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How do fertilizer runoff and untreated sewage change coastal marine ecosystems?

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Diagram showing ocean zonation by depth and region, with labeled layers: littoral, sublittoral, bathyal, abyssal, and hadal along the seafloor, and epipelagic, mesopelagic, bathypelagic, and abyssalpelagic in open water. The littoral zone is at the high-tide mark. Vertical scale marks photic and aphotic zones from surface to 10,000 meters. — Marine ecosystem Zonation of the ocean. Note that in the littoral zone the water is at the high-tide mark.

Fertilizer runoff and untreated sewage significantly degrade coastal marine ecosystems through a process known as cultural eutrophication. By introducing excess nitrogen and phosphorus into coastal waters, these pollutants act as fertilizers for phytoplankton and algae, triggering excessive population growth.12

As these massive algal blooms die and decompose, the process consumes vast amounts of dissolved oxygen. This leads to hypoxia, creating "dead zones" where oxygen levels are too low to support marine life. Bottom-dwelling organisms such as shrimp, crabs, and marine worms often suffocate in these conditions, causing massive die-offs that disrupt the entire food chain and local economies.213

Furthermore, nutrient enrichment can trigger toxic water blooms, such as red tides, which may poison wildlife and threaten human health. Over time, these shifts can fundamentally transform ecosystems, such as turning diverse coral reefs into algae-dominated environments that support far fewer species. Protecting these vital habitats requires improved pollution control and more sustainable agricultural and municipal practices.12