How can agricultural runoff lead to eutrophication in freshwater bodies?

Agricultural runoff can lead to eutrophication in freshwater bodies by introducing excessive nutrients, particularly nitrogen and phosphorus.

Agricultural runoff is primarily water that has been used for irrigation and carries with it fertilisers and other agricultural chemicals from the fields. These substances, rich in nutrients like nitrogen and phosphorus, then flow into nearby freshwater bodies such as rivers, lakes, and ponds. This process is known as nutrient pollution or nutrient loading.

The sudden influx of nutrients stimulates an overgrowth of algae and other aquatic plants, a phenomenon known as an algal bloom. This rapid growth of algae is the first stage of eutrophication. The algae form a dense mat on the surface of the water, blocking sunlight from reaching the aquatic plants below. As a result, these plants die off due to lack of sunlight, which is necessary for photosynthesis.

When these plants die, they sink to the bottom of the water body and begin to decompose. The decomposition process is facilitated by bacteria, which consume oxygen in the water. This leads to a significant decrease in the levels of dissolved oxygen, creating a condition known as hypoxia or oxygen depletion.

Aquatic animals, such as fish and invertebrates, require dissolved oxygen to survive. When the oxygen levels drop drastically, it can lead to large-scale death of these organisms, resulting in a dead zone where no aquatic life can survive. This is the final stage of eutrophication.

In addition, some types of algae that thrive in nutrient-rich conditions can produce toxins that are harmful to both aquatic life and humans. These harmful algal blooms, or HABs, can cause illness in humans who come into contact with the contaminated water or consume tainted seafood.

In conclusion, agricultural runoff plays a significant role in the eutrophication of freshwater bodies. The excessive nutrients it carries stimulate algal blooms, which lead to oxygen depletion and the creation of dead zones, severely impacting aquatic ecosystems.