Why would a decrease in phytoplankton affect the entire ocean food chain? . a) crustaceans and fish eat phytoplankton, and large ocean mammals eat fish. b) phytoplankton are the base of the food chain, and all ocean life depend on them for survival. c) zooplankton would have to replace phytoplankton, and there is a limited amount of zooplankton. d) every ocean animal eats phytoplankton including krill, crustaceans, fish, and large mammals