Biology HL

1. 1.

   A sealed transparent mesocosm contains algae, decomposers, small grazers, water and mineral nutrients. Explain why it still needs an energy input, how matter can be recycled inside it, and why sealing the vessel does not guarantee long-term stability.

   [4 marks] · no calculator

   Worked explanation

   1. Distinguish a closed system from an isolated one using the transparent boundary.
   2. Atoms can cycle, but dissipated energy cannot be recycled into photosynthetic input without a new energy source.
   3. Stability depends on rates and environmental conditions, not simply on preventing leaks.

   Common mistake: Matter retention does not make energy recyclable or ensure populations remain balanced indefinitely.

   Marking points

   • Light entering through the glass can supply energy for photosynthesis; the vessel is closed to matter but not isolated from energy.
   • Energy transferred through feeding and respiration is dissipated as heat and must be replenished.
   • Decomposers and producers recycle carbon and mineral nutrients through biological and chemical processes.
   • Unsuitable temperature, population imbalance or oxygen/nutrient constraints can destabilize the system despite matter retention.

   Examiner tip: Matter retention does not make energy recyclable or ensure populations remain balanced indefinitely.

2. 2.

   A hypothetical fish stock gains 900 tonnes through recruitment and growth in a year and loses 300 tonnes through natural mortality. Harvest is 750 tonnes. Ignore migration. Calculate the net biomass change and evaluate a proposal to call this harvest sustainable merely because the stock was large at the start.

   [4 marks]

   Worked explanation

   1. Subtract natural losses before judging how much biomass can be removed without decline under the stated assumptions.
   2. The stock falls by 150 tonnes, not rises by the 150-tonne difference between gains and harvest alone.
   3. A future quota must consider changing recruitment and uncertainty, not treat the hypothetical annual gain as constant.

   Common mistake: Gross production is not net replacement; ignoring natural mortality overestimates the removable surplus.

   Marking points

   • Net replacement before harvest = 900 - 300 = 600 tonnes.
   • Net stock change = 600 - 750 = -150 tonnes that year.
   • A harvest exceeding net replacement reduces stock even if initial biomass is large.
   • Sustainability needs repeated estimates, uncertainty and breeding-stock/environmental considerations; one year's balance is not a guaranteed safe long-term quota.

   Examiner tip: Gross production is not net replacement; ignoring natural mortality overestimates the removable surplus.

3. 3.

   After fertiliser runoff enters a hypothetical lake, algal biomass rises and later dissolved oxygen falls as much of the algae dies. Explain the causal sequence and predict how reducing nutrient inputs could affect recurrence, without claiming immediate recovery is guaranteed.

   [4 marks] · no calculator

   Worked explanation

   1. Distinguish growth stimulated by nutrients from the later oxygen demand of decomposing biomass.
   2. Track the mechanism from runoff to microbial respiration rather than saying fertiliser directly consumes all oxygen.
   3. A reduction in the external driver may prevent recurrence while internal nutrient stores sustain a lagged response.

   Common mistake: The main oxygen-demand explanation is respiration during decomposition, not the claim that fertiliser itself respires.

   Marking points

   • Added nitrate/phosphate can remove nutrient limitation and increase algal growth.
   • Death of algae supplies organic matter for decomposers.
   • Aerobic decomposition increases biochemical oxygen demand and can lower dissolved oxygen, threatening aquatic animals.
   • Reducing nutrient loading can limit future blooms, but stored nutrients and existing organic matter can delay recovery.

   Examiner tip: The main oxygen-demand explanation is respiration during decomposition, not the claim that fertiliser itself respires.

4. 4.

   A hypothetical fish stock begins a year at 2400 tonnes. Recruitment and growth add 500 tonnes, natural mortality removes 200 tonnes and harvest removes 450 tonnes. Ignore migration. Calculate the final biomass and the largest harvest that would leave biomass unchanged under these fixed one-year assumptions. Explain one reason this is not a guaranteed safe future quota.

   [4 marks]

   Worked explanation

   1. Keep the initial stock separate from annual additions and removals.
   2. The proposed 450-tonne removal exceeds the 300-tonne net replacement by 150 tonnes, matching the fall from 2400 to 2250.
   3. The balancing harvest is conditional on fixed flows, so a management recommendation also needs uncertainty and population structure.

   Common mistake: Initial biomass is not annual replacement, and gross growth must not be used as a harvest limit without subtracting other losses.

   Marking points

   • Net replacement before harvest is 500 - 200 = 300 tonnes.
   • Final biomass is 2400 + 500 - 200 - 450 = 2250 tonnes.
   • A 300-tonne harvest would leave biomass unchanged under the supplied one-year assumptions.
   • Recruitment, mortality, breeding-stock structure and environmental conditions can vary; uncertainty means that this arithmetic is not a guaranteed safe long-term quota.

   Examiner tip: Initial biomass is not annual replacement, and gross growth must not be used as a harvest limit without subtracting other losses.

5. 5.

   After nutrient runoff is reduced, a hypothetical lake continues to show low oxygen for several weeks. Suggest two mechanisms consistent with delayed recovery and explain why this observation alone does not show that reducing nutrient input was ineffective.

   [3 marks] · no calculator

   Worked explanation

   1. Separate a change in external nutrient loading from the remaining material already inside the lake.
   2. Both decomposition and internal recycling can maintain the effects after the external driver is reduced.
   3. A lag is compatible with a useful intervention, although it does not guarantee eventual recovery or exclude other causes.

   Common mistake: Do not assume environmental state must recover instantly when one input changes, or claim that a lag proves the intervention succeeded.

   Marking points

   • Previously accumulated organic matter can continue to support oxygen-consuming decomposition.
   • Stored nutrients, including sediment release under suitable conditions, can continue supporting algal production.
   • Recovery can lag behind reduced input; repeated measurements of nutrient loads, oxygen and relevant conditions are needed to assess the intervention.

   Examiner tip: Do not assume environmental state must recover instantly when one input changes, or claim that a lag proves the intervention succeeded.

Original, unofficial practice questions from SubjectScout — not official past-paper questions or complete syllabus coverage. Added English teaching explanations are AI-authored; independent teacher review is pending.