Which of the following best describes the term 'eutrophication'?

Eutrophication is a natural process that occurs when water bodies receive excessive amounts of nutrients, primarily phosphorus and nitrogen, often from fertilizers, agricultural runoff, and sewage. This nutrient overload stimulates the rapid growth of algae, known as algal blooms. As these algae die and decompose, they consume a significant amount of dissolved oxygen in the water, leading to conditions of hypoxia or even anoxia, where the water becomes depleted of oxygen. This depletion can harm aquatic life, as fish and other organisms require oxygen to survive. Thus, the process of eutrophication effectively describes the sequence of nutrient enrichment followed by oxygen loss and its detrimental effects on the ecosystem.

In contrast, other options do not accurately encompass the concept of eutrophication. The first option suggests a rise in fish populations, which can occur for various reasons but is not a defining characteristic of eutrophication. The third option mentions acidity, which is unrelated to the primary effects of eutrophication, and while changes in pH can occur, they are not central to the definition. Lastly, the idea of natural filtration pertains more to the cleansing or purification processes in water systems rather than the nutrient-driven process of eutrophication.