IB ESS HL 2.3 Biogeochemical Cycles Questions

Analyse carbon and nitrogen cycle diagrams, storage data, bacterial transformations, ocean acidification and human impacts on cycle balance.

Exam points

classify and interpret organic/inorganic stores, stores/sinks/sources and long-term carbon storage in cycle diagrams
draw, label and trace carbon-cycle stores and flows, including photosynthesis, feeding, respiration, decomposition, sequestration and mitigation pathways
calculate and compare carbon storage or net carbon gain between ecosystems or treatment conditions, retaining units and interpreting sink effectiveness
explain ocean carbon exchange and acidification by linking atmospheric CO₂, dissolution, pH change and effects on marine calcifiers
explain how human activities alter carbon stores and flows, then outline mitigation through reduced fossil-carbon release or increased capture and storage
complete and interpret nitrogen-cycle diagrams by tracing bacterial transformations, plant assimilation and transfer into decomposer protein
analyse how agriculture, deforestation, aquaculture, urbanization, fertilizer use, combustion and sewage alter nitrogen stores and flows, including flow-versus-storage evidence and carbon-cycle comparisons

Covered syllabus points

2.3.1—Biogeochemical cycles purpose 2.3.2—Cycle components 2.3.3—Carbon stores 2.3.4—Carbon flows 2.3.5—Carbon sequestration 2.3.6—Ecosystems as carbon stores/sinks/sources 2.3.7—Fossil fuels as carbon stores 2.3.8—Agriculture as carbon store/source/sink 2.3.9—Ocean carbon exchange 2.3.10—Ocean acidification 2.3.11—Alleviating carbon cycle impacts 2.3.12 (HL)—Lithosphere carbon stores 2.3.13 (HL)—Carbonate fossilization 2.3.14 (HL)—Fossil fuel formation 2.3.15 (HL)—Methane production 2.3.16 (HL)—Methane cycle 2.3.17 (HL)—Nitrogen cycle stores 2.3.18 (HL)—Bacterial roles in nitrogen cycle 2.3.19 (HL)—Denitrification conditions 2.3.20 (HL)—Plant nitrogen fixation 2.3.21 (HL)—Nitrogen cycle flows 2.3.22 (HL)—Human impacts on nitrogen cycle 2.3.23 (HL)—Haber process 2.3.24 (HL)—Nitrogen planetary boundary 2.3.25 (HL)—Global collaboration needed

[Maximum number: 7]

Explain how a community of decomposers contributes to the stability of the whole ecosystem.

a. Decomposers will feed on dead organic matter/detritus (like dead leaves, wood, carcasses, feces)...;
b. ... reducing their accumulation / cleaning up Earth;
c. They will break down organic matter into inorganic matter…;
d. ...enhancing soil fertility / increasing nutrient availability;
e. The inorganic minerals (nitrates, phosphates, metals) will then feed plant communities / will be absorbed by plant roots...;
f. ...supporting high vegetation/plant abundance;
g. ...and may support high habitat/niche diversity;
h. Decomposers are responsible for nutrient cycling / e.g. nitrogen cycling (nitrogen fixation/ammonification/nitrification/denitrification/phosphorus cycling/carbon cycle;
i. ...high flow of matter may support complex food webs;
j. Decomposers are the basis of a detritus-based food web / important part of ecosystem flow of energy / may be eaten/pass biomass on to carnivore community...;
k. ...supporting/stabilizing carnivore abundance;
l. As decomposer numbers decrease due to predation, so do carnivores;
m. ... thereby contribute to regulation of the carnivore community / which is an example of negative feedback (leading to stability);
n. Decomposer will release organic matter through their own death/defecation...;
o. ... which would provide food for other decomposers;
p. Decomposers participate in humus formation...;
q. ...increasing water holding capacity of soil (and soil fertility = MPd);
r. Decomposers may compete with one another regulating their populations (another example of negative feedback = MPm);
s. Decomposers, like earthworms, loosen soil / disperse nutrients / create space for air and water circulation...;
t. ...helping root growth / improving soil structure;
u. Fragmentation: larger invertebrates initially fragment larger chunks of organic matter (then bacteria and fungi complete decomposition into nutrients = MPc);
v. After a disturbance, decomposers will increase their metabolic/respiration rate/feeding on detritus...;
w. ...contributing to a faster return to original state (resilience entailing stability);

[Maximum number: 7]

Figure 1: Carbon cycle

(a)

Identify one inorganic carbon storage in Figure 1.

[ 1 ]

(b)

Draw a labelled diagram to illustrate the flows of carbon between plants and the atmosphere shown in Figure 1.

[ 2 ]

(c)

Using the data in Figure 1, calculate the net gain in carbon in the oceans in $10^{15}\,\mathrm{g}$ .

[ 1 ]

(92+0.8)-(90+0.1)=2.7 in units of $10^{15}\,\mathrm{g}$ (2.7 Pg).

(d)

Describe one advantage and one disadvantage of the oceans as a carbon sink.

Advantage:

Disadvantage:

[ 2 ]

Advantage:
oceans are large so absorb a lot of CO2 / increases primary productivity/photosynthesis / reduces global warming/climate change;

Disadvantage:
increasing $\mathrm{CO}_{2}$ in oceans causes acidification / ocean acidification harms sensitive marine species/ecosystems like coral reefs;

(e)

Outline one method to mitigate the effects of atmospheric carbon storage.

[ 1 ]

a. reforestation/afforestation initiatives so that more carbon can be absorbed;
b. carbon capture and storage/CCS to bury $\mathrm{CO}_{2}$ underground;
c. land use change/urban renewal leading to revegetation/green roofs to absorb $\mathrm{CO}_{2}$ ;
d. switch to renewable energy to reduce use of fossil fuels;

Note: There are multiple acceptable responses to this question i.e. any process that reduces release of $\mathrm{CO}_{2}$ or absorbs $\mathrm{CO}_{2}$ from atmosphere
NB: This is an "outline" question, not "identify", so requires a little more than "reforestation" (i.e. WHY reforestation?) or "reduce use of fossil fuels" (...need to state HOW fossil fuel use might be reduced),

[Maximum number: 7]

Figure 1: Carbon cycle

(a)

Identify one carbon flow caused by human activities in Figure 1.

[ 1 ]

(b)

Draw a labelled diagram to illustrate the flows of carbon between plants and the atmosphere shown in Figure 1.

[ 2 ]

(c)

Using the data in Figure 1, calculate the net gain in carbon in the oceans in $10^{15}\,\mathrm{g}$ .

[ 1 ]

(92+0.8)-(90+0.1)=2.7 in units of $10^{15}\,\mathrm{g\,C\,yr^{-1}}$ .

(d)

Describe one advantage and one disadvantage of the oceans as a carbon sink.

Advantage:

Disadvantage:

[ 2 ]

Advantage:
oceans are large so absorb a lot of $\mathrm{CO}_{2}$ / increases primary productivity/photosynthesis / reduces global warming/climate change;

Disadvantage:
increasing $\mathrm{CO}_{2}$ in oceans causes acidification / ocean acidification harms sensitive marine species/ecosystems like coral reefs;

(e)

Outline one method to mitigate the effects of atmospheric carbon storage.

[ 1 ]

Note: There are multiple acceptable responses to this question i.e. any process that reduces release of CO2 or absorbs CO2 from atmosphere
NB: This is an "outline" question, not "identify", so requires a little more than "reforestation" (i.e. WHY reforestation?) or "reduce use of fossil fuels" (...need to state HOW fossil fuel use might be reduced),

[Maximum number: 1]

Figure 1: Carbon cycle

Identify one inorganic carbon storage in Figure 1.

[Maximum number: 2]

Figure 1: Carbon cycle

Draw a labelled diagram to illustrate the flows of carbon between plants and the atmosphere shown in Figure 1.

[Maximum number: 2]

Ecosystem category	Ecosystem	Soil organic carbon (metric ton carbon per hectare)	Living biomass (metric ton carbon per hectare)	Total (metric ton carbon per hectare)
Terrestrial forest ecosystems	Boreal forest	150	94	244
Terrestrial forest ecosystems	Tropical forest	83	167	250
Coastal marine ecosystems	Mangroves	1030	464	1494
Coastal marine ecosystems	Tidal marsh	920	31	951
Coastal marine ecosystems	Seagrass meadows	608	10	618

Figure:Carbon storage in different ecosystems

With reference to Figure 6, outline the effectiveness of mangroves and tropical rainforests in the mitigation of climate change.

mangroves/tropical rainforest mitigate climate change/reduce $\mathrm{CO}_{2}$ in the atmosphere by absorbing $\mathrm{CO}_{2}$ ;
mangroves sequester/remove a greater amount of $\mathrm{CO}_{2}$ (per unit area) than the other ecosystems/tropical forest/ mangroves are more efficient at storing carbon than tropical forests / mangroves are more effective carbon sinks than tropical forests;
tropical forest hold more carbon in their living biomass / mangroves hold more carbon in the soil;
both tropical rainforest and mangroves store the highest amount of $\mathrm{CO}_{2}$ within their living biomass for their own ecosystem category;
mangroves hold approximately 1500 metric ton carbon per hectare / tropical forest hold approx 230 metric ton carbon per hectare;
tropical forests may remove less $\mathrm{CO}_{2}$ per unit area but occupy a far greater area globally / mangroves remove more $\mathrm{CO}_{2}$ per unit area but occupy less area globally;

Marking guidance:

Accept quantification without units.

[Maximum number: 1]

Figure 3(a): A simplified food web in a kelp forest ecosystem

Figure 3(b): Kelp forest ecosystems with and without the presence of sea otters

The arrows indicate the flow of carbon between the atmosphere and the carbon stored in kelp.

Calculate the difference between carbon stored in kelp in the ecosystem with and without sea otters.

$180\left(\mathrm{~g} \mathrm{C} \mathrm{m}^{-2}\right)-14\left(\mathrm{~g} \mathrm{C} \mathrm{m}^{-2}\right)=166\left(\mathrm{~g} \mathrm{C} \mathrm{m}^{-2}\right)$ ;

[Maximum number: 1]

Figure 1: Carbon cycle

Identify one carbon flow caused by human activities in Figure 1.

[Maximum number: 7]

Describe how the use of fossil fuels may impact the abiotic conditions of oceanic systems.

a. $\mathrm{CO}_{2}$ / greenhouse gas emissions lead to increasing global/atmospheric temperatures;
b. higher levels of atmospheric $\mathrm{CO}_{2}$ lead to more $\mathrm{CO}_{2}$ dissolved in ocean water…;
c. ...which leads to ocean acidification / reduced pH;
d. increased global temperatures cause rise in ocean temperature...;
e. ...also cause expansion/increase in volume of ocean/rising sea levels (same MP if linked to melting of glaciers MPg);
f. ...also may increase algal growth, which absorbs more sunlight / making water even warmer / increasing oxygen availability in water (locally);
g. ...also cause melting of ice caps/glaciers...;
h. ... which, in turn, dilutes salinity of surrounding sea;
i. rising sea temperatures reduce solubility of gases/available oxygen [no contradiction with local impact in MPf];
j. changing salinity/temperatures will cause changes in ocean currents/conveyor belt;
k. oil spills may cause pollution/reduce light penetration...;
l. ...regionally reduced light penetration/killing of phytoplankton results in reduced oxygen levels (due to reduced photosynthesis) [no contradiction with local impact in MPf];
m. changing weather patterns/events may affect wind-driven oceanic currents;
n. lower pH may cause death of sea life (e.g. coral) which will decompose (aerobically), thus increasing $\mathrm{CO}_{2}$ in oceans / or decompose anaerobically in deep ocean, thus increasing $\mathrm{CH}_{4}$ ;
o. Increased GHGs/temperature may increase ocean evaporation leading to loss of water / increased salinity;

Note: Do not credit mention of acid rain, NOx, SO2 emissions affecting ocean pH. Do not credit use of oil in manufacturing petrochemicals (e.g. plastics) Credit may be given for any other valid responses linking fossil fuel use to some abiotic change in oceans.

[Maximum number: 1]

Figure 1: Carbon cycle

Outline one method to mitigate the effects of atmospheric carbon storage.

[Maximum number: 4]

Outline the processes by which nitrogen in the atmosphere may eventually be found in the protein of a decomposer.

[Maximum number: 7]

Explain how human activities can impact the nitrogen cycle.

All question bank results loaded

Covered syllabus points

Question 1

Question 2

Question (a)

Question (b)

Question (c)

Question (d)

Question (e)

Question 3

Question (a)

Question (b)

Question (c)

Question (d)

Question (e)

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12