Answer:
They do not get enough oxygen, and die.
Explanation:
When fertilizer flows into rivers and lakes, they trigger the process of eutrophication, which is the increased concentration of nutrients in the water environment. This high concentration of nutrients encourages the overproduction of algae that often completely covers the surface of the aquatic environment.
The large amount of nutrients, also increased by the death of these algae, helps in reproduction and exaggerated increase of aerobic bacteria. By using oxygen in their breathing, these bacteria end up consuming a large amount of this gas, which causes the death of several other species, such as fish and other aquatic beings. It is noteworthy that, with the lack of oxygen, aerobic bacteria also end up dying.
In addition, excessive algae build-up forms a layer that prevents sunlight from passing through, which consequently affects the photosynthesis process performed by algae and aquatic plants. This causes the death of photosynthetic organisms that are not located in the most superficial layers.
