HNLC regions are ocean areas characterized by which combination?

• A. High Nutrients and Low Chlorophyll
• B. High Chlorophyll and Low Nutrients
• C. Low Nutrients and High Temperature
• D. High Temperature and High Nutrients

Answer: (A)

HNLC regions are ocean areas where nutrients are abundant, but phytoplankton biomass remains low. The reason is that a key necessary nutrient for phytoplankton—iron—is in short supply there. Iron is essential for photosynthesis and for using the available macronutrients like nitrate and phosphate. When iron is scarce, phytoplankton can’t fully utilize those nutrients, so chlorophyll levels stay low even though nutrients are plentiful. In some experiments, adding iron to these waters triggers a bloom, confirming that the limiting factor isn’t the macronutrients but the trace iron.

So the combination described is high nutrients with low chlorophyll. The other patterns would describe different ocean conditions (for example, high chlorophyll when nutrients are abundant, or low nutrients when productivity is typically low), but they don’t define HNLC.