Fertilizers give nutrients into the water. Since there are more nutrients, more algae and other plants grow. But eventually the nutrients are consumed and then the algae die. Then microorganisms decompose the dead algae/plants and in this process oxygen is consumed.
EpidemicRage t1_jaahwb2 wrote
Reply to ELI5: Why is it that when fertilizers make their way into waterways, all the oxygen disappears, killing the fish? by Psychological-Dog994
Fertilizers give nutrients into the water. Since there are more nutrients, more algae and other plants grow. But eventually the nutrients are consumed and then the algae die. Then microorganisms decompose the dead algae/plants and in this process oxygen is consumed.
Hence, overall oxygen levels decrease.