What might happen to the organisms in the food web below if the number of phytoplankton and vegetation drastically decreased?

i don't now exactly how to answer this when theres' no food chart however as the vegetation and phytoplankton drastically decrease everything decreases. This is because the producer the bottom of the food chain also known as vegetation is the prime source of energy in every food chain. when there's scarce amounts of producers a domino affect will occur. There will be no food for the first consumers to eat leading their population to decrease and the animals that feed off them will decrease then the animals that feed of them will decrease and so on until the organisms of the food web die out.