure, and optimizing agricultural nutrient management.
- Option A: Hypoxia strictly denotes a state of severely reduced dissolved oxygen concentrations (typically below 2-3 mg/L), which is a secondary consequence or symptom resulting from eutrophication.
- Option B: Anoxia represents the absolute complete absence of oxygen in the water column; like hypoxia, it is a catastrophic end-stage result of the eutrophication process rather than the process itself.
- Option D: Solifluction is a geological phenomenon involving the slow, downward flow of water-saturated soil over an impermeable layer (like permafrost), entirely unrelated to aquatic nutrient dynamics.
- Lake oligotrophy represents the exact opposite ecological state, characterized by naturally low nutrient levels, deep light penetration, and high dissolved oxygen.
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