Why can excess nitrogen and phosphates create low-oxygen coastal waters?

Excess nitrogen and phosphates, primarily from agricultural runoff, sewage, and industrial waste, trigger a process known as cultural eutrophication in coastal waters.12

When these nutrients enter the ocean, they act as fertilizer for phytoplankton and algae, causing them to grow in excessive quantities, often forming large water blooms. As these massive populations of organisms die, they sink to the bottom and decompose. The decomposition process consumes vast amounts of dissolved oxygen, leading to hypoxia—a state of extremely low oxygen concentration.132

Because these oxygen levels fall below the threshold necessary to support marine life, bottom-dwelling organisms such as crabs, shrimp, and fish are unable to survive, resulting in the formation of "dead zones". This degradation of coastal ecosystems is a significant environmental concern, particularly in poorly mixed waters near river mouths and industrial centers.432