Abiotic factors: oxygen levels for aquatic animals.

The idea

Dissolved oxygen is an abiotic factor because it is a non-living physical condition of water. Aquatic animals such as fish, mayfly nymphs and water snails depend on oxygen gas dissolved in the water for aerobic respiration. Oxygen enters water by diffusion from the air and as a by-product of photosynthesis by aquatic plants and algae. Its concentration falls when water is warm, when organic pollution is broken down by microorganisms, or when ice and slow flow reduce mixing.

What earns the marks

• Dissolved oxygen is a non-living, physical component of water, so it is classified as an abiotic factor.
• Aquatic animals absorb dissolved oxygen, often across gills, and use it in aerobic respiration to release energy for life processes.
• Oxygen dissolves into water from the atmosphere and is also produced by photosynthesis of aquatic plants and algae.
• Warm water holds less dissolved oxygen than cold water, so oxygen levels can fall as temperature rises.
• Microorganisms decomposing sewage or other organic waste use up dissolved oxygen, lowering the amount available to aquatic animals.
• Different species tolerate different oxygen concentrations, so oxygen level affects which organisms can survive and where they are found in a water body.

Common mistakes and how to put them right

• Thinking aquatic animals breathe oxygen from water molecules rather than dissolved oxygen gas.

  Put it right by stating that oxygen gas, O₂, is dissolved between water molecules.

• Assuming plants only add oxygen and never remove it.

  Put it right by noting that aquatic plants respire day and night and only photosynthesise in light.

• Confusing the effect of temperature on oxygen solubility with the effect on animal metabolism.

  Put it right by separating lower solubility in warm water from faster respiration rates in warm conditions.

Exam tips

• Use the phrase dissolved oxygen and name the gas as O₂ to show precise scientific vocabulary.
• When comparing sites, quote oxygen concentrations from the data and link each value to a named organism or group.
• Explain a chain of reasoning: warmer water or pollution lowers dissolved oxygen, so aerobic respiration is limited and sensitive animals die or move away.