A chemical spill has just killed off all the algae in a part of the open ocean. how will this affect the food webs in that part?

Most affected will be the creatures that eat algae (like whales, flamingo etc.)

next the creature that eat them and that they also eat

then gradually the web spreads affecting both prey and predator of the animals

Well algae is a main source of food for many herbivores in the ocean. the herbivores would have no food causing them to die off which would led to the carnivores to die off as well. the food web would become unstable