Questions: Marine Food Webs and Trophic Structure

Question 1

Two ocean regions each produce 10,000 units of carbon via photosynthesis annually. Region A has a 3-step food chain (phytoplankton → zooplankton → fish). Region B has a 5-step food chain. Assuming 10% trophic efficiency, how much more energy reaches the top predator in Region A than Region B?

• A. The same — total primary production determines total biomass at every level
• B. 10× more — each extra trophic step multiplies available energy by 10
• C. 100× more — two extra trophic steps each lose 90%, compounding to 1% of Region A's top-level energy
• D. 2× more — each trophic level removes a fixed amount, not a percentage

Answer: (C)

At 10% trophic efficiency, each step retains 10% of the previous level's energy. Region A (3 steps): 10,000 × 0.1 × 0.1 = 10 units at the top. Region B (5 steps): 10,000 × 0.1⁴ = 0.1 units at the top. Region A has 100× more energy available to top predators. This is why the world's most productive fisheries are in upwelling zones with short food chains, not in vast oligotrophic gyres with long chains.

Question 2

What is the role of the microbial loop in marine ecosystems?

• A. It decomposes organic matter into inorganic nutrients that sink to the seafloor permanently
• B. It recycles dissolved organic carbon through bacteria and protists back into the classical food web
• C. It short-circuits the food web by converting phytoplankton directly into fish biomass
• D. It is a dead-end pathway that consumes but does not return energy to higher trophic levels

Answer: (B)

The microbial loop is not a dead end. When phytoplankton and other organisms release dissolved organic matter, bacteria consume it and convert it back to particulate biomass. These bacteria are then grazed by protists, which are eaten by larger zooplankton — funneling dissolved carbon back into the classical food web. In nutrient-poor oligotrophic waters, the microbial loop can process more carbon than the direct phytoplankton-to-zooplankton pathway.

Question 3

The vast oligotrophic open ocean gyres produce more total fish biomass than coastal upwelling zones because they cover a much larger ocean area.

• T. True
• F. False

Answer: False

Coverage area does not compensate for the combination of low primary productivity AND long food chains in oligotrophic gyres. These regions have sparse phytoplankton (starting energy is low) and support long food chains with many trophic steps (compounding energy loss). Upwelling zones have high primary productivity and typically short food chains (phytoplankton → zooplankton → fish), making them far more productive per unit area despite covering a fraction of the ocean. The world's major commercial fisheries concentrate in upwelling zones and continental shelves.

Question 4

Dissolved organic matter released by phytoplankton (through excretion and cell lysis) eventually re-enters the classical food web through bacterial processing.

• T. True
• F. False

Answer: True

This is the microbial loop: bacteria consume dissolved organic matter and convert it back into particulate biomass, which is then grazed by protists (nanoflagellates, ciliates), which are in turn consumed by larger zooplankton that connect back to the classical food web. This pathway was only discovered in the 1980s but is now understood to be the dominant energy conduit in nutrient-poor waters, not a decomposition dead-end.

Question 5

Explain why shortening a marine food chain by one trophic level dramatically increases the amount of fish biomass available for harvest from the same primary production.

Model answer: Trophic efficiency is approximately 10%, meaning 90% of energy is lost at each transfer as metabolic heat, waste, and incomplete digestion. Removing one trophic level multiplies the energy reaching the harvest level by ~10×. For example, if phytoplankton produce 10,000 units: a 4-step chain yields ~10 units at the top, while a 3-step chain yields ~100 units — a 10-fold increase. This is why krill-eating species (like herring, anchovies, and baleen whales) support far more biomass than tuna, which sit multiple steps above the base.

This principle has direct fisheries implications. Upwelling zones (like Peru/Chile, Benguela current, Somali coast) have short food chains because large diatom phytoplankton bloom directly support large zooplankton like copepods and krill, which in turn support anchovies and sardines. These regions produce disproportionate fractions of global fish catch despite covering a small fraction of ocean area. In contrast, oligotrophic gyres cover ~40% of Earth's surface but contribute minimally to fisheries.