What factor is most likely to decrease freshwater biodiversity?

Eutrophication is a process that occurs when excess nutrients, particularly nitrogen and phosphorus, enter freshwater ecosystems. This nutrient overload typically originates from agricultural runoff, wastewater, and other human activities. As a result, it promotes excessive growth of algae, leading to algal blooms. These blooms can significantly reduce water clarity and deplete oxygen levels in the water when the algae die and decompose.

The decline in oxygen levels, known as hypoxia, can create "dead zones" where aquatic life struggles to survive, ultimately leading to decreased biodiversity. Many species of fish and invertebrates are sensitive to changes in oxygen and are unable to cope with rapid shifts in their environments, resulting in population declines or local extinctions. Such changes can disrupt the balance of the ecosystem, affecting not only the species directly impacted but also the broader food web. Thus, eutrophication stands out as a significant threat to freshwater biodiversity due to its cascading effects on aquatic life, which ultimately degrades the ecosystem's health and biodiversity.