What is an algal bloom?

• A. A zone of no oxygen in water
• B. A decrease in algae due to nutrient shortage
• C. Rapid growth of algae caused by too many nutrients in water
• D. A type of sediment pollution

Answer: (C)

An algal bloom is a rapid growth of algae in a water body driven by an excess of nutrients, especially nitrogen and phosphorus, from sources like fertilizer runoff, wastewater, and detergents. When these nutrients are plentiful and conditions are warm with plenty of sunlight, algae can multiply quickly, often turning the water green or brown and making it look visibly crowded with plant-like matter. This sudden surge in algae can be problematic because when the blooms die and decompose, they consume a lot of dissolved oxygen, which can create low-oxygen conditions that stress or kill fish and other aquatic organisms. The occurrence of low oxygen is a consequence of the bloom’s aftermath rather than the bloom itself, so the term describes the rapid, nutrient-driven growth rather than the hypoxic zone that can result later. The other options describe different things: a decrease in algae due to nutrient shortage isn’t a bloom, and sediment pollution refers to solid particles in the water rather than the rapid growth of algae.