Questions: Zooplankton and Marine Food Web Structure

Question 1

Why do highly productive upwelling zones support major commercial fisheries while the vast subtropical gyres, despite their enormous area, support comparatively little harvestable fish biomass?

• A. Upwelling zones have warmer water, which fish prefer over cold gyre waters
• B. Subtropical gyres lack zooplankton entirely because salinity is too high
• C. Upwelling zones have shorter food chains, so more of the primary production energy reaches fish
• D. Gyres have too many large predators that consume fish before they can be harvested

Answer: (C)

Each trophic transfer loses roughly 80–90% of energy as metabolic heat. Upwelling zones support large diatoms eaten directly by large copepods and krill, which are then eaten by fish — only two or three steps from sunlight to harvestable fish. Subtropical gyres have longer chains: tiny picoplankton → microzooplankton → small copepods → larger zooplankton → fish, with each additional step multiplying the energy loss. Food chain length, not area, determines fish productivity.

Question 2

A warming ocean causes a shift from large diatom-dominated phytoplankton to smaller picoplankton in a previously productive region. What would you predict for fish biomass at higher trophic levels?

• A. Fish biomass increases because more phytoplankton organisms are now available at the base
• B. Fish biomass stays the same because total phytoplankton carbon is unchanged
• C. Fish biomass decreases because the food chain becomes longer and each additional step loses 80–90% of energy
• D. Fish biomass increases because picoplankton are more nutritious per unit mass than diatoms

Answer: (C)

Smaller phytoplankton cannot be eaten efficiently by large zooplankton — they require intermediate steps through microzooplankton, adding trophic levels and compounding energy loss. Even with the same primary production at the base, each additional link in the chain multiplies the energy lost to metabolism. A chain of five steps delivers far less energy to fish than a chain of three, even starting from the same amount of sunlight-fixed carbon.

Question 3

Zooplankton diel vertical migration actively transports carbon from surface waters to the deep ocean.

• T. True
• F. False

Answer: True

During diel vertical migration, zooplankton feed on phytoplankton in the surface layer at night, then descend hundreds of meters at dawn to avoid visual predators. At depth, they metabolize, respire, and excrete the carbon they consumed at the surface. This physically carries carbon from the sunlit surface layer to the deep ocean — a process called the 'biological pump.' It is one reason this nightly migration has major consequences for ocean carbon cycling and climate.

Question 4

Copepods are the dominant zooplankton in polar waters and serve as the primary food source for baleen whales.

• T. True
• F. False

Answer: False

Krill — small shrimp-like crustaceans 1–6 cm long — dominate polar waters and are the primary food of baleen whales, penguins, and many fish. Copepods (tiny crustaceans, 1–5 mm) are the most abundant zooplankton globally across most ocean regions, but in polar systems, krill fill that role at a larger body size. The distinction matters because krill's larger size makes them an efficient single-step link between phytoplankton and large marine mammals.

Question 5

Explain why food chain length matters for the amount of energy available to top marine predators like tuna and baleen whales.

Model answer: Each trophic transfer is highly inefficient — only about 10% of the energy at one level passes to the next (roughly 80–90% is lost as metabolic heat, respiration, and excretion). So a longer food chain means more multiplied inefficiencies. Starting with 1,000 units of primary production: a 3-step chain (phytoplankton → zooplankton → fish) delivers about 10 units to fish. A 5-step chain delivers only about 0.1 units. Top predators eating from shorter chains therefore have far more energy available, which is why productive upwelling zones with short chains support major fisheries while nutrient-poor gyres with long chains do not.

This principle — trophic efficiency — explains the fishery productivity paradox: enormous areas of open ocean can be biologically 'empty' for large animals despite having phytoplankton at the base, simply because the energy path to top predators is too long and lossy.