Eutrophication: get the sequence in the right order

The chain, in the order the mark scheme awards:

  1. Excess nitrate (from fertiliser run-off) or sewage enters the water.
  2. It causes rapid growth of algae and water plants. An algal bloom.
  3. The bloom blocks light, so plants below cannot photosynthesise and die.
  4. Decomposers (bacteria) feed on the dead plants and algae, multiply, and respire, using up oxygen.
  5. The dissolved oxygen concentration falls, so fish and other aquatic animals die.

The step that separates a full-mark answer from a half one is step 4: it is the decomposers respiring that removes the oxygen, not the algae 'using up oxygen' directly. Name nitrate as the ion responsible, 'fertiliser' alone is weaker.

The greenhouse effect and climate change

The mechanism: greenhouse gases. Mainly carbon dioxide (from burning fossil fuels and deforestation) and methane (from cattle and rice fields/landfill). Absorb and re-emit heat (long-wave radiation) that would otherwise leave the Earth, so the atmosphere warms. Consequences examiners accept: climate change, melting ice caps, rising sea levels, changing rainfall and more extreme weather affecting habitats. Be precise that the gases trap heat/long-wave radiation: 'CO2 traps the sun's rays' is too loose. The link to deforestation is double: cutting trees releases stored carbon AND removes the trees that would absorb CO2 in photosynthesis.

Deforestation and pollution

Deforestation consequences to list with reasons: loss of habitat and reduced biodiversity (species lose where they live); soil erosion (no roots to bind soil) and flooding; less photosynthesis so more CO2 remains, adding to the greenhouse effect; disruption of the water cycle. Plastic pollution in water: plastics are non-biodegradable, persist for a long time, and harm aquatic animals (entanglement, blocked guts when eaten). Water pollution from sewage links back to eutrophication and to spreading pathogens. For each, the exam wants the pollutant, the mechanism of harm, and the effect on organisms. Three linked parts, not a single word.

Conservation and sustainable resources

The positive half of the topic. Sustainable development means meeting present needs without stopping future generations meeting theirs. Sustainable resource use examples: replanting forests (reforestation) and quotas/protected areas for fish. Reasons for conserving species and habitats that score: maintaining biodiversity, protecting food chains, potential medicines and genetic resources, and keeping ecosystems functioning. Recycling and reducing fossil-fuel use connect to the carbon dioxide and pollution sections. Questions often ask you to 'suggest how a named resource could be managed sustainably'. Give a specific method (e.g. Fishing quotas, restocking, net mesh size to let young fish through) rather than 'don't overuse it'.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

  • Acid rain: Rain that has a pH lower than normal (below pH 5.6), caused by sulfur dioxide and nitrogen oxides dissolving in atmospheric water. These gases come from burning fossil fuels. Acid rain damages vegetation, acidifies lakes (killing aquatic life), and erodes buildings and statues.
  • Conservation: The management and protection of ecosystems, habitats and species to maintain biodiversity and ensure sustainable use of natural resources. Conservation strategies include establishing protected areas, breeding programmes for endangered species, seed banks and legislation against poaching.
  • Deforestation: The large-scale removal of forests by cutting down or burning trees, often to clear land for agriculture, cattle ranching or urban development. Deforestation leads to habitat loss, reduced biodiversity, soil erosion, flooding and increased atmospheric carbon dioxide.
  • Greenhouse effect: The natural process by which certain gases in the Earth's atmosphere (greenhouse gases such as carbon dioxide, methane and water vapour) trap heat energy radiated from the Earth's surface, keeping the planet warm enough to support life. The enhanced greenhouse effect caused by increased greenhouse gas emissions leads to global warming.
  • Pollution: The introduction of harmful substances or excessive energy into the environment, causing damage to living organisms and ecosystems. Pollution can affect air (e.g. Sulfur dioxide, carbon monoxide), water (e.g. Sewage, fertiliser runoff) or land (e.g. Pesticides, landfill waste).
  • Sustainability: Using natural resources in a way that meets present needs without compromising the ability of future generations to meet their own needs. Sustainable practices include replanting forests, regulating fishing quotas and using renewable energy sources.

Browse the full IGCSE Biology glossary when a related term needs checking.

Apply the topic in practical work

These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.

  • Sampling with Quadrats: Quadrats are square frames placed on the ground to sample organisms in a defined area. Random sampling uses randomly generated coordinates to place quadrats. Systematic sampling places quadrats at regular intervals along a transect line to study how distribution changes across a habitat.