What is eutrophication and what nutrient is typically the limiting factor in freshwater systems?

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At the core, eutrophication is about how nutrient availability drives aquatic production. When a water body receives excess nutrients—especially nitrogen and phosphorus—algae can grow rapidly, producing blooms. As these blooms die and decompose, the process consumes dissolved oxygen, often leading to oxygen depletion that stresses or kills other aquatic organisms.

In freshwater systems, the limiting nutrient—the one whose availability most controls growth—is usually phosphorus. However, some lakes or rivers can be nitrogen-limited, depending on their specific nutrient balance and sources. So the best description is that eutrophication is nutrient-driven algal blooms and oxygen depletion, with phosphorus often being the limiting nutrient, though nitrogen can also limit in some systems.

The other statements don’t fit: eutrophication isn’t a reduction in nutrients from dilution, it isn’t about heavy metal deposition, and it doesn’t increase water clarity—algal blooms typically reduce clarity.