Questions: Phytoplankton Productivity and Limiting Factors

Question 1

A region of the open ocean receives abundant sunlight year-round but has very low phytoplankton productivity. The most likely explanation is:

• A. Water temperatures are too high for phytoplankton photosynthesis to function efficiently
• B. Intense grazing by zooplankton removes phytoplankton as fast as they grow
• C. Strong thermal stratification prevents nutrient-rich deep water from reaching the sunlit surface
• D. Light penetrates too deeply, diluting its intensity below the threshold for net photosynthesis

Answer: (C)

This describes the subtropical gyres — warm, clear, and sunlit, yet biological deserts. Strong thermal stratification creates a permanent pycnocline that decouples the nutrient-depleted surface from the nutrient-rich deep ocean. Light and temperature are not the limiting factors here; nutrients are. Option B (grazing) can also suppress biomass, but the primary reason for low productivity in subtropical gyres is nutrient depletion driven by stratification.

Question 2

Scientists add small amounts of iron to a patch of the Southern Ocean and observe a massive phytoplankton bloom. This result most directly demonstrates that:

• A. The Southern Ocean was previously light-limited, and iron improved light-harvesting efficiency
• B. Iron was the primary limiting nutrient, even though it is required only in trace quantities
• C. Iron warms the surface water, accelerating phytoplankton metabolism
• D. Iron suppresses zooplankton grazing, allowing standing stocks to accumulate

Answer: (B)

The iron fertilization experiments (e.g., IRONEX, SOIREE) directly confirmed the iron hypothesis: iron is the limiting micronutrient in large areas of the Southern Ocean, equatorial Pacific, and subarctic Pacific. Despite being needed only in trace amounts (for photosynthetic enzymes), its absence caps productivity across enormous regions. More light, more warmth, or less grazing are not what was missing — a single trace element was. This is a classic illustration of Liebig's law of the minimum: the scarcest required resource sets the ceiling.

Question 3

In the subtropical gyres, phytoplankton productivity is low despite ample light, because strong thermal stratification prevents nutrient replenishment from deep water.

• T. True
• F. False

Answer: True

Correct. The subtropical gyres are warm, highly stratified, and sunlit — but chronically nutrient-starved. The thermocline acts as a physical barrier preventing upward mixing of the cold, nutrient-rich deep water. Without nitrogen, phosphorus, and iron, phytoplankton cannot grow even in ideal light conditions. These regions are the ocean's 'blue deserts' — clear and beautiful but nearly devoid of biological productivity.

Question 4

Warmer ocean temperatures consistently increase marine phytoplankton productivity because higher temperatures accelerate photosynthetic reaction rates.

• T. True
• F. False

Answer: False

This is a common but incorrect generalization. While warmer temperatures do accelerate metabolic rates up to a thermal optimum, they also strengthen thermal stratification, which reduces the upward supply of nutrients from depth. In much of the ocean, this tradeoff means warming reduces productivity by cutting off nutrient resupply — the opposite of what intuition suggests. Predicted ocean warming under climate change is expected to expand nutrient-poor stratified zones and reduce overall marine primary productivity in many regions.

Question 5

Why are coastal upwelling zones — such as the Peruvian coast or the Antarctic divergence — among the most biologically productive regions in the ocean?

Model answer: Upwelling brings cold, nutrient-rich water from depth to the sunlit surface layer, simultaneously supplying nitrogen, phosphorus, iron, and other nutrients that are chronically depleted in the euphotic zone. With both light and nutrients available at the same time, phytoplankton can grow rapidly, supporting dense zooplankton populations and the entire food web above them.

Productivity requires both energy (light) and raw materials (nutrients) simultaneously in the same location. Upwelling solves the nutrient limitation problem that makes most of the open ocean a biological desert. The Peruvian upwelling system, driven by wind-induced Ekman transport, is among the most productive on Earth and supports enormous fisheries — a direct consequence of nutrient delivery from depth.