(a) Fig. 6.1 shows part of the carbon cycle in the ocean.
(a)(i) Phytoplankton contain chlorophyll. Explain how phytoplankton lower carbon dioxide concentrations in the atmosphere.
(a)(ii) Suggest the role of zooplankton in this ecosystem.
(a)(iii) State the names of two processes that add carbon dioxide to the atmosphere.
(a)(iv) Describe the effect on the environment of additional carbon dioxide in the atmosphere.
(b) Fig. 6.2 shows the pyramid of energy for a food chain in an ocean ecosystem.
(b)(i) On Fig. 6.2, label the trophic level containing herbivores.
(b)(ii) Explain why pyramids of energy usually have fewer than five trophic levels.
(b)(iii) State two advantages of using a pyramid of energy rather than a pyramid of biomass to represent a food chain.
(c) The oceans contain fish stocks that can be managed as a sustainable resource to provide food for humans.
(c)(i) State what is meant by the term sustainable resource.
(c)(ii) Fishing can be managed to promote the conservation of fish stocks. Explain one way that fishing can be managed to conserve fish stocks.
▶️ Answer/Explanation
(a)(i) Phytoplankton lower CO2 concentrations by photosynthesis, where they use CO2 and sunlight to produce carbohydrates, storing carbon in their biomass.
(a)(ii) Zooplankton act as primary consumers, feeding on phytoplankton and transferring energy to higher trophic levels. They also contribute to the carbon cycle through respiration and decomposition.
(a)(iii) Two processes adding CO2 to the atmosphere:
1. Respiration (by organisms).
2. Combustion (burning fossil fuels).
(a)(iv) Additional CO2 enhances the greenhouse effect, leading to:
1. Global warming.
2. Climate change (e.g., extreme weather).
3. Ocean acidification (harmful to marine life).
(b)(i) The middle layer of the pyramid (labeled “Zooplankton”) represents herbivores.
(b)(ii) Energy pyramids rarely exceed five levels because ~90% of energy is lost at each trophic level (as heat/metabolism), leaving insufficient energy to sustain higher levels.
(b)(iii) Advantages of energy pyramids:
1. Quantifies actual energy flow (in kJ/m2/year).
2. Accounts for time, unlike biomass pyramids which are static.
(c)(i) A sustainable resource is one replenished at the same rate it is used, ensuring long-term availability (e.g., managed fish stocks).
(c)(ii) Example management method:
Fishing quotas limit catches, preventing overfishing and allowing fish populations to reproduce sustainably.
(a) The flow chart in Fig. 6.1 shows one pathway of nitrogen as it travels through the nitrogen cycle.
Complete the flow chart in Fig. 6.1.
(b) State the names of two processes that occur in both the carbon and nitrogen cycles.
(c) State the principal source of energy input to biological systems.
▶️ Answer/Explanation
(a) The nitrogen cycle involves several key steps: lightning/root nodules fix atmospheric nitrogen, converting it into nitrate. Ammonia is converted to urea, which is further processed into nitrate. Finally, denitrification returns nitrogen to the atmosphere.
(b) Both the carbon and nitrogen cycles involve decomposition (breakdown of organic matter) and feeding (transfer of nutrients through trophic levels). Another common process is excretion, which releases nutrients back into the environment.
(c) The principal source of energy for biological systems is the Sun. Solar energy drives photosynthesis, forming the basis of energy flow in ecosystems.
Final Answers:
(a) lightning / root nodules ; nitrate ; urea ; nitrate ; denitrification ;
(b) any two from: decomposition / decay ; feeding ; excretion ;
(c) (the) Sun ;