CLEP Biology Ecosystems and Energy Flow 4 — Questions and Answers
Question 1: In the carbon cycle, which process is responsible for transferring carbon from the atmosphere into marine organisms?
- Methanogenesis by archaea
- Photosynthesis by marine phytoplankton (Correct answer)
- Chemosynthesis at hydrothermal vents
- Decomposition by marine bacteria
Correct answer: Photosynthesis by marine phytoplankton
Marine phytoplankton fix atmospheric CO₂ dissolved in seawater through photosynthesis, forming the base of marine food webs.
Question 2: Eutrophication of a lake following nutrient runoff leads to fish kills primarily because:
- High phosphorus levels are directly toxic to fish gills
- Algal blooms block sunlight, killing aquatic plants and fish eggs
- Decomposition of dying algae depletes dissolved oxygen (Correct answer)
- Nitrogen gas bubbles out of solution and suffocates fish
Correct answer: Decomposition of dying algae depletes dissolved oxygen
Massive algal blooms die and sink; aerobic decomposition of this organic matter consumes most dissolved oxygen, creating hypoxic conditions that kill fish.
Question 3: Which of the following is an example of a keystone species exerting top-down control on an ecosystem?
- Grasses producing large amounts of biomass that support herbivores
- Sea otters limiting sea urchin populations, preserving kelp forests (Correct answer)
- Nitrogen-fixing bacteria enabling plant growth in poor soils
- Decomposers recycling nutrients back to primary producers
Correct answer: Sea otters limiting sea urchin populations, preserving kelp forests
Sea otters are a classic keystone predator; by controlling sea urchin numbers, they prevent overgrazing of kelp, maintaining entire kelp forest ecosystems.
Question 4: In comparing aquatic and terrestrial ecosystems, which statement is generally true about energy pyramids?
- Aquatic ecosystems always have more trophic levels than terrestrial ones
- In aquatic ecosystems, biomass of producers often equals or is less than that of consumers (Correct answer)
- Terrestrial ecosystems have higher energy transfer efficiency
- Aquatic ecosystems have no detrital food chain
Correct answer: In aquatic ecosystems, biomass of producers often equals or is less than that of consumers
In many aquatic ecosystems, especially open oceans, phytoplankton reproduce so rapidly that consumers (zooplankton) can maintain biomass equal to or greater than producers.
Question 5: Sulfur enters the biotic component of an ecosystem primarily through:
- Volcanic eruptions releasing SO₂ into the atmosphere
- Plant uptake of sulfate (SO₄²⁻) ions from soil solution (Correct answer)
- Animal respiration releasing hydrogen sulfide
- Lightning converting atmospheric sulfur into bioavailable forms
Correct answer: Plant uptake of sulfate (SO₄²⁻) ions from soil solution
Plants absorb sulfate ions dissolved in soil water and incorporate sulfur into amino acids such as cysteine and methionine, entering the food web.
Question 6: A researcher finds that doubling the phosphorus input to a lake doubles NPP. This suggests the lake ecosystem is:
- Energy-limited
- Phosphorus-limited (Correct answer)
- Nitrogen-limited
- Light-limited
Correct answer: Phosphorus-limited
If productivity responds proportionally to phosphorus additions, phosphorus is the limiting nutrient controlling primary production in that system.
Question 7: Which type of symbiosis best describes the relationship between a cleaner wrasse fish and larger reef fish that it removes parasites from?
- Parasitism
- Commensalism
- Mutualism (Correct answer)
- Amensalism
Correct answer: Mutualism
The cleaner wrasse gains food (parasites) while the host fish benefits from parasite removal — both organisms benefit, defining mutualism.
CLEP Biology Ecosystems and Energy Flow 4
In the carbon cycle, which process is responsible for transferring carbon from the atmosphere into marine organisms?