Seasonal dead zones in certain coastal waters can be traced to nutrient-driven algal blooms. Which change would most likely trigger rapid phytoplankton growth?

• A. Increase nutrients
• B. Decrease competition
• C. Increase predation
• D. Decrease light level

Answer: (A)

Nutrient availability is what governs how fast phytoplankton can grow. In coastal waters, nitrogen and phosphorus often limit photosynthetic algae. When these nutrients become more abundant, phytoplankton can divide more quickly, leading to a rapid bloom. Later, when the bloom dies and the organic matter decomposes, microbes consume a lot of the dissolved oxygen, creating a hypoxic or “dead” zone.

So increasing nutrients is the change most likely to trigger fast phytoplankton growth because it removes a limiting factor and allows photosynthesis and reproduction to accelerate. Decreasing competition could help some organisms, but blooms in nutrient-rich coastal waters are typically driven by more available nutrients. Decreasing light would slow growth, and increasing predation would suppress phytoplankton numbers rather than promote a bloom.