3.5Unit 3 — Life on Earth

Food Production

5 min read 4 parts Lesson 23 of 31

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Part 1 of 2Fertilisers and eutrophication

Fertilisers

Human population growth requires increased food yields. Farmers use fertilisers (providing nitrates for protein synthesis) and pesticides (killing crop pests).

Sequence of freshwater algal blooms and eutrophication

1. LeachingHeavy rainfall washes excess synthetic fertilisers from agricultural fields into nearby freshwater streams, rivers, and lochs.
2. Algal BloomThe high nitrate and phosphate concentrations cause rapid, uncontrolled proliferation of surface algae, forming a thick algal bloom.
3. Light BlockageThe dense blanket of algae blocks sunlight from penetrating the water, killing submerged aquatic plants as they can no longer photosynthesise.
4. Bacterial DecompositionDead plants and algae provide a vast food source for aerobic bacteria, whose populations explode.
- 5. Oxygen Depletion & DeathMultiplying bacteria consume vast quantities of dissolved oxygen for aerobic respiration.
- Dissolved oxygen levels plummet, causing fish and other aquatic invertebrates to suffocate and die.

Part 2 of 2Pesticides, bioaccumulation, and alternatives

1Pesticides and bioaccumulation

Pesticides sprayed onto crops can be washed into rivers and absorbed by microscopic aquatic organisms.
Many pesticides are persistent (non-biodegradable): they cannot be broken down by animal metabolism.
As energy transfers along the food chain, the concentration of the toxic pesticide increases at each successive trophic level (bioaccumulation / biomagnification).
Apex predators at the top of the food chain accumulate lethal or sub-lethal toxic concentrations, causing eggshell thinning or mortality.

2Sustainable alternatives

Biological controlIntroducing a natural predator, herbivore, or pathogen to suppress pest populations (e.g. ladybirds introduced to feed on aphid pests, or parasitic wasps against whiteflies in greenhouses).
Genetically Modified (GM) cropsEngineering crops with natural pest resistance (e.g. Bt corn expressing a bacterial toxin harmless to mammals but lethal to insect borers), eliminating the need for chemical pesticide sprays.

Key terms

Term	Meaning
Leaching	The washing of dissolved agricultural nutrients out of soil into aquatic waterways.
Algal bloom	A rapid proliferation and accumulation of algae on the surface of water caused by nutrient enrichment.
Bioaccumulation	The progressive increase in concentration of a non-biodegradable chemical within organisms higher in a food chain.
Biological control	The intentional release of a natural predator or parasite to regulate a target pest population.

Avoid these mistakes

Claiming the algae consume all the oxygen — it is the multiplying decomposer bacteria feeding on dead algae that exhaust dissolved oxygen through aerobic respiration.

Confusing bioaccumulation with leaching — leaching is washing of chemicals into water; bioaccumulation is the buildup of persistent toxins inside living animal tissues up the food chain.