Eutrophication

It starts with nutrients. Fertilizer runoff, sewage, or animal waste adds nitrogen and phosphorus to a lake, far more than the algae and plants there normally get. Algae treat this like an all-you-can-eat buffet and explode into a dense bloom covering the surface. That bloom doesn't last. Algae have short lifespans, and a population that big runs out of nutrients or light fast, so huge amounts of it die at once and sink.

That dead algae becomes food for decomposer bacteria, and their populations explode too. Decomposition is aerobic respiration: bacteria consume oxygen as they break down the organic matter. With so much dead algae to process, bacterial oxygen demand skyrockets and dissolved oxygen in the water plummets. Fish and other aerobic organisms are left gasping in water that can no longer support them. This is why the algae itself usually is not the poison. The killer is oxygen starvation caused by the decomposers, a step removed from the bloom. The oxygen crash hits hardest at the bottom, where dead algae accumulate and bacteria work, so a lake can show a sharp drop in oxygen with depth, from normal near the surface to nearly zero below, a pattern called stratified oxygen depletion.

This same chain plays out at continental scale. Fertilizer spread on Iowa cornfields washes into streams, into the Mississippi River, and down to the Gulf of Mexico, feeding blooms far from the original field. The result is the Gulf's seasonal dead zone, where oxygen drops so low that shrimp and fish must flee or die, linking a single farming decision to an ecosystem collapse a thousand miles downstream.