Option C: Ecology and conservation (AHL) :C.6 Nitrogen and phosphorus cycles HL Paper 3

Date	November 2017	Marks available	6	Reference code	17N.3.HL.TZ0.18
Level	Higher level	Paper	Paper 3	Time zone	TZ0
Command term	Discuss	Question number	18	Adapted from	N/A

Question

Discuss the causes and consequences of eutrophication.

Markscheme

causes:

a. excess nutrients/nitrates/phosphates in an aquatic system

b. natural runoff from soil/erosion/weathering of rocks

c. runoff of fertilizers «from agricultural land/golf courses»

d. partially treated sewage/animal waste discharged into waterways

consequences: [4 max]

e. algal blooms

f. blocks light for photosynthetic organisms

g. dead organisms sink to bottom of water and decompose

h. decomposers/microorganisms increase BOD/use oxygen

i. oxygen/DO availability for other organisms decreases

j. decrease in biodiversity/disappearance of organisms

OWTTE

[Max 6 Marks]

Date	May 2017	Marks available	2	Reference code	17M.3.HL.TZ2.15
Level	Higher level	Paper	Paper 3	Time zone	Time zone 2
Command term	Explain	Question number	15	Adapted from	N/A

Question

Most reef-building corals contain photosynthetic algae, called Zooxanthellae, that live in their cells. Coral bleaching can occur as a result of human-induced changes leading to the Zooxanthellae being ejected from the coral.

State the type of interaction that occurs between Zooxanthellae and reef-building corals.

[1]

State the trophic level of Zooxanthellae.

[1]

When coral is bleached, certain organisms become more common in the ecosystem such as the cnidarian Gorgonia, the echinoderm Diadema, other algae and certain sponges. State the term that is used for organisms whose presence provides evidence of the existence of a particular environmental condition.

[1]

A coat of algae builds up on coral reefs as a consequence of eutrophication. Explain the relationship between eutrophication and algal growth.

[2]

Explain how an excessive growth of algae on coral reefs can be controlled by top-down factors.

[2]

Markscheme

symbiosis/mutualism

producers

indicator species

a. eutrophication is nutrient enrichment of a body of water

b. example of nutrients eg: nitrates

c. «nutrients» serve as fertilizer for the algae «promoting growth»

a. top-down factors refer to predation/herbivory/trophic level above another one

b. which limit/control population growth

c. named example of a top-down predator eg: parrotfish. Do not accept general names, like “fish”.

Date	May 2017	Marks available	2	Reference code	17M.3.HL.TZ1.16
Level	Higher level	Paper	Paper 3	Time zone	Time zone 1
Command term	Suggest	Question number	16	Adapted from	N/A

Question

The predominant source of phosphorus is rock containing phosphate (phosphate rock). The graph below shows the world production between 1900 and 2009.

Some scientists estimate that available phosphorus reserves in the Earth will be completely depleted within approximately 100 years. Discuss the implications of these estimates.

[2]

The percentage of phosphorus in an ecosystem that is recycled per year is in most cases very small, and far smaller than the percentage of nitrogen that is recycled. Suggest reasons for this difference.

[2]

Nitrates (NO ) are components of the nitrogen cycle. Outline the possible conversions of NO in the nitrogen cycle.

[2]

Markscheme

a. production of fertilizers will decrease/price of fertilizers will rise

b. less food production / increase in cost of foods

c. development of alternative methods of agriculture

d. Phosphate needed by living organisms for nucleic acids/ATP so lack will affect growth negatively

a. largest store of phosphorus «in ecosystems» is in marine sediments and minerals/phosphate rock while nitrogen is in the atmosphere

b. main source of release of phosphorous is by weathering of rocks «very slow process»/ nitrogen is by bacterial action

c. high concentrations of nitrogen/low concentration of phosphorous «compounds» in living organism

d. phosphorus is not a very soluble mineral

a. assimilation by plants / conversion to amino acids

b. denitrification to nitrogen gas / reduction to nitrogen «N2» by denitrifying bacteria

c. reduction of nitrates to nitrites

Date	November 2016	Marks available	6	Reference code	16N.3.HL.TZ0.18
Level	Higher level	Paper	Paper 3	Time zone	TZ0
Command term	Discuss	Question number	18	Adapted from	N/A

Question

Discuss how crop plants obtain the phosphorus that they need to grow and whether the supply of phosphorus to crops is sustainable.

Markscheme

Natural cycle

a. plants absorb phosphorus from the soil by the roots

b. soil phosphorus comes from weathered «phosphate» rocks

c. «soil/organic» phosphorus «also» comes from humus/plant/animal residues/guano/microorganisms

d. phosphorus is a limiting «macronutrient» factor in plant growth
OR
phosphorus is necessary for vital functions/ATP/DNA
OR
deficiency of phosphorus «in soil» limits plant growth/production

Sustainability

e. «natural» phosphorus cycle is slow/not sustainable with increased crop production

f. phosphorus is replenished «in the soil» by use of mineral/organic fertilizers

g. phosphorus/phosphate is «mainly» mined from «phosphate» rocks

h. rocks are becoming used up/inaccessible so non-sustainable/non-renewable
OR
the use of fertilizers is non-sustainable «with increased crop production»

i. increased demand for food/meat/dairy/grain increases demand for fertilizers

j. runoff/erosion/leaching of fertilizers decreases potential supply for crops/OWTTE

Date	May 2016	Marks available	1	Reference code	16M.3.HL.TZ0.14
Level	Higher level	Paper	Paper 3	Time zone	TZ0
Command term	Suggest	Question number	14	Adapted from	N/A

Question

Where high amounts of ammonium ions are present in agricultural areas, gaseous ammonia can be released into the atmosphere. This ammonia can dissolve and be carried across distances and then be deposited through precipitation. In a study of the effects of deposition of ammonium in a forest, soil samples were taken starting at the forest edge next to an open field and moving toward the centre of the forest.

Outline the procedure that was most likely used by the researchers to decide where to take the samples.

[2]

List two sources of the ammonium in the forest soils apart from deposition in rainfall.

[2]

Suggest one reason for ammonium levels in the interior of the forest being lower than the soil ammonium close to the edge.

[1]

Markscheme

Line transect
OR
a line is identified and individual walks the line

Decision on where to start and end the line/transect

Samples taken every 20m/at regular intervals

Activity of soil microorganisms/ammonification/nitrogen fixation

Urine/feces/other nitrogenous waste

Dead organisms

Fertilizers «on edge of forest»

Greater distance from an open field where they are applying fertilizer

More ammonium absorbed/recycled

Less ammonium in rain reaches soil/trapped in treetops

Date	November 2015	Marks available	2	Reference code	15N.3.HL.TZ0.8
Level	Higher level	Paper	Paper 3	Time zone	TZ0
Command term	Outline	Question number	8	Adapted from	N/A

Question

State one example of a bacterium that forms aggregates.

[1]

Outline the process of nitrogen fixation by a named free-living bacterium.

[2]

The image shows part of a sewage treatment plant.

Outline the role of bacteria in trickling filter bed treatment of sewage.

[3]

Markscheme

Pseudomonas aeruginosa / Vibrio fischeri

Accept other correct answers.

a. (atmospheric) nitrogen is converted to ammonia;

b. by Azotobacter;

Do not accept Rhizobium.

a. (saprotrophic) bacteria/biofilm fix on the surface of the rocks/material in the trickling filter;

b. bacteria decompose the sewage/organic matter as it runs over the filter bed;

c. bacteria break down organic matter aerobically;

d. the rocks increase the surface area for the decomposition of organic matter;

e. filter bed can treat high amounts of sewage quickly;

Date	May 2011	Marks available	2	Reference code	11M.3.HL.TZ1.8
Level	Higher level	Paper	Paper 3	Time zone	Time zone 1
Command term	State	Question number	8	Adapted from	N/A

Question

Outline the diversity of Eubacteria according to cell wall structure.

[2]

State the role of Rhizobium and Nitrobacter in the nitrogen cycle.

Rhizobium: …………………………………………………..

Nitrobacter: …………………………………………………..

[2]

Explain the use of bacteria in bioremediation.

[2]

Markscheme

can be Gram-positive or Gram-negative;
Gram-negative have a thinner wall/less peptidoglycan/converse;
Gram-negative have an outer layer of lipopolysaccharide and protein;

Rhizobium: converts atmospheric nitrogen to ammonia / nitrogen fixation;
Nitrobacter: oxidizes nitrite into nitrate / nitrification;

bacteria remove contaminants from the environment;
by using them as energy sources;
(or) by converting them to a soluble/ harmless form;
example of bioremediation (e.g. Pseudomonas is used to clean up oil spills);

Date	May 2012	Marks available	1	Reference code	12M.3.HL.TZ1.10
Level	Higher level	Paper	Paper 3	Time zone	Time zone 1
Command term	Deduce	Question number	10	Adapted from	N/A

Question

Mercury is a toxic substance and its biomagnification in aquatic food chains is a global concern. A study tested the effects of inorganic phosphorus levels on both algal growth and mercury accumulation by Daphnia mendotae feeding on the algae which absorb mercury from the water. Daphnia may subsequently be eaten by fish.

Experiments were performed in six different tanks with increasing phosphorus concentrations. The same amount of mercury was added to each tank. The quantity of algae, determined by measuring the amount of chlorophyll a, and the accumulation of mercury by Daphnia was measured in each tank. Chlorophyll a and mercury levels were measured twice at each of the six different phosphorus concentrations.

Deduce the tank in which the quantity of algae was highest.

[1]

a (i).

Deduce the tank in which the level of mercury accumulation in Daphnia was lowest.

[1]

a (ii).

Outline the relationship between phosphorus concentration in the water and the accumulation of mercury by Daphnia.

[2]

Using the data, suggest reasons for the relationship between phosphorus concentration in the tanks and mercury concentration in Daphnia.

[2]

Markscheme

(tank) 6

a (i).

(tank) 4

a (ii).

increasing phosphorus decreases mercury accumulation by Daphnia;
increasing concentration of phosphorus above tank 4 has little effect;
highest levels of mercury accumulation recorded at low phosphorus concentrations;

increased phosphorus concentrations produce algal blooms/increased algae;
larger populations of algae result in smaller concentrations of mercury in the (individual) algae;
less mercury taken in/accumulated by Daphnia from their food;

Date	November 2010	Marks available	2	Reference code	10N.3.HL.TZ0.8
Level	Higher level	Paper	Paper 3	Time zone	TZ0
Command term	Outline	Question number	8	Adapted from	N/A

Question

State two nitrogen-fixing bacteria.

[1]

a(i).

Outline the conditions that favour denitrification.

[2]

a(ii).

Markscheme

Award [1] for any two of the following.

Rhizobium / Azotobacter / Cyanobacteria / Frankia

a(i).

anaerobic conditions;

due to waterlogged soils;

a(ii).