How can reduced dissolved oxygen levels affect aquatic life?

Reduced dissolved oxygen levels directly impact aerobic organisms, which depend on oxygen to survive and function. Aquatic organisms, such as fish and many invertebrates, rely on dissolved oxygen in water for respiration. When oxygen levels decline, these organisms struggle to extract enough oxygen, leading to stress, reduced growth rates, and even mortality. Consequently, this decline in aerobic organisms can disrupt the entire aquatic ecosystem, as many species rely on these organisms for food or as part of their ecological interactions.

In contrast, the other options misrepresent the effects of low dissolved oxygen. Enhanced fish population growth or increased fertility of water bodies contradicts the necessary conditions for a healthy aquatic environment, while stating that there is no significant effect on aquatic organisms overlooks the critical role that oxygen plays in sustaining life in aquatic ecosystems.