Which phenomenon results from elevated nutrient levels leading to algal growth that harms water quality?

• A. Oxygen-Depleted Zones
• B. Nutrient Runoff
• C. Saltwater Intrusion
• D. Harmful Algal Blooms

Answer: (D)

Elevated nutrients in a water body spark rapid growth of algae. When that growth becomes excessive and harms water quality—for example by releasing toxins or by consuming a lot of oxygen as the algae die and decompose—it’s called a Harmful Algal Bloom. The nutrients most often responsible are nitrogen and phosphorus from sources like fertilizers, agricultural runoff, and wastewater. The harm includes poorer water clarity, potential toxins that affect wildlife and people, and lower dissolved oxygen levels that can create dead zones. Oxygen-depleted zones are a downstream consequence of these blooms, not the initiating phenomenon itself. Nutrient runoff explains where the nutrients come from, while saltwater intrusion is not tied to this nutrient-driven bloom.