Why are nutrients from fertilizer harmful to aquatic ecosystems? They cause an increase in fish populations. The increase in algae population reduces oxygen in the water. They can infect plants, which causes animals and humans to become sick. Fish and other organisms grow too large and can no longer live in the ecosystem.

The nutrients from fertilizers are harmful to aquatic ecosystems because they increase the algae population which reduces oxygen in the water. This phenomena is called an algae bloom and results in the loss of oxygen in the water. With the subsequent decline in oxygen within the water animal life is often hurt and many species can be killed. This phenomena of algae blooms and harm of the aquatic system can occur both in fresh and saltwater environments.