OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What is eutrophication?

1. The process of converting waste into energy

2. When a thriving ecosystem becomes a dead ecosystem

3. The gradual increase of biodiversity in lakes

4. The introduction of new species into an ecosystem

The correct answer is: When a thriving ecosystem becomes a dead ecosystem

Eutrophication refers to the process that typically occurs when water bodies such as lakes and rivers receive an excess of nutrients, particularly nitrogen and phosphorus, often from fertilizers, agricultural runoff, and sewage. This nutrient overload leads to excessive growth of algae, which can create dense algal blooms. When these algae die, they decompose, a process that consumes oxygen in the water. The reduction in oxygen levels can result in hypoxic conditions, making it difficult for aquatic life, such as fish and other organisms, to survive. As a consequence, the once-thriving ecosystem can become depleted of oxygen and unable to support its previous variety of life forms, leading to a dead ecosystem. This chain reaction captures the essence of what eutrophication entails, marking a significant ecological imbalance. Thus, the definition surrounding eutrophication highlights the detrimental transformation from a healthy ecosystem to one that struggles to sustain its biological community.