Phytoplankton is the first level of our food chain, followed by the zooplankton, which feeds on the phytoplankton. The zooplankton are then eaten by krill, fish and other crustaceans, which all go on to be eaten by big fish, penguins, seals, walruses and whales. The food chain continues when these are eaten by mammals like polar bears.(Vegas)
If you think about the food chain logically it is easy to understand how, without plankton, all of the oceans animals would die. Without phytoplankton, zooplankton would not have food and die. Without zooplankton, krill, smaller fish and other crustaceans would have nothing to eat and they would die, etc, etc. Until finally you get all the way out to large mammals like whales, dolphins, and manatees. All animals in the ocean depend on plankton for survival.
Although humans don't currently feed off polar bears and fish as their entire diet, at one time we did. Hunter-gatherers relied on meat and fish as part of their daily diet. Without plankton they would not have had access to many kinds of meat and fish. Meaning, hunter-gatherers would have had to rely solely on the food that they gathered, rather than caught and killed; these include roots, and berries and nuts. These foods would eventually have become so sparse that large groups of hunter-gatherers would have starved to death.(Pearson) Seafood has played an important role throughout history and still does today.