I need help with this question Why would you expect a dead zone to start near the mouth of a river, where the river flows into a body of water?

Water pollution comes from many sources. One of the biggest sources is runoff. Runoff moves over the surface of the land and picks up chemicals such as fertilizer (contains vital nutrients such as nitrogen, phosphorous, and potassium - NPK) from agricultural fields, lawns, and golf courses. It carries the chemicals to bodies of water. The added nutrients from fertilizer often cause the excessive growth of algae, creating algal blooms which are temporary phenomena because the large population of algae use up the available nutrients (Figure 2.4c). When the algae die, the decomposing bacteria use up oxygen in the water so that other aquatic organisms cannot survive. This has occurred over large areas of the ocean, creating dead zones, where low oxygen levels have killed all ocean life. A very large dead zone exists in the Gulf of Mexico. Measures that can help prevent these problems include cutting down on fertilizer use. Preserving wetlands also helps because wetlands filter runoff.

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