In HNLC regions, adding which micronutrient can trigger phytoplankton blooms?

• A. Iron
• B. Nitrogen
• C. Phosphorus
• D. Copper

Answer: (A)

In many ocean regions described as High-Nutrient, Low-Chlorophyll, the growth of phytoplankton is limited not by the amount of nitrogen or phosphorus, but by the availability of iron, a trace micronutrient needed for key cellular processes like photosynthesis and nutrient uptake. When iron is scarce, phytoplankton can’t make full use of the abundant macronutrients, so growth stays low. Introducing iron removes this bottleneck, allowing phytoplankton to multiply and form blooms, which then use up the available nutrients and pigment production increases (chlorophyll). Nitrogen and phosphorus are often plentiful there, so adding them alone doesn’t reliably trigger blooms. Copper isn’t typically the limiting factor and can even be harmful at higher concentrations, so it wouldn’t be the nutrient that initiates blooms.