Eutrophication

Nutrient enrichment triggers a chain that ends in oxygen collapse. The killing step is not the algae but their decomposition.

The limiting nutrient: Phosphorus usually limits growth in fresh water and nitrogen in the sea. Reducing the limiting one is what breaks the chain.
Why summer is worst: Warm water holds less oxygen, stratification cuts off the bottom layer from the surface, and light drives faster growth. All three push the same way.
Recovery is slow: Phosphorus accumulated in sediment continues to release for years after external inputs stop — internal loading.

Text description

Nitrogen and phosphorus from agriculture and wastewater enter a lake or slow river. The added nutrients trigger a rapid algal bloom, which shades out submerged plants below. When the bloom dies, bacteria decompose the mass of dead algae and consume dissolved oxygen in the process. Oxygen falls, fish and invertebrates die, and the decomposition of those in turn consumes more oxygen, reinforcing the collapse.