Phytoplanktons are very important component of the marine ecosystem. They are the producers of the marine ecosystem.
The green plants make food by the help of photosynthesis and ultimately serves as the food for small fishes and zooplanktons.
These small fishes are eaten by the large fishes.
In this way the whales eats the small fishes so it can be said that the whales are indirectly dependent on the Phytoplanktons.