Fig. 1.3 shows a freshwater pond ecosystem. Water is essential for all life on Earth and is used for drinking, agriculture, industry and energy production. ...

(a) Two abiotic factors affecting organisms in the pond [2]

Abiotic factors are non-living components of the environment. Any two of the following are acceptable:

• Water temperature [1]
• Light intensity / availability [1]
• Dissolved oxygen concentration [1]
• pH of the water [1]
• Turbidity (cloudiness of water) [1]

Each of these physical or chemical factors affects the types and abundance of organisms that can survive in the pond. For example, dissolved oxygen concentration determines which aquatic animals can thrive, and light intensity affects the rate of photosynthesis by algae and aquatic plants.

(b) Role of decomposers in the pond ecosystem [3]

1. Decomposers (bacteria and fungi) break down dead organic matter, including dead plants, dead animals and faeces, in the mud at the bottom of the pond. [1]
2. As they decompose this material, they release nutrients and mineral ions (such as nitrates and phosphates) back into the water and mud. [1]
3. These released nutrients are then available for uptake by producers such as algae and water lilies, completing the nutrient cycle and sustaining continued primary production. [1]

Without decomposers, nutrients would remain locked in dead organisms and the pond would gradually run out of the mineral ions needed by plants for growth.

(c) How eutrophication could affect the pond [6]

Eutrophication is a process triggered by excess nutrients entering a body of water. It follows a predictable sequence:

1. Excess nutrients, particularly nitrates and phosphates, enter the pond from sources such as agricultural fertiliser runoff or untreated sewage. [1]
2. These nutrients stimulate rapid growth of algae on the water surface, producing an algal bloom. [1]
3. The dense layer of algae blocks sunlight from reaching submerged plants (such as the water lilies shown in the diagram). Without light, these plants cannot photosynthesise and they die. [1]
4. Aerobic bacteria multiply rapidly to decompose the large quantity of dead plant material. This decomposition consumes dissolved oxygen from the water (biological oxygen demand rises). [1]
5. Dissolved oxygen levels in the pond fall dramatically. The water becomes deoxygenated (anoxic). [1]
6. Fish, dragonfly larvae and other aerobic organisms die because they cannot obtain enough oxygen for respiration. Biodiversity in the pond decreases sharply. [1]