Questions: Marine Food Web Structure and Energy Transfer

Question 1

A coastal upwelling region and a tropical oligotrophic gyre each fix exactly the same amount of primary production per year. Which ecosystem produces more harvestable fish per unit of primary production, and why?

• A. The tropical gyre — greater biodiversity creates more complex food webs that ultimately support more fish
• B. The upwelling region — its shorter food chain (fewer trophic steps from phytoplankton to fish) means more energy reaches the fish level
• C. Both produce the same amount — total fish production depends only on total primary production, not food chain length
• D. The upwelling region, but only because of higher water temperatures that accelerate fish metabolism

Answer: (B)

With 10% efficiency per trophic level, every additional step costs 90% of the energy. In an upwelling region, large diatoms are grazed by large zooplankton, which are eaten directly by schooling fish (3 links from sunlight to fish). In the oligotrophic gyre, the food chain is longer — small picoplankton → microzooplankton → mesozooplankton → small fish → larger fish (5+ links). A 3-link chain delivers ~1% of primary production to fish; a 5-link chain delivers only 0.01% — a 100-fold difference. Option A reverses the logic: biodiversity (longer webs) reduces, not increases, energy efficiency to large fish.

Question 2

A great white shark has extremely high mercury concentrations in its tissues despite living in open ocean water with very low dissolved mercury levels. What best explains this?

• A. Sharks produce mercury internally as a byproduct of their unique metabolic pathways
• B. Mercury bioaccumulates at each trophic transfer — the shark consumes many prey items, each of which concentrated mercury from their own food, amplifying the toxin exponentially up the chain
• C. Mercury in seawater is selectively absorbed through shark skin and gill membranes over decades
• D. Sharks filter large volumes of water, removing and concentrating dissolved mercury through gill filtration

Answer: (B)

Bioaccumulation follows the same logic as trophic energy transfer, but in reverse for persistent toxins. Unlike energy (which is lost at each step), toxins like mercury (as methylmercury) are not metabolized or excreted efficiently — they are retained in tissues. Each predator consumes many prey items, accumulating the mercury burden of all of them. A shark at trophic level 5 may have consumed thousands of prey items over its lifetime, each carrying concentrated mercury from their own prey. Concentrations can be 10⁷ times higher in top predators than in surrounding water.

Question 3

Removing a top predator from a marine food web (e.g., overfishing sharks) has no significant impact on organisms more than one trophic level below it.

• T. True
• F. False

Answer: False

This is incorrect — trophic cascades can propagate multiple levels through a food web. When top predators are removed, their prey populations typically explode (prey release), which then overgrazes the prey's food source, causing that population to collapse. A classic example: removing large predatory fish → explosion of mid-level fish → collapse of zooplankton → bloom of phytoplankton. Effects can propagate across 3–4 trophic levels and even alter habitat structure (e.g., sea otter removal → sea urchin explosion → loss of kelp forests).

Question 4

In a 4-trophic-level marine food web (phytoplankton → zooplankton → small fish → large fish), approximately 0.1% of the energy fixed by phytoplankton reaches the large fish, assuming 10% efficiency at each level.

• T. True
• F. False

Answer: True

With 10% efficiency per step: phytoplankton → zooplankton retains 10%; zooplankton → small fish retains another 10% (= 1% of original); small fish → large fish retains another 10% (= 0.1% of original). Three trophic transfers at 10% each: 0.1 × 0.1 × 0.1 = 0.001 = 0.1%. This exponential decay explains why top predators are rare relative to primary producers — and why fishing down the food web (targeting smaller, lower-trophic-level fish) can sustainably yield far more biomass than fishing top predators.

Question 5

Why do upwelling regions like the coast of Peru support much higher commercial fish yields than subtropical gyres, despite both being ocean systems? Use trophic efficiency in your answer.

Model answer: Upwelling regions deliver nutrient-rich deep water to the surface, fueling explosive blooms of large phytoplankton (especially diatoms). These are grazed by large zooplankton, which are eaten directly by schooling fish like anchovies — a food chain of only 3 trophic steps. Subtropical gyres support nutrient-poor waters with small picoplankton, requiring 5 or more trophic steps before energy reaches fish. Since each step loses ~90% of energy, a 3-step chain delivers ~1% of primary production as fish, while a 5-step chain delivers ~0.01% — a 100-fold difference in efficiency. Even with the same primary productivity, the shorter chain in upwelling regions produces far more harvestable fish.

This is why the Peruvian anchovy fishery was historically one of the world's most productive, despite the region's relatively modest size. The combination of high primary productivity AND a short food chain makes upwelling regions extraordinarily productive for large fish. When El Niño events suppress upwelling, the anchovy catch can collapse almost overnight — illustrating how tightly fish abundance is coupled to both nutrient supply and food chain length.