What is a significant impact of fertilizers when they enter aquatic systems?

The significant impact of fertilizers entering aquatic systems is that they lead to nutrient pollution and eutrophication. When fertilizers, which are rich in nitrogen and phosphorus, run off into lakes, rivers, and coastal waters, they introduce excessive nutrients into these ecosystems. This nutrient overload promotes rapid algal growth, leading to phytoplankton blooms.

As algae proliferate, they eventually die off, and their decomposition consumes oxygen in the water. This process can lead to hypoxic conditions or "dead zones," where oxygen levels become too low to support most marine life, resulting in fish kills and the decline of aquatic biodiversity. Eutrophication can also alter the food web, shift species composition, and negatively impact water quality and recreational activities.

In contrast, the other options do not accurately reflect the impact of fertilizers on aquatic systems: improving fish populations, increasing oxygen levels, and reducing water temperature are not direct consequences of fertilizer runoff. Instead, they typically result from more balanced and healthy aquatic environments.