Runoff gathers up fertilizers used on crops and lawns as it travels across the land. These toxins will eventually find their way into a lake, river, or stream, where they may cause toxic algae development.
Therefore the answer is the harmful growth of algae.
Algae blooms are caused by an excessive amount of nitrogen and phosphorus in a short period of time. Algal growth consumes oxygen and inhibits sunlight from reaching submerged plants. The oxygen in the water is consumed when the algae eventually die. Due to a shortage of oxygen, aquatic life cannot survive.
The Gulf of Mexico's largest dead zone, covering over 6,500 square miles, occurs every summer as a result of nutrient contamination from the Mississippi River Basin.
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