What is the primary impact of nutrient pollution on aquatic ecosystems?

Nutrient pollution, primarily from sources such as agricultural runoff and wastewater discharges, introduces excessive amounts of nitrogen and phosphorus into aquatic ecosystems. This influx of nutrients can lead to eutrophication, a process where nutrient-rich conditions promote the rapid growth of algae. Over time, as these algae die and decompose, oxygen levels in the water can significantly drop, leading to hypoxic or anoxic conditions—commonly referred to as "dead zones."

These low-oxygen conditions are detrimental to aquatic life, particularly fish, which require adequate oxygen to survive. As a result, fish populations may decline due to mortality rates increasing and reduced reproductive success. This chain reaction disrupts the entire aquatic food web, causing not just a decrease in fish populations, but also negatively impacting other organisms that rely on fish for food, thus further destabilizing the ecosystem.

The other options present potential positive outcomes related to changes in aquatic environments, but they do not accurately reflect the significant adverse effects that nutrient pollution tends to have. Thus, understanding the primary impact of nutrient pollution highlights the crucial balance needed to maintain healthy aquatic ecosystems.