Questions: Marine Nutrient Cycling and Productivity Limitation

Question 1

A region of the Southern Ocean has surface nitrate concentrations 10× higher than typical North Atlantic surface waters, yet phytoplankton biomass remains low. What is the most likely explanation?

• A. The water is too cold for phytoplankton to grow at high latitudes
• B. Nitrogen is not the limiting nutrient in this region — iron, which is scarce in the Southern Ocean, limits productivity despite the abundant nitrate
• C. Excessive grazing by zooplankton prevents phytoplankton biomass from accumulating even when nutrients are available
• D. The deep mixed layer prevents phytoplankton from staying in the euphotic zone long enough to use the nitrate

Answer: (B)

The Southern Ocean is the classic example of a 'High Nutrient Low Chlorophyll' (HNLC) region. Nitrogen and phosphorus are abundant (upwelling brings deep water rich in these nutrients), yet phytoplankton biomass is surprisingly low. The limiting factor is iron — a trace metal needed for the enzyme systems of photosynthesis (particularly for the photosynthetic electron transport chain). The Southern Ocean is far from continental dust sources, so iron inputs are minimal. Iron fertilization experiments confirmed this: adding iron to Southern Ocean water produces bloom conditions rapidly. This demonstrates that Liebig's Law must be applied regionally, not globally.

Question 2

Why do subtropical ocean gyres — the centers of the great circular current systems — support far less primary productivity than upwelling zones despite being warm and well-lit?

• A. Subtropical gyres are too warm, and phytoplankton require cold water to photosynthesize efficiently
• B. In subtropical gyres, surface water converges and sinks (downwelling), actively pushing nutrients away from the euphotic zone and preventing replenishment from the nutrient-rich deep
• C. Subtropical gyres have high salinity that is toxic to most phytoplankton species
• D. Subtropical gyres are far from continental shelves and therefore receive no river-borne nutrients

Answer: (B)

Subtropical gyres are defined by convergent surface flow (Ekman transport pushes surface water toward the gyre center) and downwelling. This circulation actively subducts surface water and its nutrients downward, away from the euphotic zone. The result is a stratified water column with a strong permanent thermocline that prevents nutrient-rich deep water from mixing upward. Despite ample sunlight and warm temperatures, phytoplankton are starved of dissolved nitrogen, phosphorus, and other nutrients. These 'blue-water deserts' are oligotrophic precisely because the physical circulation works against nutrient replenishment — the opposite of upwelling zones.

Question 3

Nitrogen is the primary limiting nutrient for marine primary productivity in most regions of the open ocean.

• T. True
• F. False

Answer: False

While nitrogen (as nitrate or ammonium) limits productivity in much of the open ocean, this is not universal. In High Nutrient Low Chlorophyll (HNLC) regions — the Southern Ocean, the equatorial Pacific, and the subarctic Pacific — iron is the limiting nutrient because nitrogen is abundant but iron is vanishingly scarce. In some coastal and estuarine environments, phosphorus can be limiting. Silica specifically limits diatoms, which require it for their frustules. Liebig's Law means productivity is constrained by whichever nutrient is in shortest supply relative to biological demand in that specific location — and that nutrient varies by ocean region and season.

Question 4

Nutrient concentrations in the open ocean typically show a characteristic vertical profile where surface waters are nutrient-depleted and deep waters are nutrient-rich.

• T. True
• F. False

Answer: True

This vertical gradient is the fundamental consequence of the biological pump. In the euphotic zone (upper ~200 m), phytoplankton consume dissolved nutrients and incorporate them into organic matter. When organisms die, sink as particles, get eaten and excreted as fecal pellets, or form marine snow, the organic matter descends into deeper water where bacteria decompose it (remineralization), releasing nutrients back into dissolved form. The result is a characteristic 'nutrient minimum at surface, maximum at depth' profile seen in virtually all open ocean nutrient datasets. The only exceptions are in upwelling zones where deep water is actively brought to the surface, and in productive coastal regions with strong horizontal nutrient inputs.

Question 5

Explain the 'biological pump' and how it creates the vertical nutrient gradient observed in the open ocean.

Model answer: The biological pump is the process by which the ocean's surface biological community converts dissolved inorganic nutrients into organic matter via photosynthesis and then exports that organic matter to depth. In the euphotic zone, phytoplankton take up dissolved nitrate, phosphate, and other nutrients and fix them into organic carbon compounds. When phytoplankton die, are eaten (with feces sinking), or aggregate into dense marine snow particles, this organic matter sinks out of the sunlit zone. In deeper waters, bacteria decompose (remineralize) the sinking particles, releasing the nutrients back into dissolved form. Because the biological pump continuously strips nutrients from the surface (where light is available) and re-releases them at depth (where light cannot reach), a stable vertical gradient develops: low nutrients at the surface, high nutrients at depth. This gradient is what makes deep-water upwelling so consequential — it is the only physical mechanism that can return those deep nutrients to where phytoplankton can use them.

The biological pump also sequesters carbon: the organic matter that sinks and is remineralized at depth represents carbon that was removed from the atmosphere-surface ocean system. Changes in pump efficiency due to ocean warming, acidification, or altered circulation could significantly affect how much CO₂ the ocean can absorb — one reason understanding marine nutrient cycling matters for climate projections.