The cows let out their poop which has nutrients and is fertilizer
The fertilizer goes into the water
The fertilizer causes over lets more plants grow which causes overgrowth.
Because of the plant overgrowth the algae levels get really high and completely cover the water which blocks the SAV
Because the algae covers the water no sun light can come through which makes all the living things in the water die. It's a dead zone. Now the decomposes are going to eat the dead things and nothing will live in that water anymore.
for an example you can now see this fish floating up in the air because it dies in the dead zone. Now the decomposes have a nice snack!